* async.c
* Asynchronous notification: NOTIFY, LISTEN, UNLISTEN
*
- * Portions Copyright (c) 1996-2003, PostgreSQL Global Development Group
+ * Portions Copyright (c) 1996-2010, PostgreSQL Global Development Group
* Portions Copyright (c) 1994, Regents of the University of California
*
* IDENTIFICATION
- * $Header: /cvsroot/pgsql/src/backend/commands/async.c,v 1.103 2003/11/09 21:30:36 tgl Exp $
+ * $PostgreSQL: pgsql/src/backend/commands/async.c,v 1.151 2010/02/16 22:34:43 tgl Exp $
*
*-------------------------------------------------------------------------
*/
/*-------------------------------------------------------------------------
- * New Async Notification Model:
- * 1. Multiple backends on same machine. Multiple backends listening on
- * one relation. (Note: "listening on a relation" is not really the
- * right way to think about it, since the notify names need not have
- * anything to do with the names of relations actually in the database.
- * But this terminology is all over the code and docs, and I don't feel
- * like trying to replace it.)
- *
- * 2. There is a tuple in relation "pg_listener" for each active LISTEN,
- * ie, each relname/listenerPID pair. The "notification" field of the
- * tuple is zero when no NOTIFY is pending for that listener, or the PID
- * of the originating backend when a cross-backend NOTIFY is pending.
- * (We skip writing to pg_listener when doing a self-NOTIFY, so the
- * notification field should never be equal to the listenerPID field.)
- *
- * 3. The NOTIFY statement itself (routine Async_Notify) just adds the target
- * relname to a list of outstanding NOTIFY requests. Actual processing
- * happens if and only if we reach transaction commit. At that time (in
- * routine AtCommit_Notify) we scan pg_listener for matching relnames.
- * If the listenerPID in a matching tuple is ours, we just send a notify
- * message to our own front end. If it is not ours, and "notification"
- * is not already nonzero, we set notification to our own PID and send a
- * SIGUSR2 signal to the receiving process (indicated by listenerPID).
- * BTW: if the signal operation fails, we presume that the listener backend
- * crashed without removing this tuple, and remove the tuple for it.
- *
- * 4. Upon receipt of a SIGUSR2 signal, the signal handler can call inbound-
- * notify processing immediately if this backend is idle (ie, it is
- * waiting for a frontend command and is not within a transaction block).
- * Otherwise the handler may only set a flag, which will cause the
- * processing to occur just before we next go idle.
+ * Async Notification Model as of 9.0:
+ *
+ * 1. Multiple backends on same machine. Multiple backends listening on
+ * several channels. (Channels are also called "conditions" in other
+ * parts of the code.)
+ *
+ * 2. There is one central queue in disk-based storage (directory pg_notify/),
+ * with actively-used pages mapped into shared memory by the slru.c module.
+ * All notification messages are placed in the queue and later read out
+ * by listening backends.
+ *
+ * There is no central knowledge of which backend listens on which channel;
+ * every backend has its own list of interesting channels.
+ *
+ * Although there is only one queue, notifications are treated as being
+ * database-local; this is done by including the sender's database OID
+ * in each notification message. Listening backends ignore messages
+ * that don't match their database OID. This is important because it
+ * ensures senders and receivers have the same database encoding and won't
+ * misinterpret non-ASCII text in the channel name or payload string.
+ *
+ * Since notifications are not expected to survive database crashes,
+ * we can simply clean out the pg_notify data at any reboot, and there
+ * is no need for WAL support or fsync'ing.
+ *
+ * 3. Every backend that is listening on at least one channel registers by
+ * entering its PID into the array in AsyncQueueControl. It then scans all
+ * incoming notifications in the central queue and first compares the
+ * database OID of the notification with its own database OID and then
+ * compares the notified channel with the list of channels that it listens
+ * to. In case there is a match it delivers the notification event to its
+ * frontend. Non-matching events are simply skipped.
*
- * 5. Inbound-notify processing consists of scanning pg_listener for tuples
- * matching our own listenerPID and having nonzero notification fields.
- * For each such tuple, we send a message to our frontend and clear the
- * notification field. BTW: this routine has to start/commit its own
- * transaction, since by assumption it is only called from outside any
- * transaction.
+ * 4. The NOTIFY statement (routine Async_Notify) stores the notification in
+ * a backend-local list which will not be processed until transaction end.
+ *
+ * Duplicate notifications from the same transaction are sent out as one
+ * notification only. This is done to save work when for example a trigger
+ * on a 2 million row table fires a notification for each row that has been
+ * changed. If the application needs to receive every single notification
+ * that has been sent, it can easily add some unique string into the extra
+ * payload parameter.
+ *
+ * When the transaction is ready to commit, PreCommit_Notify() adds the
+ * pending notifications to the head of the queue. The head pointer of the
+ * queue always points to the next free position and a position is just a
+ * page number and the offset in that page. This is done before marking the
+ * transaction as committed in clog. If we run into problems writing the
+ * notifications, we can still call elog(ERROR, ...) and the transaction
+ * will roll back.
+ *
+ * Once we have put all of the notifications into the queue, we return to
+ * CommitTransaction() which will then do the actual transaction commit.
+ *
+ * After commit we are called another time (AtCommit_Notify()). Here we
+ * make the actual updates to the effective listen state (listenChannels).
+ *
+ * Finally, after we are out of the transaction altogether, we check if
+ * we need to signal listening backends. In SignalBackends() we scan the
+ * list of listening backends and send a PROCSIG_NOTIFY_INTERRUPT signal
+ * to every listening backend (we don't know which backend is listening on
+ * which channel so we must signal them all). We can exclude backends that
+ * are already up to date, though. We don't bother with a self-signal
+ * either, but just process the queue directly.
+ *
+ * 5. Upon receipt of a PROCSIG_NOTIFY_INTERRUPT signal, the signal handler
+ * can call inbound-notify processing immediately if this backend is idle
+ * (ie, it is waiting for a frontend command and is not within a transaction
+ * block). Otherwise the handler may only set a flag, which will cause the
+ * processing to occur just before we next go idle.
*
- * Although we grab AccessExclusiveLock on pg_listener for any operation,
- * the lock is never held very long, so it shouldn't cause too much of
- * a performance problem.
+ * Inbound-notify processing consists of reading all of the notifications
+ * that have arrived since scanning last time. We read every notification
+ * until we reach either a notification from an uncommitted transaction or
+ * the head pointer's position. Then we check if we were the laziest
+ * backend: if our pointer is set to the same position as the global tail
+ * pointer is set, then we move the global tail pointer ahead to where the
+ * second-laziest backend is (in general, we take the MIN of the current
+ * head position and all active backends' new tail pointers). Whenever we
+ * move the global tail pointer we also truncate now-unused pages (i.e.,
+ * delete files in pg_notify/ that are no longer used).
*
- * An application that listens on the same relname it notifies will get
+ * An application that listens on the same channel it notifies will get
* NOTIFY messages for its own NOTIFYs. These can be ignored, if not useful,
* by comparing be_pid in the NOTIFY message to the application's own backend's
* PID. (As of FE/BE protocol 2.0, the backend's PID is provided to the
* frontend during startup.) The above design guarantees that notifies from
- * other backends will never be missed by ignoring self-notifies. Note,
- * however, that we do *not* guarantee that a separate frontend message will
- * be sent for every outside NOTIFY. Since there is only room for one
- * originating PID in pg_listener, outside notifies occurring at about the
- * same time may be collapsed into a single message bearing the PID of the
- * first outside backend to perform the NOTIFY.
+ * other backends will never be missed by ignoring self-notifies.
+ *
+ * The amount of shared memory used for notify management (NUM_ASYNC_BUFFERS)
+ * can be varied without affecting anything but performance. The maximum
+ * amount of notification data that can be queued at one time is determined
+ * by slru.c's wraparound limit; see QUEUE_MAX_PAGE below.
*-------------------------------------------------------------------------
*/
#include "postgres.h"
+#include <limits.h>
#include <unistd.h>
#include <signal.h>
-#include <errno.h>
-#include <netinet/in.h>
-#include "access/heapam.h"
-#include "catalog/catname.h"
-#include "catalog/pg_listener.h"
-#include "catalog/pg_type.h"
+#include "access/slru.h"
+#include "access/transam.h"
+#include "access/xact.h"
+#include "catalog/pg_database.h"
#include "commands/async.h"
+#include "funcapi.h"
#include "libpq/libpq.h"
#include "libpq/pqformat.h"
#include "miscadmin.h"
#include "storage/ipc.h"
+#include "storage/lmgr.h"
+#include "storage/procsignal.h"
+#include "storage/sinval.h"
#include "tcop/tcopprot.h"
-#include "utils/fmgroids.h"
+#include "utils/builtins.h"
+#include "utils/memutils.h"
#include "utils/ps_status.h"
-#include "utils/syscache.h"
/*
- * State for outbound notifies consists of a list of all relnames NOTIFYed
- * in the current transaction. We do not actually perform a NOTIFY until
- * and unless the transaction commits. pendingNotifies is NIL if no
- * NOTIFYs have been done in the current transaction. The List nodes and
- * referenced strings are all palloc'd in TopTransactionContext.
+ * Maximum size of a NOTIFY payload, including terminating NULL. This
+ * must be kept small enough so that a notification message fits on one
+ * SLRU page.
+ */
+#define NOTIFY_PAYLOAD_MAX_LENGTH 8000
+
+/*
+ * Struct representing an entry in the global notify queue
+ *
+ * This struct declaration has the maximal length, but in a real queue entry
+ * the data area is only big enough for the actual channel and payload strings
+ * (each null-terminated). AsyncQueueEntryEmptySize is the minimum possible
+ * entry size, if both channel and payload strings are empty (but note it
+ * doesn't include alignment padding).
+ *
+ * The "length" field should always be rounded up to the next QUEUEALIGN
+ * multiple so that all fields are properly aligned.
+ */
+typedef struct AsyncQueueEntry
+{
+ int length; /* total allocated length of entry */
+ Oid dboid; /* sender's database OID */
+ TransactionId xid; /* sender's XID */
+ int32 srcPid; /* sender's PID */
+ char data[NAMEDATALEN + NOTIFY_PAYLOAD_MAX_LENGTH];
+} AsyncQueueEntry;
+
+/* Currently, no field of AsyncQueueEntry requires more than int alignment */
+#define QUEUEALIGN(len) INTALIGN(len)
+
+#define AsyncQueueEntryEmptySize (offsetof(AsyncQueueEntry, data) + 2)
+
+/*
+ * Struct describing a queue position, and assorted macros for working with it
+ */
+typedef struct QueuePosition
+{
+ int page; /* SLRU page number */
+ int offset; /* byte offset within page */
+} QueuePosition;
+
+#define QUEUE_POS_PAGE(x) ((x).page)
+#define QUEUE_POS_OFFSET(x) ((x).offset)
+
+#define SET_QUEUE_POS(x,y,z) \
+ do { \
+ (x).page = (y); \
+ (x).offset = (z); \
+ } while (0)
+
+#define QUEUE_POS_EQUAL(x,y) \
+ ((x).page == (y).page && (x).offset == (y).offset)
+
+/* choose logically smaller QueuePosition */
+#define QUEUE_POS_MIN(x,y) \
+ (asyncQueuePagePrecedesLogically((x).page, (y).page) ? (x) : \
+ (x).page != (y).page ? (y) : \
+ (x).offset < (y).offset ? (x) : (y))
+
+/*
+ * Struct describing a listening backend's status
+ */
+typedef struct QueueBackendStatus
+{
+ int32 pid; /* either a PID or InvalidPid */
+ QueuePosition pos; /* backend has read queue up to here */
+} QueueBackendStatus;
+
+#define InvalidPid (-1)
+
+/*
+ * Shared memory state for LISTEN/NOTIFY (excluding its SLRU stuff)
+ *
+ * The AsyncQueueControl structure is protected by the AsyncQueueLock.
+ *
+ * When holding the lock in SHARED mode, backends may only inspect their own
+ * entries as well as the head and tail pointers. Consequently we can allow a
+ * backend to update its own record while holding only SHARED lock (since no
+ * other backend will inspect it).
+ *
+ * When holding the lock in EXCLUSIVE mode, backends can inspect the entries
+ * of other backends and also change the head and tail pointers.
+ *
+ * In order to avoid deadlocks, whenever we need both locks, we always first
+ * get AsyncQueueLock and then AsyncCtlLock.
+ *
+ * Each backend uses the backend[] array entry with index equal to its
+ * BackendId (which can range from 1 to MaxBackends). We rely on this to make
+ * SendProcSignal fast.
+ */
+typedef struct AsyncQueueControl
+{
+ QueuePosition head; /* head points to the next free location */
+ QueuePosition tail; /* the global tail is equivalent to the
+ tail of the "slowest" backend */
+ TimestampTz lastQueueFillWarn; /* time of last queue-full msg */
+ QueueBackendStatus backend[1]; /* actually of length MaxBackends+1 */
+ /* DO NOT ADD FURTHER STRUCT MEMBERS HERE */
+} AsyncQueueControl;
+
+static AsyncQueueControl *asyncQueueControl;
+
+#define QUEUE_HEAD (asyncQueueControl->head)
+#define QUEUE_TAIL (asyncQueueControl->tail)
+#define QUEUE_BACKEND_PID(i) (asyncQueueControl->backend[i].pid)
+#define QUEUE_BACKEND_POS(i) (asyncQueueControl->backend[i].pos)
+
+/*
+ * The SLRU buffer area through which we access the notification queue
+ */
+static SlruCtlData AsyncCtlData;
+
+#define AsyncCtl (&AsyncCtlData)
+#define QUEUE_PAGESIZE BLCKSZ
+#define QUEUE_FULL_WARN_INTERVAL 5000 /* warn at most once every 5s */
+
+/*
+ * slru.c currently assumes that all filenames are four characters of hex
+ * digits. That means that we can use segments 0000 through FFFF.
+ * Each segment contains SLRU_PAGES_PER_SEGMENT pages which gives us
+ * the pages from 0 to SLRU_PAGES_PER_SEGMENT * 0x10000 - 1.
+ *
+ * It's of course possible to enhance slru.c, but this gives us so much
+ * space already that it doesn't seem worth the trouble.
+ *
+ * The most data we can have in the queue at a time is QUEUE_MAX_PAGE/2
+ * pages, because more than that would confuse slru.c into thinking there
+ * was a wraparound condition. With the default BLCKSZ this means there
+ * can be up to 8GB of queued-and-not-read data.
+ *
+ * Note: it's possible to redefine QUEUE_MAX_PAGE with a smaller multiple of
+ * SLRU_PAGES_PER_SEGMENT, for easier testing of queue-full behaviour.
+ */
+#define QUEUE_MAX_PAGE (SLRU_PAGES_PER_SEGMENT * 0x10000 - 1)
+
+/*
+ * listenChannels identifies the channels we are actually listening to
+ * (ie, have committed a LISTEN on). It is a simple list of channel names,
+ * allocated in TopMemoryContext.
+ */
+static List *listenChannels = NIL; /* list of C strings */
+
+/*
+ * State for pending LISTEN/UNLISTEN actions consists of an ordered list of
+ * all actions requested in the current transaction. As explained above,
+ * we don't actually change listenChannels until we reach transaction commit.
+ *
+ * The list is kept in CurTransactionContext. In subtransactions, each
+ * subtransaction has its own list in its own CurTransactionContext, but
+ * successful subtransactions attach their lists to their parent's list.
+ * Failed subtransactions simply discard their lists.
+ */
+typedef enum
+{
+ LISTEN_LISTEN,
+ LISTEN_UNLISTEN,
+ LISTEN_UNLISTEN_ALL
+} ListenActionKind;
+
+typedef struct
+{
+ ListenActionKind action;
+ char channel[1]; /* actually, as long as needed */
+} ListenAction;
+
+static List *pendingActions = NIL; /* list of ListenAction */
+
+static List *upperPendingActions = NIL; /* list of upper-xact lists */
+
+/*
+ * State for outbound notifies consists of a list of all channels+payloads
+ * NOTIFYed in the current transaction. We do not actually perform a NOTIFY
+ * until and unless the transaction commits. pendingNotifies is NIL if no
+ * NOTIFYs have been done in the current transaction.
+ *
+ * The list is kept in CurTransactionContext. In subtransactions, each
+ * subtransaction has its own list in its own CurTransactionContext, but
+ * successful subtransactions attach their lists to their parent's list.
+ * Failed subtransactions simply discard their lists.
+ *
+ * Note: the action and notify lists do not interact within a transaction.
+ * In particular, if a transaction does NOTIFY and then LISTEN on the same
+ * condition name, it will get a self-notify at commit. This is a bit odd
+ * but is consistent with our historical behavior.
*/
-static List *pendingNotifies = NIL;
+typedef struct Notification
+{
+ char *channel; /* channel name */
+ char *payload; /* payload string (can be empty) */
+} Notification;
+
+static List *pendingNotifies = NIL; /* list of Notifications */
+
+static List *upperPendingNotifies = NIL; /* list of upper-xact lists */
/*
- * State for inbound notifies consists of two flags: one saying whether
+ * State for inbound notifications consists of two flags: one saying whether
* the signal handler is currently allowed to call ProcessIncomingNotify
* directly, and one saying whether the signal has occurred but the handler
* was not allowed to call ProcessIncomingNotify at the time.
* does not grok "volatile", you'd be best advised to compile this file
* with all optimization turned off.
*/
-static volatile int notifyInterruptEnabled = 0;
-static volatile int notifyInterruptOccurred = 0;
+static volatile sig_atomic_t notifyInterruptEnabled = 0;
+static volatile sig_atomic_t notifyInterruptOccurred = 0;
/* True if we've registered an on_shmem_exit cleanup */
static bool unlistenExitRegistered = false;
+/* has this backend sent notifications in the current transaction? */
+static bool backendHasSentNotifications = false;
+/* has this backend executed its first LISTEN in the current transaction? */
+static bool backendHasExecutedInitialListen = false;
+/* GUC parameter */
bool Trace_notify = false;
-
-static void Async_UnlistenAll(void);
-static void Async_UnlistenOnExit(void);
+/* local function prototypes */
+static bool asyncQueuePagePrecedesPhysically(int p, int q);
+static bool asyncQueuePagePrecedesLogically(int p, int q);
+static void queue_listen(ListenActionKind action, const char *channel);
+static void Async_UnlistenOnExit(int code, Datum arg);
+static void Exec_ListenPreCommit(void);
+static void Exec_ListenCommit(const char *channel);
+static void Exec_UnlistenCommit(const char *channel);
+static void Exec_UnlistenAllCommit(void);
+static bool IsListeningOn(const char *channel);
+static void asyncQueueUnregister(void);
+static bool asyncQueueIsFull(void);
+static bool asyncQueueAdvance(QueuePosition *position, int entryLength);
+static void asyncQueueNotificationToEntry(Notification *n, AsyncQueueEntry *qe);
+static ListCell *asyncQueueAddEntries(ListCell *nextNotify);
+static void asyncQueueFillWarning(void);
+static bool SignalBackends(void);
+static void asyncQueueReadAllNotifications(void);
+static bool asyncQueueProcessPageEntries(QueuePosition *current,
+ QueuePosition stop,
+ char *page_buffer);
+static void asyncQueueAdvanceTail(void);
static void ProcessIncomingNotify(void);
-static void NotifyMyFrontEnd(char *relname, int32 listenerPID);
-static bool AsyncExistsPendingNotify(const char *relname);
-static void ClearPendingNotifies(void);
+static void NotifyMyFrontEnd(const char *channel,
+ const char *payload,
+ int32 srcPid);
+static bool AsyncExistsPendingNotify(const char *channel, const char *payload);
+static void ClearPendingActionsAndNotifies(void);
+
+
+/*
+ * We will work on the page range of 0..QUEUE_MAX_PAGE.
+ *
+ * asyncQueuePagePrecedesPhysically just checks numerically without any magic
+ * if one page precedes another one. This is wrong for normal operation but
+ * is helpful when clearing pg_notify/ during startup.
+ *
+ * asyncQueuePagePrecedesLogically compares using wraparound logic, as is
+ * required by slru.c.
+ */
+static bool
+asyncQueuePagePrecedesPhysically(int p, int q)
+{
+ return p < q;
+}
+
+static bool
+asyncQueuePagePrecedesLogically(int p, int q)
+{
+ int diff;
+
+ /*
+ * We have to compare modulo (QUEUE_MAX_PAGE+1)/2. Both inputs should
+ * be in the range 0..QUEUE_MAX_PAGE.
+ */
+ Assert(p >= 0 && p <= QUEUE_MAX_PAGE);
+ Assert(q >= 0 && q <= QUEUE_MAX_PAGE);
+
+ diff = p - q;
+ if (diff >= ((QUEUE_MAX_PAGE+1)/2))
+ diff -= QUEUE_MAX_PAGE+1;
+ else if (diff < -((QUEUE_MAX_PAGE+1)/2))
+ diff += QUEUE_MAX_PAGE+1;
+ return diff < 0;
+}
+
+/*
+ * Report space needed for our shared memory area
+ */
+Size
+AsyncShmemSize(void)
+{
+ Size size;
+
+ /* This had better match AsyncShmemInit */
+ size = mul_size(MaxBackends, sizeof(QueueBackendStatus));
+ size = add_size(size, sizeof(AsyncQueueControl));
+
+ size = add_size(size, SimpleLruShmemSize(NUM_ASYNC_BUFFERS, 0));
+
+ return size;
+}
+
+/*
+ * Initialize our shared memory area
+ */
+void
+AsyncShmemInit(void)
+{
+ bool found;
+ int slotno;
+ Size size;
+
+ /*
+ * Create or attach to the AsyncQueueControl structure.
+ *
+ * The used entries in the backend[] array run from 1 to MaxBackends.
+ * sizeof(AsyncQueueControl) already includes space for the unused zero'th
+ * entry, but we need to add on space for the used entries.
+ */
+ size = mul_size(MaxBackends, sizeof(QueueBackendStatus));
+ size = add_size(size, sizeof(AsyncQueueControl));
+
+ asyncQueueControl = (AsyncQueueControl *)
+ ShmemInitStruct("Async Queue Control", size, &found);
+
+ if (!asyncQueueControl)
+ elog(ERROR, "out of shared memory");
+
+ if (!found)
+ {
+ /* First time through, so initialize it */
+ int i;
+
+ SET_QUEUE_POS(QUEUE_HEAD, 0, 0);
+ SET_QUEUE_POS(QUEUE_TAIL, 0, 0);
+ asyncQueueControl->lastQueueFillWarn = 0;
+ /* zero'th entry won't be used, but let's initialize it anyway */
+ for (i = 0; i <= MaxBackends; i++)
+ {
+ QUEUE_BACKEND_PID(i) = InvalidPid;
+ SET_QUEUE_POS(QUEUE_BACKEND_POS(i), 0, 0);
+ }
+ }
+
+ /*
+ * Set up SLRU management of the pg_notify data.
+ */
+ AsyncCtl->PagePrecedes = asyncQueuePagePrecedesLogically;
+ SimpleLruInit(AsyncCtl, "Async Ctl", NUM_ASYNC_BUFFERS, 0,
+ AsyncCtlLock, "pg_notify");
+ /* Override default assumption that writes should be fsync'd */
+ AsyncCtl->do_fsync = false;
+
+ if (!found)
+ {
+ /*
+ * During start or reboot, clean out the pg_notify directory.
+ *
+ * Since we want to remove every file, we temporarily use
+ * asyncQueuePagePrecedesPhysically() and pass INT_MAX as the
+ * comparison value; every file in the directory should therefore
+ * appear to be less than that.
+ */
+ AsyncCtl->PagePrecedes = asyncQueuePagePrecedesPhysically;
+ (void) SlruScanDirectory(AsyncCtl, INT_MAX, true);
+ AsyncCtl->PagePrecedes = asyncQueuePagePrecedesLogically;
+
+ /* Now initialize page zero to empty */
+ LWLockAcquire(AsyncCtlLock, LW_EXCLUSIVE);
+ slotno = SimpleLruZeroPage(AsyncCtl, QUEUE_POS_PAGE(QUEUE_HEAD));
+ /* This write is just to verify that pg_notify/ is writable */
+ SimpleLruWritePage(AsyncCtl, slotno, NULL);
+ LWLockRelease(AsyncCtlLock);
+ }
+}
+
+
+/*
+ * pg_notify -
+ * SQL function to send a notification event
+ */
+Datum
+pg_notify(PG_FUNCTION_ARGS)
+{
+ const char *channel;
+ const char *payload;
+
+ if (PG_ARGISNULL(0))
+ channel = "";
+ else
+ channel = text_to_cstring(PG_GETARG_TEXT_PP(0));
+
+ if (PG_ARGISNULL(1))
+ payload = "";
+ else
+ payload = text_to_cstring(PG_GETARG_TEXT_PP(1));
+
+ Async_Notify(channel, payload);
+
+ PG_RETURN_VOID();
+}
/*
- *--------------------------------------------------------------
* Async_Notify
*
* This is executed by the SQL notify command.
*
- * Adds the relation to the list of pending notifies.
+ * Adds the message to the list of pending notifies.
* Actual notification happens during transaction commit.
* ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+ */
+void
+Async_Notify(const char *channel, const char *payload)
+{
+ Notification *n;
+ MemoryContext oldcontext;
+
+ if (Trace_notify)
+ elog(DEBUG1, "Async_Notify(%s)", channel);
+
+ /* a channel name must be specified */
+ if (!channel || !strlen(channel))
+ ereport(ERROR,
+ (errcode(ERRCODE_INVALID_PARAMETER_VALUE),
+ errmsg("channel name cannot be empty")));
+
+ if (strlen(channel) >= NAMEDATALEN)
+ ereport(ERROR,
+ (errcode(ERRCODE_INVALID_PARAMETER_VALUE),
+ errmsg("channel name too long")));
+
+ if (payload)
+ {
+ if (strlen(payload) >= NOTIFY_PAYLOAD_MAX_LENGTH)
+ ereport(ERROR,
+ (errcode(ERRCODE_INVALID_PARAMETER_VALUE),
+ errmsg("payload string too long")));
+ }
+
+ /* no point in making duplicate entries in the list ... */
+ if (AsyncExistsPendingNotify(channel, payload))
+ return;
+
+ /*
+ * The notification list needs to live until end of transaction, so store
+ * it in the transaction context.
+ */
+ oldcontext = MemoryContextSwitchTo(CurTransactionContext);
+
+ n = (Notification *) palloc(sizeof(Notification));
+ n->channel = pstrdup(channel);
+ if (payload)
+ n->payload = pstrdup(payload);
+ else
+ n->payload = "";
+
+ /*
+ * We want to preserve the order so we need to append every
+ * notification. See comments at AsyncExistsPendingNotify().
+ */
+ pendingNotifies = lappend(pendingNotifies, n);
+
+ MemoryContextSwitchTo(oldcontext);
+}
+
+/*
+ * queue_listen
+ * Common code for listen, unlisten, unlisten all commands.
*
- * Results:
- * XXX
+ * Adds the request to the list of pending actions.
+ * Actual update of the listenChannels list happens during transaction
+ * commit.
+ */
+static void
+queue_listen(ListenActionKind action, const char *channel)
+{
+ MemoryContext oldcontext;
+ ListenAction *actrec;
+
+ /*
+ * Unlike Async_Notify, we don't try to collapse out duplicates. It would
+ * be too complicated to ensure we get the right interactions of
+ * conflicting LISTEN/UNLISTEN/UNLISTEN_ALL, and it's unlikely that there
+ * would be any performance benefit anyway in sane applications.
+ */
+ oldcontext = MemoryContextSwitchTo(CurTransactionContext);
+
+ /* space for terminating null is included in sizeof(ListenAction) */
+ actrec = (ListenAction *) palloc(sizeof(ListenAction) + strlen(channel));
+ actrec->action = action;
+ strcpy(actrec->channel, channel);
+
+ pendingActions = lappend(pendingActions, actrec);
+
+ MemoryContextSwitchTo(oldcontext);
+}
+
+/*
+ * Async_Listen
*
- *--------------------------------------------------------------
+ * This is executed by the SQL listen command.
*/
void
-Async_Notify(char *relname)
+Async_Listen(const char *channel)
{
if (Trace_notify)
- elog(DEBUG1, "Async_Notify(%s)", relname);
+ elog(DEBUG1, "Async_Listen(%s,%d)", channel, MyProcPid);
- /* no point in making duplicate entries in the list ... */
- if (!AsyncExistsPendingNotify(relname))
+ queue_listen(LISTEN_LISTEN, channel);
+}
+
+/*
+ * Async_Unlisten
+ *
+ * This is executed by the SQL unlisten command.
+ */
+void
+Async_Unlisten(const char *channel)
+{
+ if (Trace_notify)
+ elog(DEBUG1, "Async_Unlisten(%s,%d)", channel, MyProcPid);
+
+ /* If we couldn't possibly be listening, no need to queue anything */
+ if (pendingActions == NIL && !unlistenExitRegistered)
+ return;
+
+ queue_listen(LISTEN_UNLISTEN, channel);
+}
+
+/*
+ * Async_UnlistenAll
+ *
+ * This is invoked by UNLISTEN * command, and also at backend exit.
+ */
+void
+Async_UnlistenAll(void)
+{
+ if (Trace_notify)
+ elog(DEBUG1, "Async_UnlistenAll(%d)", MyProcPid);
+
+ /* If we couldn't possibly be listening, no need to queue anything */
+ if (pendingActions == NIL && !unlistenExitRegistered)
+ return;
+
+ queue_listen(LISTEN_UNLISTEN_ALL, "");
+}
+
+/*
+ * SQL function: return a set of the channel names this backend is actively
+ * listening to.
+ *
+ * Note: this coding relies on the fact that the listenChannels list cannot
+ * change within a transaction.
+ */
+Datum
+pg_listening_channels(PG_FUNCTION_ARGS)
+{
+ FuncCallContext *funcctx;
+ ListCell **lcp;
+
+ /* stuff done only on the first call of the function */
+ if (SRF_IS_FIRSTCALL())
{
- /*
- * The name list needs to live until end of transaction, so store
- * it in the top transaction context.
- */
- MemoryContext oldcontext;
+ MemoryContext oldcontext;
+
+ /* create a function context for cross-call persistence */
+ funcctx = SRF_FIRSTCALL_INIT();
- oldcontext = MemoryContextSwitchTo(TopTransactionContext);
+ /* switch to memory context appropriate for multiple function calls */
+ oldcontext = MemoryContextSwitchTo(funcctx->multi_call_memory_ctx);
- pendingNotifies = lcons(pstrdup(relname), pendingNotifies);
+ /* allocate memory for user context */
+ lcp = (ListCell **) palloc(sizeof(ListCell *));
+ *lcp = list_head(listenChannels);
+ funcctx->user_fctx = (void *) lcp;
MemoryContextSwitchTo(oldcontext);
}
+
+ /* stuff done on every call of the function */
+ funcctx = SRF_PERCALL_SETUP();
+ lcp = (ListCell **) funcctx->user_fctx;
+
+ while (*lcp != NULL)
+ {
+ char *channel = (char *) lfirst(*lcp);
+
+ *lcp = lnext(*lcp);
+ SRF_RETURN_NEXT(funcctx, CStringGetTextDatum(channel));
+ }
+
+ SRF_RETURN_DONE(funcctx);
}
/*
- *--------------------------------------------------------------
- * Async_Listen
+ * Async_UnlistenOnExit
*
- * This is executed by the SQL listen command.
+ * This is executed at backend exit if we have done any LISTENs in this
+ * backend. It might not be necessary anymore, if the user UNLISTENed
+ * everything, but we don't try to detect that case.
+ */
+static void
+Async_UnlistenOnExit(int code, Datum arg)
+{
+ Exec_UnlistenAllCommit();
+}
+
+/*
+ * AtPrepare_Notify
*
- * Register a backend (identified by its Unix PID) as listening
- * on the specified relation.
+ * This is called at the prepare phase of a two-phase
+ * transaction. Save the state for possible commit later.
+ */
+void
+AtPrepare_Notify(void)
+{
+ /* It's not allowed to have any pending LISTEN/UNLISTEN/NOTIFY actions */
+ if (pendingActions || pendingNotifies)
+ ereport(ERROR,
+ (errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
+ errmsg("cannot PREPARE a transaction that has executed LISTEN, UNLISTEN or NOTIFY")));
+}
+
+/*
+ * PreCommit_Notify
*
- * Results:
- * XXX
+ * This is called at transaction commit, before actually committing to
+ * clog.
*
- * Side effects:
- * pg_listener is updated.
+ * If there are pending LISTEN actions, make sure we are listed in the
+ * shared-memory listener array. This must happen before commit to
+ * ensure we don't miss any notifies from transactions that commit
+ * just after ours.
*
- *--------------------------------------------------------------
+ * If there are outbound notify requests in the pendingNotifies list,
+ * add them to the global queue. We do that before commit so that
+ * we can still throw error if we run out of queue space.
*/
void
-Async_Listen(char *relname, int pid)
+PreCommit_Notify(void)
{
- Relation lRel;
- HeapScanDesc scan;
- HeapTuple tuple;
- Datum values[Natts_pg_listener];
- char nulls[Natts_pg_listener];
- int i;
- bool alreadyListener = false;
+ ListCell *p;
+
+ if (pendingActions == NIL && pendingNotifies == NIL)
+ return; /* no relevant statements in this xact */
if (Trace_notify)
- elog(DEBUG1, "Async_Listen(%s,%d)", relname, pid);
+ elog(DEBUG1, "PreCommit_Notify");
- lRel = heap_openr(ListenerRelationName, AccessExclusiveLock);
+ Assert(backendHasExecutedInitialListen == false);
- /* Detect whether we are already listening on this relname */
- scan = heap_beginscan(lRel, SnapshotNow, 0, (ScanKey) NULL);
- while ((tuple = heap_getnext(scan, ForwardScanDirection)) != NULL)
+ /* Preflight for any pending listen/unlisten actions */
+ foreach(p, pendingActions)
{
- Form_pg_listener listener = (Form_pg_listener) GETSTRUCT(tuple);
+ ListenAction *actrec = (ListenAction *) lfirst(p);
- if (listener->listenerpid == pid &&
- strncmp(NameStr(listener->relname), relname, NAMEDATALEN) == 0)
+ switch (actrec->action)
{
- alreadyListener = true;
- /* No need to scan the rest of the table */
- break;
+ case LISTEN_LISTEN:
+ Exec_ListenPreCommit();
+ break;
+ case LISTEN_UNLISTEN:
+ /* there is no Exec_UnlistenPreCommit() */
+ break;
+ case LISTEN_UNLISTEN_ALL:
+ /* there is no Exec_UnlistenAllPreCommit() */
+ break;
}
}
- heap_endscan(scan);
- if (alreadyListener)
+ /* Queue any pending notifies */
+ if (pendingNotifies)
{
- heap_close(lRel, AccessExclusiveLock);
- return;
+ ListCell *nextNotify;
+
+ /*
+ * Make sure that we have an XID assigned to the current transaction.
+ * GetCurrentTransactionId is cheap if we already have an XID, but
+ * not so cheap if we don't, and we'd prefer not to do that work
+ * while holding AsyncQueueLock.
+ */
+ (void) GetCurrentTransactionId();
+
+ /*
+ * Serialize writers by acquiring a special lock that we hold till
+ * after commit. This ensures that queue entries appear in commit
+ * order, and in particular that there are never uncommitted queue
+ * entries ahead of committed ones, so an uncommitted transaction
+ * can't block delivery of deliverable notifications.
+ *
+ * We use a heavyweight lock so that it'll automatically be released
+ * after either commit or abort. This also allows deadlocks to be
+ * detected, though really a deadlock shouldn't be possible here.
+ *
+ * The lock is on "database 0", which is pretty ugly but it doesn't
+ * seem worth inventing a special locktag category just for this.
+ * (Historical note: before PG 9.0, a similar lock on "database 0" was
+ * used by the flatfiles mechanism.)
+ */
+ LockSharedObject(DatabaseRelationId, InvalidOid, 0,
+ AccessExclusiveLock);
+
+ /* Now push the notifications into the queue */
+ backendHasSentNotifications = true;
+
+ nextNotify = list_head(pendingNotifies);
+ while (nextNotify != NULL)
+ {
+ /*
+ * Add the pending notifications to the queue. We acquire and
+ * release AsyncQueueLock once per page, which might be overkill
+ * but it does allow readers to get in while we're doing this.
+ *
+ * A full queue is very uncommon and should really not happen,
+ * given that we have so much space available in the SLRU pages.
+ * Nevertheless we need to deal with this possibility. Note that
+ * when we get here we are in the process of committing our
+ * transaction, but we have not yet committed to clog, so at this
+ * point in time we can still roll the transaction back.
+ */
+ LWLockAcquire(AsyncQueueLock, LW_EXCLUSIVE);
+ asyncQueueFillWarning();
+ if (asyncQueueIsFull())
+ ereport(ERROR,
+ (errcode(ERRCODE_PROGRAM_LIMIT_EXCEEDED),
+ errmsg("too many notifications in the NOTIFY queue")));
+ nextNotify = asyncQueueAddEntries(nextNotify);
+ LWLockRelease(AsyncQueueLock);
+ }
}
+}
+
+/*
+ * AtCommit_Notify
+ *
+ * This is called at transaction commit, after committing to clog.
+ *
+ * Update listenChannels and clear transaction-local state.
+ */
+void
+AtCommit_Notify(void)
+{
+ ListCell *p;
/*
- * OK to insert a new tuple
+ * Allow transactions that have not executed LISTEN/UNLISTEN/NOTIFY to
+ * return as soon as possible
*/
+ if (!pendingActions && !pendingNotifies)
+ return;
- for (i = 0; i < Natts_pg_listener; i++)
+ if (Trace_notify)
+ elog(DEBUG1, "AtCommit_Notify");
+
+ /* Perform any pending listen/unlisten actions */
+ foreach(p, pendingActions)
{
- nulls[i] = ' ';
- values[i] = PointerGetDatum(NULL);
+ ListenAction *actrec = (ListenAction *) lfirst(p);
+
+ switch (actrec->action)
+ {
+ case LISTEN_LISTEN:
+ Exec_ListenCommit(actrec->channel);
+ break;
+ case LISTEN_UNLISTEN:
+ Exec_UnlistenCommit(actrec->channel);
+ break;
+ case LISTEN_UNLISTEN_ALL:
+ Exec_UnlistenAllCommit();
+ break;
+ }
}
- i = 0;
- values[i++] = (Datum) relname;
- values[i++] = (Datum) pid;
- values[i++] = (Datum) 0; /* no notifies pending */
+ /*
+ * If we did an initial LISTEN, listenChannels now has the entry, so
+ * we no longer need or want the flag to be set.
+ */
+ backendHasExecutedInitialListen = false;
- tuple = heap_formtuple(RelationGetDescr(lRel), values, nulls);
- simple_heap_insert(lRel, tuple);
+ /* And clean up */
+ ClearPendingActionsAndNotifies();
+}
-#ifdef NOT_USED /* currently there are no indexes */
- CatalogUpdateIndexes(lRel, tuple);
-#endif
+/*
+ * Exec_ListenPreCommit --- subroutine for PreCommit_Notify
+ *
+ * This function must make sure we are ready to catch any incoming messages.
+ */
+static void
+Exec_ListenPreCommit(void)
+{
+ /*
+ * Nothing to do if we are already listening to something, nor if we
+ * already ran this routine in this transaction.
+ */
+ if (listenChannels != NIL || backendHasExecutedInitialListen)
+ return;
- heap_freetuple(tuple);
+ if (Trace_notify)
+ elog(DEBUG1, "Exec_ListenPreCommit(%d)", MyProcPid);
- heap_close(lRel, AccessExclusiveLock);
+ /*
+ * We need this variable to detect an aborted initial LISTEN.
+ * In that case we would set up our pointer but not listen on any channel.
+ * This flag gets cleared in AtCommit_Notify or AtAbort_Notify().
+ */
+ backendHasExecutedInitialListen = true;
/*
- * now that we are listening, make sure we will unlisten before dying.
+ * Before registering, make sure we will unlisten before dying.
+ * (Note: this action does not get undone if we abort later.)
*/
if (!unlistenExitRegistered)
{
on_shmem_exit(Async_UnlistenOnExit, 0);
unlistenExitRegistered = true;
}
+
+ /*
+ * This is our first LISTEN, so establish our pointer.
+ *
+ * We set our pointer to the global tail pointer and then move it forward
+ * over already-committed notifications. This ensures we cannot miss any
+ * not-yet-committed notifications. We might get a few more but that
+ * doesn't hurt.
+ */
+ LWLockAcquire(AsyncQueueLock, LW_SHARED);
+ QUEUE_BACKEND_POS(MyBackendId) = QUEUE_TAIL;
+ QUEUE_BACKEND_PID(MyBackendId) = MyProcPid;
+ LWLockRelease(AsyncQueueLock);
+
+ /*
+ * Try to move our pointer forward as far as possible. This will skip over
+ * already-committed notifications. Still, we could get notifications that
+ * have already committed before we started to LISTEN.
+ *
+ * Note that we are not yet listening on anything, so we won't deliver
+ * any notification to the frontend.
+ *
+ * This will also advance the global tail pointer if possible.
+ */
+ asyncQueueReadAllNotifications();
}
/*
- *--------------------------------------------------------------
- * Async_Unlisten
+ * Exec_ListenCommit --- subroutine for AtCommit_Notify
*
- * This is executed by the SQL unlisten command.
+ * Add the channel to the list of channels we are listening on.
+ */
+static void
+Exec_ListenCommit(const char *channel)
+{
+ MemoryContext oldcontext;
+
+ /* Do nothing if we are already listening on this channel */
+ if (IsListeningOn(channel))
+ return;
+
+ /*
+ * Add the new channel name to listenChannels.
+ *
+ * XXX It is theoretically possible to get an out-of-memory failure here,
+ * which would be bad because we already committed. For the moment it
+ * doesn't seem worth trying to guard against that, but maybe improve this
+ * later.
+ */
+ oldcontext = MemoryContextSwitchTo(TopMemoryContext);
+ listenChannels = lappend(listenChannels, pstrdup(channel));
+ MemoryContextSwitchTo(oldcontext);
+}
+
+/*
+ * Exec_UnlistenCommit --- subroutine for AtCommit_Notify
+ *
+ * Remove the specified channel name from listenChannels.
+ */
+static void
+Exec_UnlistenCommit(const char *channel)
+{
+ ListCell *q;
+ ListCell *prev;
+
+ if (Trace_notify)
+ elog(DEBUG1, "Exec_UnlistenCommit(%s,%d)", channel, MyProcPid);
+
+ prev = NULL;
+ foreach(q, listenChannels)
+ {
+ char *lchan = (char *) lfirst(q);
+
+ if (strcmp(lchan, channel) == 0)
+ {
+ listenChannels = list_delete_cell(listenChannels, q, prev);
+ pfree(lchan);
+ break;
+ }
+ prev = q;
+ }
+
+ /*
+ * We do not complain about unlistening something not being listened;
+ * should we?
+ */
+
+ /* If no longer listening to anything, get out of listener array */
+ if (listenChannels == NIL)
+ asyncQueueUnregister();
+}
+
+/*
+ * Exec_UnlistenAllCommit --- subroutine for AtCommit_Notify
+ *
+ * Unlisten on all channels for this backend.
+ */
+static void
+Exec_UnlistenAllCommit(void)
+{
+ if (Trace_notify)
+ elog(DEBUG1, "Exec_UnlistenAllCommit(%d)", MyProcPid);
+
+ list_free_deep(listenChannels);
+ listenChannels = NIL;
+
+ asyncQueueUnregister();
+}
+
+/*
+ * ProcessCompletedNotifies --- send out signals and self-notifies
*
- * Remove the backend from the list of listening backends
- * for the specified relation.
+ * This is called from postgres.c just before going idle at the completion
+ * of a transaction. If we issued any notifications in the just-completed
+ * transaction, send signals to other backends to process them, and also
+ * process the queue ourselves to send messages to our own frontend.
*
- * Results:
- * XXX
+ * The reason that this is not done in AtCommit_Notify is that there is
+ * a nonzero chance of errors here (for example, encoding conversion errors
+ * while trying to format messages to our frontend). An error during
+ * AtCommit_Notify would be a PANIC condition. The timing is also arranged
+ * to ensure that a transaction's self-notifies are delivered to the frontend
+ * before it gets the terminating ReadyForQuery message.
*
- * Side effects:
- * pg_listener is updated.
+ * Note that we send signals and process the queue even if the transaction
+ * eventually aborted. This is because we need to clean out whatever got
+ * added to the queue.
*
- *--------------------------------------------------------------
+ * NOTE: we are outside of any transaction here.
*/
void
-Async_Unlisten(char *relname, int pid)
+ProcessCompletedNotifies(void)
{
- Relation lRel;
- HeapScanDesc scan;
- HeapTuple tuple;
+ bool signalled;
- /* Handle specially the `unlisten "*"' command */
- if ((!relname) || (*relname == '\0') || (strcmp(relname, "*") == 0))
- {
- Async_UnlistenAll();
+ /* Nothing to do if we didn't send any notifications */
+ if (!backendHasSentNotifications)
return;
- }
+
+ /*
+ * We reset the flag immediately; otherwise, if any sort of error
+ * occurs below, we'd be locked up in an infinite loop, because
+ * control will come right back here after error cleanup.
+ */
+ backendHasSentNotifications = false;
if (Trace_notify)
- elog(DEBUG1, "Async_Unlisten(%s,%d)", relname, pid);
+ elog(DEBUG1, "ProcessCompletedNotifies");
+
+ /*
+ * We must run asyncQueueReadAllNotifications inside a transaction,
+ * else bad things happen if it gets an error.
+ */
+ StartTransactionCommand();
+
+ /* Send signals to other backends */
+ signalled = SignalBackends();
- lRel = heap_openr(ListenerRelationName, AccessExclusiveLock);
+ if (listenChannels != NIL)
+ {
+ /* Read the queue ourselves, and send relevant stuff to the frontend */
+ asyncQueueReadAllNotifications();
+ }
+ else if (!signalled)
+ {
+ /*
+ * If we found no other listening backends, and we aren't listening
+ * ourselves, then we must execute asyncQueueAdvanceTail to flush
+ * the queue, because ain't nobody else gonna do it. This prevents
+ * queue overflow when we're sending useless notifies to nobody.
+ * (A new listener could have joined since we looked, but if so this
+ * is harmless.)
+ */
+ asyncQueueAdvanceTail();
+ }
+
+ CommitTransactionCommand();
+
+ /* We don't need pq_flush() here since postgres.c will do one shortly */
+}
+
+/*
+ * Test whether we are actively listening on the given channel name.
+ *
+ * Note: this function is executed for every notification found in the queue.
+ * Perhaps it is worth further optimization, eg convert the list to a sorted
+ * array so we can binary-search it. In practice the list is likely to be
+ * fairly short, though.
+ */
+static bool
+IsListeningOn(const char *channel)
+{
+ ListCell *p;
+
+ foreach(p, listenChannels)
+ {
+ char *lchan = (char *) lfirst(p);
+
+ if (strcmp(lchan, channel) == 0)
+ return true;
+ }
+ return false;
+}
+
+/*
+ * Remove our entry from the listeners array when we are no longer listening
+ * on any channel. NB: must not fail if we're already not listening.
+ */
+static void
+asyncQueueUnregister(void)
+{
+ bool advanceTail;
+
+ Assert(listenChannels == NIL); /* else caller error */
+
+ LWLockAcquire(AsyncQueueLock, LW_SHARED);
+ /* check if entry is valid and oldest ... */
+ advanceTail = (MyProcPid == QUEUE_BACKEND_PID(MyBackendId)) &&
+ QUEUE_POS_EQUAL(QUEUE_BACKEND_POS(MyBackendId), QUEUE_TAIL);
+ /* ... then mark it invalid */
+ QUEUE_BACKEND_PID(MyBackendId) = InvalidPid;
+ LWLockRelease(AsyncQueueLock);
+
+ /* If we were the laziest backend, try to advance the tail pointer */
+ if (advanceTail)
+ asyncQueueAdvanceTail();
+}
+
+/*
+ * Test whether there is room to insert more notification messages.
+ *
+ * Caller must hold at least shared AsyncQueueLock.
+ */
+static bool
+asyncQueueIsFull(void)
+{
+ int nexthead;
+ int boundary;
+
+ /*
+ * The queue is full if creating a new head page would create a page that
+ * logically precedes the current global tail pointer, ie, the head
+ * pointer would wrap around compared to the tail. We cannot create such
+ * a head page for fear of confusing slru.c. For safety we round the tail
+ * pointer back to a segment boundary (compare the truncation logic in
+ * asyncQueueAdvanceTail).
+ *
+ * Note that this test is *not* dependent on how much space there is on
+ * the current head page. This is necessary because asyncQueueAddEntries
+ * might try to create the next head page in any case.
+ */
+ nexthead = QUEUE_POS_PAGE(QUEUE_HEAD) + 1;
+ if (nexthead > QUEUE_MAX_PAGE)
+ nexthead = 0; /* wrap around */
+ boundary = QUEUE_POS_PAGE(QUEUE_TAIL);
+ boundary -= boundary % SLRU_PAGES_PER_SEGMENT;
+ return asyncQueuePagePrecedesLogically(nexthead, boundary);
+}
+
+/*
+ * Advance the QueuePosition to the next entry, assuming that the current
+ * entry is of length entryLength. If we jump to a new page the function
+ * returns true, else false.
+ */
+static bool
+asyncQueueAdvance(QueuePosition *position, int entryLength)
+{
+ int pageno = QUEUE_POS_PAGE(*position);
+ int offset = QUEUE_POS_OFFSET(*position);
+ bool pageJump = false;
+
+ /*
+ * Move to the next writing position: First jump over what we have just
+ * written or read.
+ */
+ offset += entryLength;
+ Assert(offset <= QUEUE_PAGESIZE);
+
+ /*
+ * In a second step check if another entry can possibly be written to the
+ * page. If so, stay here, we have reached the next position. If not, then
+ * we need to move on to the next page.
+ */
+ if (offset + QUEUEALIGN(AsyncQueueEntryEmptySize) > QUEUE_PAGESIZE)
+ {
+ pageno++;
+ if (pageno > QUEUE_MAX_PAGE)
+ pageno = 0; /* wrap around */
+ offset = 0;
+ pageJump = true;
+ }
+
+ SET_QUEUE_POS(*position, pageno, offset);
+ return pageJump;
+}
+
+/*
+ * Fill the AsyncQueueEntry at *qe with an outbound notification message.
+ */
+static void
+asyncQueueNotificationToEntry(Notification *n, AsyncQueueEntry *qe)
+{
+ size_t channellen = strlen(n->channel);
+ size_t payloadlen = strlen(n->payload);
+ int entryLength;
+
+ Assert(channellen < NAMEDATALEN);
+ Assert(payloadlen < NOTIFY_PAYLOAD_MAX_LENGTH);
+
+ /* The terminators are already included in AsyncQueueEntryEmptySize */
+ entryLength = AsyncQueueEntryEmptySize + payloadlen + channellen;
+ entryLength = QUEUEALIGN(entryLength);
+ qe->length = entryLength;
+ qe->dboid = MyDatabaseId;
+ qe->xid = GetCurrentTransactionId();
+ qe->srcPid = MyProcPid;
+ memcpy(qe->data, n->channel, channellen + 1);
+ memcpy(qe->data + channellen + 1, n->payload, payloadlen + 1);
+}
+
+/*
+ * Add pending notifications to the queue.
+ *
+ * We go page by page here, i.e. we stop once we have to go to a new page but
+ * we will be called again and then fill that next page. If an entry does not
+ * fit into the current page, we write a dummy entry with an InvalidOid as the
+ * database OID in order to fill the page. So every page is always used up to
+ * the last byte which simplifies reading the page later.
+ *
+ * We are passed the list cell containing the next notification to write
+ * and return the first still-unwritten cell back. Eventually we will return
+ * NULL indicating all is done.
+ *
+ * We are holding AsyncQueueLock already from the caller and grab AsyncCtlLock
+ * locally in this function.
+ */
+static ListCell *
+asyncQueueAddEntries(ListCell *nextNotify)
+{
+ AsyncQueueEntry qe;
+ int pageno;
+ int offset;
+ int slotno;
+
+ /* We hold both AsyncQueueLock and AsyncCtlLock during this operation */
+ LWLockAcquire(AsyncCtlLock, LW_EXCLUSIVE);
+
+ /* Fetch the current page */
+ pageno = QUEUE_POS_PAGE(QUEUE_HEAD);
+ slotno = SimpleLruReadPage(AsyncCtl, pageno, true, InvalidTransactionId);
+ /* Note we mark the page dirty before writing in it */
+ AsyncCtl->shared->page_dirty[slotno] = true;
+
+ while (nextNotify != NULL)
+ {
+ Notification *n = (Notification *) lfirst(nextNotify);
+
+ /* Construct a valid queue entry in local variable qe */
+ asyncQueueNotificationToEntry(n, &qe);
+
+ offset = QUEUE_POS_OFFSET(QUEUE_HEAD);
+
+ /* Check whether the entry really fits on the current page */
+ if (offset + qe.length <= QUEUE_PAGESIZE)
+ {
+ /* OK, so advance nextNotify past this item */
+ nextNotify = lnext(nextNotify);
+ }
+ else
+ {
+ /*
+ * Write a dummy entry to fill up the page. Actually readers will
+ * only check dboid and since it won't match any reader's database
+ * OID, they will ignore this entry and move on.
+ */
+ qe.length = QUEUE_PAGESIZE - offset;
+ qe.dboid = InvalidOid;
+ qe.data[0] = '\0'; /* empty channel */
+ qe.data[1] = '\0'; /* empty payload */
+ }
- scan = heap_beginscan(lRel, SnapshotNow, 0, (ScanKey) NULL);
- while ((tuple = heap_getnext(scan, ForwardScanDirection)) != NULL)
- {
- Form_pg_listener listener = (Form_pg_listener) GETSTRUCT(tuple);
+ /* Now copy qe into the shared buffer page */
+ memcpy(AsyncCtl->shared->page_buffer[slotno] + offset,
+ &qe,
+ qe.length);
- if (listener->listenerpid == pid &&
- strncmp(NameStr(listener->relname), relname, NAMEDATALEN) == 0)
+ /* Advance QUEUE_HEAD appropriately, and note if page is full */
+ if (asyncQueueAdvance(&(QUEUE_HEAD), qe.length))
{
- /* Found the matching tuple, delete it */
- simple_heap_delete(lRel, &tuple->t_self);
-
/*
- * We assume there can be only one match, so no need to scan
- * the rest of the table
+ * Page is full, so we're done here, but first fill the next
+ * page with zeroes. The reason to do this is to ensure that
+ * slru.c's idea of the head page is always the same as ours,
+ * which avoids boundary problems in SimpleLruTruncate. The
+ * test in asyncQueueIsFull() ensured that there is room to
+ * create this page without overrunning the queue.
*/
+ slotno = SimpleLruZeroPage(AsyncCtl, QUEUE_POS_PAGE(QUEUE_HEAD));
+ /* And exit the loop */
break;
}
}
- heap_endscan(scan);
- heap_close(lRel, AccessExclusiveLock);
+ LWLockRelease(AsyncCtlLock);
- /*
- * We do not complain about unlistening something not being listened;
- * should we?
- */
+ return nextNotify;
}
/*
- *--------------------------------------------------------------
- * Async_UnlistenAll
- *
- * Unlisten all relations for this backend.
+ * Check whether the queue is at least half full, and emit a warning if so.
*
- * This is invoked by UNLISTEN "*" command, and also at backend exit.
+ * This is unlikely given the size of the queue, but possible.
+ * The warnings show up at most once every QUEUE_FULL_WARN_INTERVAL.
*
- * Results:
- * XXX
- *
- * Side effects:
- * pg_listener is updated.
- *
- *--------------------------------------------------------------
+ * Caller must hold exclusive AsyncQueueLock.
*/
static void
-Async_UnlistenAll(void)
+asyncQueueFillWarning(void)
{
- Relation lRel;
- TupleDesc tdesc;
- HeapScanDesc scan;
- HeapTuple lTuple;
- ScanKeyData key[1];
+ int headPage = QUEUE_POS_PAGE(QUEUE_HEAD);
+ int tailPage = QUEUE_POS_PAGE(QUEUE_TAIL);
+ int occupied;
+ double fillDegree;
+ TimestampTz t;
+
+ occupied = headPage - tailPage;
+
+ if (occupied == 0)
+ return; /* fast exit for common case */
+
+ if (occupied < 0)
+ {
+ /* head has wrapped around, tail not yet */
+ occupied += QUEUE_MAX_PAGE+1;
+ }
- if (Trace_notify)
- elog(DEBUG1, "Async_UnlistenAll");
+ fillDegree = (double) occupied / (double) ((QUEUE_MAX_PAGE+1)/2);
+
+ if (fillDegree < 0.5)
+ return;
- lRel = heap_openr(ListenerRelationName, AccessExclusiveLock);
- tdesc = RelationGetDescr(lRel);
+ t = GetCurrentTimestamp();
- /* Find and delete all entries with my listenerPID */
- ScanKeyEntryInitialize(&key[0], 0,
- Anum_pg_listener_pid,
- BTEqualStrategyNumber, F_INT4EQ,
- Int32GetDatum(MyProcPid), INT4OID);
- scan = heap_beginscan(lRel, SnapshotNow, 1, key);
+ if (TimestampDifferenceExceeds(asyncQueueControl->lastQueueFillWarn,
+ t, QUEUE_FULL_WARN_INTERVAL))
+ {
+ QueuePosition min = QUEUE_HEAD;
+ int32 minPid = InvalidPid;
+ int i;
+
+ for (i = 1; i <= MaxBackends; i++)
+ {
+ if (QUEUE_BACKEND_PID(i) != InvalidPid)
+ {
+ min = QUEUE_POS_MIN(min, QUEUE_BACKEND_POS(i));
+ if (QUEUE_POS_EQUAL(min, QUEUE_BACKEND_POS(i)))
+ minPid = QUEUE_BACKEND_PID(i);
+ }
+ }
- while ((lTuple = heap_getnext(scan, ForwardScanDirection)) != NULL)
- simple_heap_delete(lRel, &lTuple->t_self);
+ ereport(WARNING,
+ (errmsg("pg_notify queue is %.0f%% full", fillDegree * 100),
+ (minPid != InvalidPid ?
+ errdetail("PID %d is among the slowest backends.", minPid)
+ : 0),
+ (minPid != InvalidPid ?
+ errhint("Cleanup can only proceed if this backend ends its current transaction.")
+ : 0)));
- heap_endscan(scan);
- heap_close(lRel, AccessExclusiveLock);
+ asyncQueueControl->lastQueueFillWarn = t;
+ }
}
/*
- *--------------------------------------------------------------
- * Async_UnlistenOnExit
+ * Send signals to all listening backends (except our own).
*
- * Clean up the pg_listener table at backend exit.
+ * Returns true if we sent at least one signal.
*
- * This is executed if we have done any LISTENs in this backend.
- * It might not be necessary anymore, if the user UNLISTENed everything,
- * but we don't try to detect that case.
+ * Since we need EXCLUSIVE lock anyway we also check the position of the other
+ * backends and in case one is already up-to-date we don't signal it.
+ * This can happen if concurrent notifying transactions have sent a signal and
+ * the signaled backend has read the other notifications and ours in the same
+ * step.
*
- * Results:
- * XXX
- *
- * Side effects:
- * pg_listener is updated if necessary.
- *
- *--------------------------------------------------------------
+ * Since we know the BackendId and the Pid the signalling is quite cheap.
*/
-static void
-Async_UnlistenOnExit(void)
+static bool
+SignalBackends(void)
{
+ bool signalled = false;
+ int32 *pids;
+ BackendId *ids;
+ int count;
+ int i;
+ int32 pid;
+
/*
- * We need to start/commit a transaction for the unlisten, but if
- * there is already an active transaction we had better abort that one
- * first. Otherwise we'd end up committing changes that probably
- * ought to be discarded.
+ * Identify all backends that are listening and not already up-to-date.
+ * We don't want to send signals while holding the AsyncQueueLock, so
+ * we just build a list of target PIDs.
+ *
+ * XXX in principle these pallocs could fail, which would be bad.
+ * Maybe preallocate the arrays? But in practice this is only run
+ * in trivial transactions, so there should surely be space available.
*/
- AbortOutOfAnyTransaction();
- /* Now we can do the unlisten */
- StartTransactionCommand();
- Async_UnlistenAll();
- CommitTransactionCommand();
+ pids = (int32 *) palloc(MaxBackends * sizeof(int32));
+ ids = (BackendId *) palloc(MaxBackends * sizeof(BackendId));
+ count = 0;
+
+ LWLockAcquire(AsyncQueueLock, LW_EXCLUSIVE);
+ for (i = 1; i <= MaxBackends; i++)
+ {
+ pid = QUEUE_BACKEND_PID(i);
+ if (pid != InvalidPid && pid != MyProcPid)
+ {
+ QueuePosition pos = QUEUE_BACKEND_POS(i);
+
+ if (!QUEUE_POS_EQUAL(pos, QUEUE_HEAD))
+ {
+ pids[count] = pid;
+ ids[count] = i;
+ count++;
+ }
+ }
+ }
+ LWLockRelease(AsyncQueueLock);
+
+ /* Now send signals */
+ for (i = 0; i < count; i++)
+ {
+ pid = pids[i];
+
+ /*
+ * Note: assuming things aren't broken, a signal failure here could
+ * only occur if the target backend exited since we released
+ * AsyncQueueLock; which is unlikely but certainly possible.
+ * So we just log a low-level debug message if it happens.
+ */
+ if (SendProcSignal(pid, PROCSIG_NOTIFY_INTERRUPT, ids[i]) < 0)
+ elog(DEBUG3, "could not signal backend with PID %d: %m", pid);
+ else
+ signalled = true;
+ }
+
+ pfree(pids);
+ pfree(ids);
+
+ return signalled;
}
/*
- *--------------------------------------------------------------
- * AtCommit_Notify
- *
- * This is called at transaction commit.
- *
- * If there are outbound notify requests in the pendingNotifies list,
- * scan pg_listener for matching tuples, and either signal the other
- * backend or send a message to our own frontend.
- *
- * NOTE: we are still inside the current transaction, therefore can
- * piggyback on its committing of changes.
- *
- * Results:
- * XXX
+ * AtAbort_Notify
*
- * Side effects:
- * Tuples in pg_listener that have matching relnames and other peoples'
- * listenerPIDs are updated with a nonzero notification field.
+ * This is called at transaction abort.
*
- *--------------------------------------------------------------
+ * Gets rid of pending actions and outbound notifies that we would have
+ * executed if the transaction got committed.
*/
void
-AtCommit_Notify(void)
+AtAbort_Notify(void)
{
- Relation lRel;
- TupleDesc tdesc;
- HeapScanDesc scan;
- HeapTuple lTuple,
- rTuple;
- Datum value[Natts_pg_listener];
- char repl[Natts_pg_listener],
- nulls[Natts_pg_listener];
-
- if (pendingNotifies == NIL)
- return; /* no NOTIFY statements in this
- * transaction */
-
/*
- * NOTIFY is disabled if not normal processing mode. This test used to
- * be in xact.c, but it seems cleaner to do it here.
+ * If we LISTEN but then roll back the transaction we have set our pointer
+ * but have not made any entry in listenChannels. In that case, remove
+ * our pointer again.
*/
- if (!IsNormalProcessingMode())
+ if (backendHasExecutedInitialListen)
{
- ClearPendingNotifies();
- return;
+ /*
+ * Checking listenChannels should be redundant but it can't hurt doing
+ * it for safety reasons.
+ */
+ if (listenChannels == NIL)
+ asyncQueueUnregister();
+
+ backendHasExecutedInitialListen = false;
}
- if (Trace_notify)
- elog(DEBUG1, "AtCommit_Notify");
+ /* And clean up */
+ ClearPendingActionsAndNotifies();
+}
- /* preset data to update notify column to MyProcPid */
- nulls[0] = nulls[1] = nulls[2] = ' ';
- repl[0] = repl[1] = repl[2] = ' ';
- repl[Anum_pg_listener_notify - 1] = 'r';
- value[0] = value[1] = value[2] = (Datum) 0;
- value[Anum_pg_listener_notify - 1] = Int32GetDatum(MyProcPid);
+/*
+ * AtSubStart_Notify() --- Take care of subtransaction start.
+ *
+ * Push empty state for the new subtransaction.
+ */
+void
+AtSubStart_Notify(void)
+{
+ MemoryContext old_cxt;
- lRel = heap_openr(ListenerRelationName, AccessExclusiveLock);
- tdesc = RelationGetDescr(lRel);
- scan = heap_beginscan(lRel, SnapshotNow, 0, (ScanKey) NULL);
+ /* Keep the list-of-lists in TopTransactionContext for simplicity */
+ old_cxt = MemoryContextSwitchTo(TopTransactionContext);
- while ((lTuple = heap_getnext(scan, ForwardScanDirection)) != NULL)
- {
- Form_pg_listener listener = (Form_pg_listener) GETSTRUCT(lTuple);
- char *relname = NameStr(listener->relname);
- int32 listenerPID = listener->listenerpid;
+ upperPendingActions = lcons(pendingActions, upperPendingActions);
- if (!AsyncExistsPendingNotify(relname))
- continue;
+ Assert(list_length(upperPendingActions) ==
+ GetCurrentTransactionNestLevel() - 1);
- if (listenerPID == MyProcPid)
- {
- /*
- * Self-notify: no need to bother with table update. Indeed,
- * we *must not* clear the notification field in this path, or
- * we could lose an outside notify, which'd be bad for
- * applications that ignore self-notify messages.
- */
+ pendingActions = NIL;
- if (Trace_notify)
- elog(DEBUG1, "AtCommit_Notify: notifying self");
+ upperPendingNotifies = lcons(pendingNotifies, upperPendingNotifies);
- NotifyMyFrontEnd(relname, listenerPID);
- }
- else
- {
- if (Trace_notify)
- elog(DEBUG1, "AtCommit_Notify: notifying pid %d",
- listenerPID);
+ Assert(list_length(upperPendingNotifies) ==
+ GetCurrentTransactionNestLevel() - 1);
- /*
- * If someone has already notified this listener, we don't
- * bother modifying the table, but we do still send a SIGUSR2
- * signal, just in case that backend missed the earlier signal
- * for some reason. It's OK to send the signal first, because
- * the other guy can't read pg_listener until we unlock it.
- */
- if (kill(listenerPID, SIGUSR2) < 0)
- {
- /*
- * Get rid of pg_listener entry if it refers to a PID that
- * no longer exists. Presumably, that backend crashed
- * without deleting its pg_listener entries. This code
- * used to only delete the entry if errno==ESRCH, but as
- * far as I can see we should just do it for any failure
- * (certainly at least for EPERM too...)
- */
- simple_heap_delete(lRel, &lTuple->t_self);
- }
- else if (listener->notification == 0)
- {
- ItemPointerData ctid;
- int result;
+ pendingNotifies = NIL;
- rTuple = heap_modifytuple(lTuple, lRel,
- value, nulls, repl);
- /*
- * We cannot use simple_heap_update here because the tuple
- * could have been modified by an uncommitted transaction;
- * specifically, since UNLISTEN releases exclusive lock on
- * the table before commit, the other guy could already have
- * tried to unlisten. There are no other cases where we
- * should be able to see an uncommitted update or delete.
- * Therefore, our response to a HeapTupleBeingUpdated result
- * is just to ignore it. We do *not* wait for the other
- * guy to commit --- that would risk deadlock, and we don't
- * want to block while holding the table lock anyway for
- * performance reasons. We also ignore HeapTupleUpdated,
- * which could occur if the other guy commits between our
- * heap_getnext and heap_update calls.
- */
- result = heap_update(lRel, &lTuple->t_self, rTuple,
- &ctid,
- GetCurrentCommandId(), SnapshotAny,
- false /* no wait for commit */);
- switch (result)
- {
- case HeapTupleSelfUpdated:
- /* Tuple was already updated in current command? */
- elog(ERROR, "tuple already updated by self");
- break;
-
- case HeapTupleMayBeUpdated:
- /* done successfully */
-
-#ifdef NOT_USED /* currently there are no indexes */
- CatalogUpdateIndexes(lRel, rTuple);
-#endif
- break;
-
- case HeapTupleBeingUpdated:
- /* ignore uncommitted tuples */
- break;
-
- case HeapTupleUpdated:
- /* ignore just-committed tuples */
- break;
-
- default:
- elog(ERROR, "unrecognized heap_update status: %u",
- result);
- break;
- }
- }
- }
- }
+ MemoryContextSwitchTo(old_cxt);
+}
+
+/*
+ * AtSubCommit_Notify() --- Take care of subtransaction commit.
+ *
+ * Reassign all items in the pending lists to the parent transaction.
+ */
+void
+AtSubCommit_Notify(void)
+{
+ List *parentPendingActions;
+ List *parentPendingNotifies;
+
+ parentPendingActions = (List *) linitial(upperPendingActions);
+ upperPendingActions = list_delete_first(upperPendingActions);
- heap_endscan(scan);
+ Assert(list_length(upperPendingActions) ==
+ GetCurrentTransactionNestLevel() - 2);
/*
- * We do NOT release the lock on pg_listener here; we need to hold it
- * until end of transaction (which is about to happen, anyway) to
- * ensure that notified backends see our tuple updates when they look.
- * Else they might disregard the signal, which would make the
- * application programmer very unhappy.
+ * Mustn't try to eliminate duplicates here --- see queue_listen()
*/
- heap_close(lRel, NoLock);
+ pendingActions = list_concat(parentPendingActions, pendingActions);
- ClearPendingNotifies();
+ parentPendingNotifies = (List *) linitial(upperPendingNotifies);
+ upperPendingNotifies = list_delete_first(upperPendingNotifies);
- if (Trace_notify)
- elog(DEBUG1, "AtCommit_Notify: done");
+ Assert(list_length(upperPendingNotifies) ==
+ GetCurrentTransactionNestLevel() - 2);
+
+ /*
+ * We could try to eliminate duplicates here, but it seems not worthwhile.
+ */
+ pendingNotifies = list_concat(parentPendingNotifies, pendingNotifies);
}
/*
- *--------------------------------------------------------------
- * AtAbort_Notify
- *
- * This is called at transaction abort.
- *
- * Gets rid of pending outbound notifies that we would have executed
- * if the transaction got committed.
- *
- * Results:
- * XXX
- *
- *--------------------------------------------------------------
+ * AtSubAbort_Notify() --- Take care of subtransaction abort.
*/
void
-AtAbort_Notify(void)
+AtSubAbort_Notify(void)
{
- ClearPendingNotifies();
+ int my_level = GetCurrentTransactionNestLevel();
+
+ /*
+ * All we have to do is pop the stack --- the actions/notifies made in
+ * this subxact are no longer interesting, and the space will be freed
+ * when CurTransactionContext is recycled.
+ *
+ * This routine could be called more than once at a given nesting level if
+ * there is trouble during subxact abort. Avoid dumping core by using
+ * GetCurrentTransactionNestLevel as the indicator of how far we need to
+ * prune the list.
+ */
+ while (list_length(upperPendingActions) > my_level - 2)
+ {
+ pendingActions = (List *) linitial(upperPendingActions);
+ upperPendingActions = list_delete_first(upperPendingActions);
+ }
+
+ while (list_length(upperPendingNotifies) > my_level - 2)
+ {
+ pendingNotifies = (List *) linitial(upperPendingNotifies);
+ upperPendingNotifies = list_delete_first(upperPendingNotifies);
+ }
}
/*
- *--------------------------------------------------------------
- * Async_NotifyHandler
+ * HandleNotifyInterrupt
*
- * This is the signal handler for SIGUSR2.
+ * This is called when PROCSIG_NOTIFY_INTERRUPT is received.
*
* If we are idle (notifyInterruptEnabled is set), we can safely invoke
* ProcessIncomingNotify directly. Otherwise, just set a flag
* to do it later.
- *
- * Results:
- * none
- *
- * Side effects:
- * per above
- *--------------------------------------------------------------
*/
void
-Async_NotifyHandler(SIGNAL_ARGS)
+HandleNotifyInterrupt(void)
{
- int save_errno = errno;
-
/*
- * Note: this is a SIGNAL HANDLER. You must be very wary what you do
- * here. Some helpful soul had this routine sprinkled with TPRINTFs,
- * which would likely lead to corruption of stdio buffers if they were
- * ever turned on.
+ * Note: this is called by a SIGNAL HANDLER. You must be very wary what
+ * you do here. Some helpful soul had this routine sprinkled with
+ * TPRINTFs, which would likely lead to corruption of stdio buffers if
+ * they were ever turned on.
*/
/* Don't joggle the elbow of proc_exit */
bool save_ImmediateInterruptOK = ImmediateInterruptOK;
/*
- * We may be called while ImmediateInterruptOK is true; turn it
- * off while messing with the NOTIFY state. (We would have to
- * save and restore it anyway, because PGSemaphore operations
- * inside ProcessIncomingNotify() might reset it.)
+ * We may be called while ImmediateInterruptOK is true; turn it off
+ * while messing with the NOTIFY state. (We would have to save and
+ * restore it anyway, because PGSemaphore operations inside
+ * ProcessIncomingNotify() might reset it.)
*/
ImmediateInterruptOK = false;
/*
* I'm not sure whether some flavors of Unix might allow another
- * SIGUSR2 occurrence to recursively interrupt this routine. To
- * cope with the possibility, we do the same sort of dance that
- * EnableNotifyInterrupt must do --- see that routine for
- * comments.
+ * SIGUSR1 occurrence to recursively interrupt this routine. To cope
+ * with the possibility, we do the same sort of dance that
+ * EnableNotifyInterrupt must do --- see that routine for comments.
*/
notifyInterruptEnabled = 0; /* disable any recursive signal */
notifyInterruptOccurred = 1; /* do at least one iteration */
{
/* Here, it is finally safe to do stuff. */
if (Trace_notify)
- elog(DEBUG1, "Async_NotifyHandler: perform async notify");
+ elog(DEBUG1, "HandleNotifyInterrupt: perform async notify");
ProcessIncomingNotify();
if (Trace_notify)
- elog(DEBUG1, "Async_NotifyHandler: done");
+ elog(DEBUG1, "HandleNotifyInterrupt: done");
}
}
/*
- * Restore ImmediateInterruptOK, and check for interrupts if
- * needed.
+ * Restore ImmediateInterruptOK, and check for interrupts if needed.
*/
ImmediateInterruptOK = save_ImmediateInterruptOK;
if (save_ImmediateInterruptOK)
else
{
/*
- * In this path it is NOT SAFE to do much of anything, except
- * this:
+ * In this path it is NOT SAFE to do much of anything, except this:
*/
notifyInterruptOccurred = 1;
}
-
- errno = save_errno;
}
/*
- * --------------------------------------------------------------
* EnableNotifyInterrupt
*
* This is called by the PostgresMain main loop just before waiting
*
* NOTE: the signal handler starts out disabled, and stays so until
* PostgresMain calls this the first time.
- * --------------------------------------------------------------
*/
void
EnableNotifyInterrupt(void)
return; /* not really idle */
/*
- * This code is tricky because we are communicating with a signal
- * handler that could interrupt us at any point. If we just checked
- * notifyInterruptOccurred and then set notifyInterruptEnabled, we
- * could fail to respond promptly to a signal that happens in between
- * those two steps. (A very small time window, perhaps, but Murphy's
- * Law says you can hit it...) Instead, we first set the enable flag,
- * then test the occurred flag. If we see an unserviced interrupt has
- * occurred, we re-clear the enable flag before going off to do the
- * service work. (That prevents re-entrant invocation of
- * ProcessIncomingNotify() if another interrupt occurs.) If an
- * interrupt comes in between the setting and clearing of
- * notifyInterruptEnabled, then it will have done the service work and
- * left notifyInterruptOccurred zero, so we have to check again after
- * clearing enable. The whole thing has to be in a loop in case
- * another interrupt occurs while we're servicing the first. Once we
- * get out of the loop, enable is set and we know there is no
- * unserviced interrupt.
+ * This code is tricky because we are communicating with a signal handler
+ * that could interrupt us at any point. If we just checked
+ * notifyInterruptOccurred and then set notifyInterruptEnabled, we could
+ * fail to respond promptly to a signal that happens in between those two
+ * steps. (A very small time window, perhaps, but Murphy's Law says you
+ * can hit it...) Instead, we first set the enable flag, then test the
+ * occurred flag. If we see an unserviced interrupt has occurred, we
+ * re-clear the enable flag before going off to do the service work. (That
+ * prevents re-entrant invocation of ProcessIncomingNotify() if another
+ * interrupt occurs.) If an interrupt comes in between the setting and
+ * clearing of notifyInterruptEnabled, then it will have done the service
+ * work and left notifyInterruptOccurred zero, so we have to check again
+ * after clearing enable. The whole thing has to be in a loop in case
+ * another interrupt occurs while we're servicing the first. Once we get
+ * out of the loop, enable is set and we know there is no unserviced
+ * interrupt.
*
* NB: an overenthusiastic optimizing compiler could easily break this
- * code. Hopefully, they all understand what "volatile" means these
- * days.
+ * code. Hopefully, they all understand what "volatile" means these days.
*/
for (;;)
{
}
/*
- * --------------------------------------------------------------
* DisableNotifyInterrupt
*
* This is called by the PostgresMain main loop just after receiving
* a frontend command. Signal handler execution of inbound notifies
* is disabled until the next EnableNotifyInterrupt call.
- * --------------------------------------------------------------
+ *
+ * The PROCSIG_CATCHUP_INTERRUPT signal handler also needs to call this,
+ * so as to prevent conflicts if one signal interrupts the other. So we
+ * must return the previous state of the flag.
*/
-void
+bool
DisableNotifyInterrupt(void)
{
+ bool result = (notifyInterruptEnabled != 0);
+
notifyInterruptEnabled = 0;
+
+ return result;
}
/*
- * --------------------------------------------------------------
- * ProcessIncomingNotify
- *
- * Deal with arriving NOTIFYs from other backends.
- * This is called either directly from the SIGUSR2 signal handler,
- * or the next time control reaches the outer idle loop.
- * Scan pg_listener for arriving notifies, report them to my front end,
- * and clear the notification field in pg_listener until next time.
- *
- * NOTE: since we are outside any transaction, we must create our own.
- *
- * Results:
- * XXX
- *
- * --------------------------------------------------------------
+ * Read all pending notifications from the queue, and deliver appropriate
+ * ones to my frontend. Stop when we reach queue head or an uncommitted
+ * notification.
*/
static void
-ProcessIncomingNotify(void)
+asyncQueueReadAllNotifications(void)
{
- Relation lRel;
- TupleDesc tdesc;
- ScanKeyData key[1];
- HeapScanDesc scan;
- HeapTuple lTuple,
- rTuple;
- Datum value[Natts_pg_listener];
- char repl[Natts_pg_listener],
- nulls[Natts_pg_listener];
+ QueuePosition pos;
+ QueuePosition oldpos;
+ QueuePosition head;
+ bool advanceTail;
+ /* page_buffer must be adequately aligned, so use a union */
+ union {
+ char buf[QUEUE_PAGESIZE];
+ AsyncQueueEntry align;
+ } page_buffer;
+
+ /* Fetch current state */
+ LWLockAcquire(AsyncQueueLock, LW_SHARED);
+ /* Assert checks that we have a valid state entry */
+ Assert(MyProcPid == QUEUE_BACKEND_PID(MyBackendId));
+ pos = oldpos = QUEUE_BACKEND_POS(MyBackendId);
+ head = QUEUE_HEAD;
+ LWLockRelease(AsyncQueueLock);
+
+ if (QUEUE_POS_EQUAL(pos, head))
+ {
+ /* Nothing to do, we have read all notifications already. */
+ return;
+ }
- if (Trace_notify)
- elog(DEBUG1, "ProcessIncomingNotify");
+ /*----------
+ * Note that we deliver everything that we see in the queue and that
+ * matches our _current_ listening state.
+ * Especially we do not take into account different commit times.
+ * Consider the following example:
+ *
+ * Backend 1: Backend 2:
+ *
+ * transaction starts
+ * NOTIFY foo;
+ * commit starts
+ * transaction starts
+ * LISTEN foo;
+ * commit starts
+ * commit to clog
+ * commit to clog
+ *
+ * It could happen that backend 2 sees the notification from backend 1 in
+ * the queue. Even though the notifying transaction committed before
+ * the listening transaction, we still deliver the notification.
+ *
+ * The idea is that an additional notification does not do any harm, we
+ * just need to make sure that we do not miss a notification.
+ *
+ * It is possible that we fail while trying to send a message to our
+ * frontend (for example, because of encoding conversion failure).
+ * If that happens it is critical that we not try to send the same
+ * message over and over again. Therefore, we place a PG_TRY block
+ * here that will forcibly advance our backend position before we lose
+ * control to an error. (We could alternatively retake AsyncQueueLock
+ * and move the position before handling each individual message, but
+ * that seems like too much lock traffic.)
+ *----------
+ */
+ PG_TRY();
+ {
+ bool reachedStop;
- set_ps_display("async_notify");
+ do
+ {
+ int curpage = QUEUE_POS_PAGE(pos);
+ int curoffset = QUEUE_POS_OFFSET(pos);
+ int slotno;
+ int copysize;
- notifyInterruptOccurred = 0;
+ /*
+ * We copy the data from SLRU into a local buffer, so as to avoid
+ * holding the AsyncCtlLock while we are examining the entries and
+ * possibly transmitting them to our frontend. Copy only the part
+ * of the page we will actually inspect.
+ */
+ slotno = SimpleLruReadPage_ReadOnly(AsyncCtl, curpage,
+ InvalidTransactionId);
+ if (curpage == QUEUE_POS_PAGE(head))
+ {
+ /* we only want to read as far as head */
+ copysize = QUEUE_POS_OFFSET(head) - curoffset;
+ if (copysize < 0)
+ copysize = 0; /* just for safety */
+ }
+ else
+ {
+ /* fetch all the rest of the page */
+ copysize = QUEUE_PAGESIZE - curoffset;
+ }
+ memcpy(page_buffer.buf + curoffset,
+ AsyncCtl->shared->page_buffer[slotno] + curoffset,
+ copysize);
+ /* Release lock that we got from SimpleLruReadPage_ReadOnly() */
+ LWLockRelease(AsyncCtlLock);
- StartTransactionCommand();
+ /*
+ * Process messages up to the stop position, end of page, or an
+ * uncommitted message.
+ *
+ * Our stop position is what we found to be the head's position
+ * when we entered this function. It might have changed
+ * already. But if it has, we will receive (or have already
+ * received and queued) another signal and come here again.
+ *
+ * We are not holding AsyncQueueLock here! The queue can only
+ * extend beyond the head pointer (see above) and we leave our
+ * backend's pointer where it is so nobody will truncate or
+ * rewrite pages under us. Especially we don't want to hold a lock
+ * while sending the notifications to the frontend.
+ */
+ reachedStop = asyncQueueProcessPageEntries(&pos, head,
+ page_buffer.buf);
+ } while (!reachedStop);
+ }
+ PG_CATCH();
+ {
+ /* Update shared state */
+ LWLockAcquire(AsyncQueueLock, LW_SHARED);
+ QUEUE_BACKEND_POS(MyBackendId) = pos;
+ advanceTail = QUEUE_POS_EQUAL(oldpos, QUEUE_TAIL);
+ LWLockRelease(AsyncQueueLock);
- lRel = heap_openr(ListenerRelationName, AccessExclusiveLock);
- tdesc = RelationGetDescr(lRel);
+ /* If we were the laziest backend, try to advance the tail pointer */
+ if (advanceTail)
+ asyncQueueAdvanceTail();
- /* Scan only entries with my listenerPID */
- ScanKeyEntryInitialize(&key[0], 0,
- Anum_pg_listener_pid,
- BTEqualStrategyNumber, F_INT4EQ,
- Int32GetDatum(MyProcPid), INT4OID);
- scan = heap_beginscan(lRel, SnapshotNow, 1, key);
+ PG_RE_THROW();
+ }
+ PG_END_TRY();
+
+ /* Update shared state */
+ LWLockAcquire(AsyncQueueLock, LW_SHARED);
+ QUEUE_BACKEND_POS(MyBackendId) = pos;
+ advanceTail = QUEUE_POS_EQUAL(oldpos, QUEUE_TAIL);
+ LWLockRelease(AsyncQueueLock);
+
+ /* If we were the laziest backend, try to advance the tail pointer */
+ if (advanceTail)
+ asyncQueueAdvanceTail();
+}
- /* Prepare data for rewriting 0 into notification field */
- nulls[0] = nulls[1] = nulls[2] = ' ';
- repl[0] = repl[1] = repl[2] = ' ';
- repl[Anum_pg_listener_notify - 1] = 'r';
- value[0] = value[1] = value[2] = (Datum) 0;
- value[Anum_pg_listener_notify - 1] = Int32GetDatum(0);
+/*
+ * Fetch notifications from the shared queue, beginning at position current,
+ * and deliver relevant ones to my frontend.
+ *
+ * The current page must have been fetched into page_buffer from shared
+ * memory. (We could access the page right in shared memory, but that
+ * would imply holding the AsyncCtlLock throughout this routine.)
+ *
+ * We stop if we reach the "stop" position, or reach a notification from an
+ * uncommitted transaction, or reach the end of the page.
+ *
+ * The function returns true once we have reached the stop position or an
+ * uncommitted notification, and false if we have finished with the page.
+ * In other words: once it returns true there is no need to look further.
+ */
+static bool
+asyncQueueProcessPageEntries(QueuePosition *current,
+ QueuePosition stop,
+ char *page_buffer)
+{
+ bool reachedStop = false;
+ bool reachedEndOfPage;
+ AsyncQueueEntry *qe;
- while ((lTuple = heap_getnext(scan, ForwardScanDirection)) != NULL)
+ do
{
- Form_pg_listener listener = (Form_pg_listener) GETSTRUCT(lTuple);
- char *relname = NameStr(listener->relname);
- int32 sourcePID = listener->notification;
+ if (QUEUE_POS_EQUAL(*current, stop))
+ break;
- if (sourcePID != 0)
- {
- /* Notify the frontend */
+ qe = (AsyncQueueEntry *) (page_buffer + QUEUE_POS_OFFSET(*current));
- if (Trace_notify)
- elog(DEBUG1, "ProcessIncomingNotify: received %s from %d",
- relname, (int) sourcePID);
+ /*
+ * Advance *current over this message, possibly to the next page.
+ * As noted in the comments for asyncQueueReadAllNotifications, we
+ * must do this before possibly failing while processing the message.
+ */
+ reachedEndOfPage = asyncQueueAdvance(current, qe->length);
- NotifyMyFrontEnd(relname, sourcePID);
- /*
- * Rewrite the tuple with 0 in notification column.
- *
- * simple_heap_update is safe here because no one else would
- * have tried to UNLISTEN us, so there can be no uncommitted
- * changes.
- */
- rTuple = heap_modifytuple(lTuple, lRel, value, nulls, repl);
- simple_heap_update(lRel, &lTuple->t_self, rTuple);
+ /* Ignore messages destined for other databases */
+ if (qe->dboid == MyDatabaseId)
+ {
+ if (TransactionIdDidCommit(qe->xid))
+ {
+ /* qe->data is the null-terminated channel name */
+ char *channel = qe->data;
+
+ if (IsListeningOn(channel))
+ {
+ /* payload follows channel name */
+ char *payload = qe->data + strlen(channel) + 1;
-#ifdef NOT_USED /* currently there are no indexes */
- CatalogUpdateIndexes(lRel, rTuple);
-#endif
+ NotifyMyFrontEnd(channel, payload, qe->srcPid);
+ }
+ }
+ else if (TransactionIdDidAbort(qe->xid))
+ {
+ /*
+ * If the source transaction aborted, we just ignore its
+ * notifications.
+ */
+ }
+ else
+ {
+ /*
+ * The transaction has neither committed nor aborted so far,
+ * so we can't process its message yet. Break out of the loop.
+ */
+ reachedStop = true;
+ break;
+ }
}
+
+ /* Loop back if we're not at end of page */
+ } while (!reachedEndOfPage);
+
+ if (QUEUE_POS_EQUAL(*current, stop))
+ reachedStop = true;
+
+ return reachedStop;
+}
+
+/*
+ * Advance the shared queue tail variable to the minimum of all the
+ * per-backend tail pointers. Truncate pg_notify space if possible.
+ */
+static void
+asyncQueueAdvanceTail(void)
+{
+ QueuePosition min;
+ int i;
+ int oldtailpage;
+ int newtailpage;
+ int boundary;
+
+ LWLockAcquire(AsyncQueueLock, LW_EXCLUSIVE);
+ min = QUEUE_HEAD;
+ for (i = 1; i <= MaxBackends; i++)
+ {
+ if (QUEUE_BACKEND_PID(i) != InvalidPid)
+ min = QUEUE_POS_MIN(min, QUEUE_BACKEND_POS(i));
+ }
+ oldtailpage = QUEUE_POS_PAGE(QUEUE_TAIL);
+ QUEUE_TAIL = min;
+ LWLockRelease(AsyncQueueLock);
+
+ /*
+ * We can truncate something if the global tail advanced across an SLRU
+ * segment boundary.
+ *
+ * XXX it might be better to truncate only once every several segments,
+ * to reduce the number of directory scans.
+ */
+ newtailpage = QUEUE_POS_PAGE(min);
+ boundary = newtailpage - (newtailpage % SLRU_PAGES_PER_SEGMENT);
+ if (asyncQueuePagePrecedesLogically(oldtailpage, boundary))
+ {
+ /*
+ * SimpleLruTruncate() will ask for AsyncCtlLock but will also
+ * release the lock again.
+ */
+ SimpleLruTruncate(AsyncCtl, newtailpage);
}
- heap_endscan(scan);
+}
+
+/*
+ * ProcessIncomingNotify
+ *
+ * Deal with arriving NOTIFYs from other backends.
+ * This is called either directly from the PROCSIG_NOTIFY_INTERRUPT
+ * signal handler, or the next time control reaches the outer idle loop.
+ * Scan the queue for arriving notifications and report them to my front
+ * end.
+ *
+ * NOTE: since we are outside any transaction, we must create our own.
+ */
+static void
+ProcessIncomingNotify(void)
+{
+ bool catchup_enabled;
+
+ /* Do nothing if we aren't actively listening */
+ if (listenChannels == NIL)
+ return;
+
+ /* Must prevent catchup interrupt while I am running */
+ catchup_enabled = DisableCatchupInterrupt();
+
+ if (Trace_notify)
+ elog(DEBUG1, "ProcessIncomingNotify");
+
+ set_ps_display("notify interrupt", false);
+
+ notifyInterruptOccurred = 0;
/*
- * We do NOT release the lock on pg_listener here; we need to hold it
- * until end of transaction (which is about to happen, anyway) to
- * ensure that other backends see our tuple updates when they look.
- * Otherwise, a transaction started after this one might mistakenly
- * think it doesn't need to send this backend a new NOTIFY.
+ * We must run asyncQueueReadAllNotifications inside a transaction,
+ * else bad things happen if it gets an error.
*/
- heap_close(lRel, NoLock);
+ StartTransactionCommand();
+
+ asyncQueueReadAllNotifications();
CommitTransactionCommand();
/*
- * Must flush the notify messages to ensure frontend gets them
- * promptly.
+ * Must flush the notify messages to ensure frontend gets them promptly.
*/
pq_flush();
- set_ps_display("idle");
+ set_ps_display("idle", false);
if (Trace_notify)
elog(DEBUG1, "ProcessIncomingNotify: done");
+
+ if (catchup_enabled)
+ EnableCatchupInterrupt();
}
/*
* Send NOTIFY message to my front end.
*/
static void
-NotifyMyFrontEnd(char *relname, int32 listenerPID)
+NotifyMyFrontEnd(const char *channel, const char *payload, int32 srcPid)
{
- if (whereToSendOutput == Remote)
+ if (whereToSendOutput == DestRemote)
{
StringInfoData buf;
pq_beginmessage(&buf, 'A');
- pq_sendint(&buf, listenerPID, sizeof(int32));
- pq_sendstring(&buf, relname);
+ pq_sendint(&buf, srcPid, sizeof(int32));
+ pq_sendstring(&buf, channel);
if (PG_PROTOCOL_MAJOR(FrontendProtocol) >= 3)
- {
- /* XXX Add parameter string here later */
- pq_sendstring(&buf, "");
- }
+ pq_sendstring(&buf, payload);
pq_endmessage(&buf);
/*
* NOTE: we do not do pq_flush() here. For a self-notify, it will
* happen at the end of the transaction, and for incoming notifies
- * ProcessIncomingNotify will do it after finding all the
- * notifies.
+ * ProcessIncomingNotify will do it after finding all the notifies.
*/
}
else
- elog(INFO, "NOTIFY for %s", relname);
+ elog(INFO, "NOTIFY for \"%s\" payload \"%s\"", channel, payload);
}
-/* Does pendingNotifies include the given relname? */
+/* Does pendingNotifies include the given channel/payload? */
static bool
-AsyncExistsPendingNotify(const char *relname)
+AsyncExistsPendingNotify(const char *channel, const char *payload)
{
- List *p;
+ ListCell *p;
+ Notification *n;
+
+ if (pendingNotifies == NIL)
+ return false;
+
+ if (payload == NULL)
+ payload = "";
+
+ /*----------
+ * We need to append new elements to the end of the list in order to keep
+ * the order. However, on the other hand we'd like to check the list
+ * backwards in order to make duplicate-elimination a tad faster when the
+ * same condition is signaled many times in a row. So as a compromise we
+ * check the tail element first which we can access directly. If this
+ * doesn't match, we check the whole list.
+ *
+ * As we are not checking our parents' lists, we can still get duplicates
+ * in combination with subtransactions, like in:
+ *
+ * begin;
+ * notify foo '1';
+ * savepoint foo;
+ * notify foo '1';
+ * commit;
+ *----------
+ */
+ n = (Notification *) llast(pendingNotifies);
+ if (strcmp(n->channel, channel) == 0 &&
+ strcmp(n->payload, payload) == 0)
+ return true;
foreach(p, pendingNotifies)
{
- /* Use NAMEDATALEN for relname comparison. DZ - 26-08-1996 */
- if (strncmp((const char *) lfirst(p), relname, NAMEDATALEN) == 0)
+ n = (Notification *) lfirst(p);
+
+ if (strcmp(n->channel, channel) == 0 &&
+ strcmp(n->payload, payload) == 0)
return true;
}
return false;
}
-/* Clear the pendingNotifies list. */
+/* Clear the pendingActions and pendingNotifies lists. */
static void
-ClearPendingNotifies(void)
+ClearPendingActionsAndNotifies(void)
{
/*
- * We used to have to explicitly deallocate the list members and
- * nodes, because they were malloc'd. Now, since we know they are
- * palloc'd in TopTransactionContext, we need not do that --- they'll
- * go away automatically at transaction exit. We need only reset the
- * list head pointer.
+ * We used to have to explicitly deallocate the list members and nodes,
+ * because they were malloc'd. Now, since we know they are palloc'd in
+ * CurTransactionContext, we need not do that --- they'll go away
+ * automatically at transaction exit. We need only reset the list head
+ * pointers.
*/
+ pendingActions = NIL;
pendingNotifies = NIL;
}
projects / postgresql / blobdiff