[postgresql] / src / backend / port / sysv_shmem.c

/*-------------------------------------------------------------------------
 *
 * sysv_shmem.c
 *        Implement shared memory using SysV facilities
 *
 * These routines represent a fairly thin layer on top of SysV shared
 * memory functionality.
 *
 * Portions Copyright (c) 1996-2008, PostgreSQL Global Development Group
 * Portions Copyright (c) 1994, Regents of the University of California
 *
 * IDENTIFICATION
 *        $PostgreSQL: pgsql/src/backend/port/sysv_shmem.c,v 1.53 2008/01/01 19:45:51 momjian Exp $
 *
 *-------------------------------------------------------------------------
 */
#include "postgres.h"

#include <signal.h>
#include <unistd.h>
#include <sys/file.h>
#include <sys/stat.h>
#ifdef HAVE_SYS_IPC_H
#include <sys/ipc.h>
#endif
#ifdef HAVE_SYS_SHM_H
#include <sys/shm.h>
#endif
#ifdef HAVE_KERNEL_OS_H
#include <kernel/OS.h>
#endif

#include "miscadmin.h"
#include "storage/ipc.h"
#include "storage/pg_shmem.h"


typedef key_t IpcMemoryKey;             /* shared memory key passed to shmget(2) */
typedef int IpcMemoryId;                /* shared memory ID returned by shmget(2) */

#define IPCProtection   (0600)  /* access/modify by user only */

#ifdef SHM_SHARE_MMU                    /* use intimate shared memory on Solaris */
#define PG_SHMAT_FLAGS                  SHM_SHARE_MMU
#else
#define PG_SHMAT_FLAGS                  0
#endif


unsigned long UsedShmemSegID = 0;
void       *UsedShmemSegAddr = NULL;

static void *InternalIpcMemoryCreate(IpcMemoryKey memKey, Size size);
static void IpcMemoryDetach(int status, Datum shmaddr);
static void IpcMemoryDelete(int status, Datum shmId);
static PGShmemHeader *PGSharedMemoryAttach(IpcMemoryKey key,
                                         IpcMemoryId *shmid);


/*
 *      InternalIpcMemoryCreate(memKey, size)
 *
 * Attempt to create a new shared memory segment with the specified key.
 * Will fail (return NULL) if such a segment already exists.  If successful,
 * attach the segment to the current process and return its attached address.
 * On success, callbacks are registered with on_shmem_exit to detach and
 * delete the segment when on_shmem_exit is called.
 *
 * If we fail with a failure code other than collision-with-existing-segment,
 * print out an error and abort.  Other types of errors are not recoverable.
 */
static void *
InternalIpcMemoryCreate(IpcMemoryKey memKey, Size size)
{
        IpcMemoryId shmid;
        void       *memAddress;

        shmid = shmget(memKey, size, IPC_CREAT | IPC_EXCL | IPCProtection);

        if (shmid < 0)
        {
                /*
                 * Fail quietly if error indicates a collision with existing segment.
                 * One would expect EEXIST, given that we said IPC_EXCL, but perhaps
                 * we could get a permission violation instead?  Also, EIDRM might
                 * occur if an old seg is slated for destruction but not gone yet.
                 */
                if (errno == EEXIST || errno == EACCES
#ifdef EIDRM
                        || errno == EIDRM
#endif
                        )
                        return NULL;

                /*
                 * Else complain and abort
                 */
                ereport(FATAL,
                                (errmsg("could not create shared memory segment: %m"),
                  errdetail("Failed system call was shmget(key=%lu, size=%lu, 0%o).",
                                        (unsigned long) memKey, (unsigned long) size,
                                        IPC_CREAT | IPC_EXCL | IPCProtection),
                                 (errno == EINVAL) ?
                                 errhint("This error usually means that PostgreSQL's request for a shared memory "
                  "segment exceeded your kernel's SHMMAX parameter.  You can either "
                                                 "reduce the request size or reconfigure the kernel with larger SHMMAX.  "
                                  "To reduce the request size (currently %lu bytes), reduce "
                           "PostgreSQL's shared_buffers parameter (currently %d) and/or "
                                                 "its max_connections parameter (currently %d).\n"
                                                 "If the request size is already small, it's possible that it is less than "
                                                 "your kernel's SHMMIN parameter, in which case raising the request size or "
                                                 "reconfiguring SHMMIN is called for.\n"
                "The PostgreSQL documentation contains more information about shared "
                                                 "memory configuration.",
                                                 (unsigned long) size, NBuffers, MaxBackends) : 0,
                                 (errno == ENOMEM) ?
                                 errhint("This error usually means that PostgreSQL's request for a shared "
                                   "memory segment exceeded available memory or swap space. "
                                  "To reduce the request size (currently %lu bytes), reduce "
                           "PostgreSQL's shared_buffers parameter (currently %d) and/or "
                                                 "its max_connections parameter (currently %d).\n"
                "The PostgreSQL documentation contains more information about shared "
                                                 "memory configuration.",
                                                 (unsigned long) size, NBuffers, MaxBackends) : 0,
                                 (errno == ENOSPC) ?
                                 errhint("This error does *not* mean that you have run out of disk space. "
                                                 "It occurs either if all available shared memory IDs have been taken, "
                                                 "in which case you need to raise the SHMMNI parameter in your kernel, "
                  "or because the system's overall limit for shared memory has been "
                                 "reached.  If you cannot increase the shared memory limit, "
                  "reduce PostgreSQL's shared memory request (currently %lu bytes), "
                        "by reducing its shared_buffers parameter (currently %d) and/or "
                                                 "its max_connections parameter (currently %d).\n"
                "The PostgreSQL documentation contains more information about shared "
                                                 "memory configuration.",
                                                 (unsigned long) size, NBuffers, MaxBackends) : 0));
        }

        /* Register on-exit routine to delete the new segment */
        on_shmem_exit(IpcMemoryDelete, Int32GetDatum(shmid));

        /* OK, should be able to attach to the segment */
        memAddress = shmat(shmid, NULL, PG_SHMAT_FLAGS);

        if (memAddress == (void *) -1)
                elog(FATAL, "shmat(id=%d) failed: %m", shmid);

        /* Register on-exit routine to detach new segment before deleting */
        on_shmem_exit(IpcMemoryDetach, PointerGetDatum(memAddress));

        /* Record key and ID in lockfile for data directory. */
        RecordSharedMemoryInLockFile((unsigned long) memKey,
                                                                 (unsigned long) shmid);

        return memAddress;
}

/****************************************************************************/
/*      IpcMemoryDetach(status, shmaddr)        removes a shared memory segment         */
/*                                                                              from process' address spaceq            */
/*      (called as an on_shmem_exit callback, hence funny argument list)                */
/****************************************************************************/
static void
IpcMemoryDetach(int status, Datum shmaddr)
{
        if (shmdt(DatumGetPointer(shmaddr)) < 0)
                elog(LOG, "shmdt(%p) failed: %m", DatumGetPointer(shmaddr));
}

/****************************************************************************/
/*      IpcMemoryDelete(status, shmId)          deletes a shared memory segment         */
/*      (called as an on_shmem_exit callback, hence funny argument list)                */
/****************************************************************************/
static void
IpcMemoryDelete(int status, Datum shmId)
{
        if (shmctl(DatumGetInt32(shmId), IPC_RMID, NULL) < 0)
                elog(LOG, "shmctl(%d, %d, 0) failed: %m",
                         DatumGetInt32(shmId), IPC_RMID);
}

/*
 * PGSharedMemoryIsInUse
 *
 * Is a previously-existing shmem segment still existing and in use?
 *
 * The point of this exercise is to detect the case where a prior postmaster
 * crashed, but it left child backends that are still running.  Therefore
 * we only care about shmem segments that are associated with the intended
 * DataDir.  This is an important consideration since accidental matches of
 * shmem segment IDs are reasonably common.
 */
bool
PGSharedMemoryIsInUse(unsigned long id1, unsigned long id2)
{
        IpcMemoryId shmId = (IpcMemoryId) id2;
        struct shmid_ds shmStat;
        struct stat statbuf;
        PGShmemHeader *hdr;

        /*
         * We detect whether a shared memory segment is in use by seeing whether
         * it (a) exists and (b) has any processes attached to it.
         */
        if (shmctl(shmId, IPC_STAT, &shmStat) < 0)
        {
                /*
                 * EINVAL actually has multiple possible causes documented in the
                 * shmctl man page, but we assume it must mean the segment no longer
                 * exists.
                 */
                if (errno == EINVAL)
                        return false;

                /*
                 * EACCES implies that the segment belongs to some other userid, which
                 * means it is not a Postgres shmem segment (or at least, not one that
                 * is relevant to our data directory).
                 */
                if (errno == EACCES)
                        return false;

                /*
                 * Some Linux kernel versions (in fact, all of them as of July 2007)
                 * sometimes return EIDRM when EINVAL is correct.  The Linux kernel
                 * actually does not have any internal state that would justify
                 * returning EIDRM, so we can get away with assuming that EIDRM is
                 * equivalent to EINVAL on that platform.
                 */
#ifdef HAVE_LINUX_EIDRM_BUG
                if (errno == EIDRM)
                        return false;
#endif

                /*
                 * Otherwise, we had better assume that the segment is in use. The
                 * only likely case is EIDRM, which implies that the segment has been
                 * IPC_RMID'd but there are still processes attached to it.
                 */
                return true;
        }

        /* If it has no attached processes, it's not in use */
        if (shmStat.shm_nattch == 0)
                return false;

        /*
         * Try to attach to the segment and see if it matches our data directory.
         * This avoids shmid-conflict problems on machines that are running
         * several postmasters under the same userid.
         */
        if (stat(DataDir, &statbuf) < 0)
                return true;                    /* if can't stat, be conservative */

        hdr = (PGShmemHeader *) shmat(shmId, NULL, PG_SHMAT_FLAGS);

        if (hdr == (PGShmemHeader *) -1)
                return true;                    /* if can't attach, be conservative */

        if (hdr->magic != PGShmemMagic ||
                hdr->device != statbuf.st_dev ||
                hdr->inode != statbuf.st_ino)
        {
                /*
                 * It's either not a Postgres segment, or not one for my data
                 * directory.  In either case it poses no threat.
                 */
                shmdt((void *) hdr);
                return false;
        }

        /* Trouble --- looks a lot like there's still live backends */
        shmdt((void *) hdr);

        return true;
}


/*
 * PGSharedMemoryCreate
 *
 * Create a shared memory segment of the given size and initialize its
 * standard header.  Also, register an on_shmem_exit callback to release
 * the storage.
 *
 * Dead Postgres segments are recycled if found, but we do not fail upon
 * collision with non-Postgres shmem segments.  The idea here is to detect and
 * re-use keys that may have been assigned by a crashed postmaster or backend.
 *
 * makePrivate means to always create a new segment, rather than attach to
 * or recycle any existing segment.
 *
 * The port number is passed for possible use as a key (for SysV, we use
 * it to generate the starting shmem key).      In a standalone backend,
 * zero will be passed.
 */
PGShmemHeader *
PGSharedMemoryCreate(Size size, bool makePrivate, int port)
{
        IpcMemoryKey NextShmemSegID;
        void       *memAddress;
        PGShmemHeader *hdr;
        IpcMemoryId shmid;
        struct stat statbuf;

        /* Room for a header? */
        Assert(size > MAXALIGN(sizeof(PGShmemHeader)));

        /* Make sure PGSharedMemoryAttach doesn't fail without need */
        UsedShmemSegAddr = NULL;

        /* Loop till we find a free IPC key */
        NextShmemSegID = port * 1000;

        for (NextShmemSegID++;; NextShmemSegID++)
        {
                /* Try to create new segment */
                memAddress = InternalIpcMemoryCreate(NextShmemSegID, size);
                if (memAddress)
                        break;                          /* successful create and attach */

                /* Check shared memory and possibly remove and recreate */

                if (makePrivate)                /* a standalone backend shouldn't do this */
                        continue;

                if ((memAddress = PGSharedMemoryAttach(NextShmemSegID, &shmid)) == NULL)
                        continue;                       /* can't attach, not one of mine */

                /*
                 * If I am not the creator and it belongs to an extant process,
                 * continue.
                 */
                hdr = (PGShmemHeader *) memAddress;
                if (hdr->creatorPID != getpid())
                {
                        if (kill(hdr->creatorPID, 0) == 0 || errno != ESRCH)
                        {
                                shmdt(memAddress);
                                continue;               /* segment belongs to a live process */
                        }
                }

                /*
                 * The segment appears to be from a dead Postgres process, or from a
                 * previous cycle of life in this same process.  Zap it, if possible.
                 * This probably shouldn't fail, but if it does, assume the segment
                 * belongs to someone else after all, and continue quietly.
                 */
                shmdt(memAddress);
                if (shmctl(shmid, IPC_RMID, NULL) < 0)
                        continue;

                /*
                 * Now try again to create the segment.
                 */
                memAddress = InternalIpcMemoryCreate(NextShmemSegID, size);
                if (memAddress)
                        break;                          /* successful create and attach */

                /*
                 * Can only get here if some other process managed to create the same
                 * shmem key before we did.  Let him have that one, loop around to try
                 * next key.
                 */
        }

        /*
         * OK, we created a new segment.  Mark it as created by this process. The
         * order of assignments here is critical so that another Postgres process
         * can't see the header as valid but belonging to an invalid PID!
         */
        hdr = (PGShmemHeader *) memAddress;
        hdr->creatorPID = getpid();
        hdr->magic = PGShmemMagic;

        /* Fill in the data directory ID info, too */
        if (stat(DataDir, &statbuf) < 0)
                ereport(FATAL,
                                (errcode_for_file_access(),
                                 errmsg("could not stat data directory \"%s\": %m",
                                                DataDir)));
        hdr->device = statbuf.st_dev;
        hdr->inode = statbuf.st_ino;

        /*
         * Initialize space allocation status for segment.
         */
        hdr->totalsize = size;
        hdr->freeoffset = MAXALIGN(sizeof(PGShmemHeader));

        /* Save info for possible future use */
        UsedShmemSegAddr = memAddress;
        UsedShmemSegID = (unsigned long) NextShmemSegID;

        return hdr;
}

#ifdef EXEC_BACKEND

/*
 * PGSharedMemoryReAttach
 *
 * Re-attach to an already existing shared memory segment.      In the non
 * EXEC_BACKEND case this is not used, because postmaster children inherit
 * the shared memory segment attachment via fork().
 *
 * UsedShmemSegID and UsedShmemSegAddr are implicit parameters to this
 * routine.  The caller must have already restored them to the postmaster's
 * values.
 */
void
PGSharedMemoryReAttach(void)
{
        IpcMemoryId shmid;
        void       *hdr;
        void       *origUsedShmemSegAddr = UsedShmemSegAddr;

        Assert(UsedShmemSegAddr != NULL);
        Assert(IsUnderPostmaster);

#ifdef __CYGWIN__
        /* cygipc (currently) appears to not detach on exec. */
        PGSharedMemoryDetach();
        UsedShmemSegAddr = origUsedShmemSegAddr;
#endif

        elog(DEBUG3, "attaching to %p", UsedShmemSegAddr);
        hdr = (void *) PGSharedMemoryAttach((IpcMemoryKey) UsedShmemSegID, &shmid);
        if (hdr == NULL)
                elog(FATAL, "could not reattach to shared memory (key=%d, addr=%p): %m",
                         (int) UsedShmemSegID, UsedShmemSegAddr);
        if (hdr != origUsedShmemSegAddr)
                elog(FATAL, "reattaching to shared memory returned unexpected address (got %p, expected %p)",
                         hdr, origUsedShmemSegAddr);

        UsedShmemSegAddr = hdr;         /* probably redundant */
}
#endif   /* EXEC_BACKEND */

/*
 * PGSharedMemoryDetach
 *
 * Detach from the shared memory segment, if still attached.  This is not
 * intended for use by the process that originally created the segment
 * (it will have an on_shmem_exit callback registered to do that).      Rather,
 * this is for subprocesses that have inherited an attachment and want to
 * get rid of it.
 */
void
PGSharedMemoryDetach(void)
{
        if (UsedShmemSegAddr != NULL)
        {
                if ((shmdt(UsedShmemSegAddr) < 0)
#if defined(EXEC_BACKEND) && defined(__CYGWIN__)
                /* Work-around for cygipc exec bug */
                        && shmdt(NULL) < 0
#endif
                        )
                        elog(LOG, "shmdt(%p) failed: %m", UsedShmemSegAddr);
                UsedShmemSegAddr = NULL;
        }
}


/*
 * Attach to shared memory and make sure it has a Postgres header
 *
 * Returns attach address if OK, else NULL
 */
static PGShmemHeader *
PGSharedMemoryAttach(IpcMemoryKey key, IpcMemoryId *shmid)
{
        PGShmemHeader *hdr;

        if ((*shmid = shmget(key, sizeof(PGShmemHeader), 0)) < 0)
                return NULL;

        hdr = (PGShmemHeader *) shmat(*shmid, UsedShmemSegAddr, PG_SHMAT_FLAGS);

        if (hdr == (PGShmemHeader *) -1)
                return NULL;                    /* failed: must be some other app's */

        if (hdr->magic != PGShmemMagic)
        {
                shmdt((void *) hdr);
                return NULL;                    /* segment belongs to a non-Postgres app */
        }

        return hdr;
}