Make XactLockTableWait work for transactions that are not yet self-locked

XactLockTableWait assumed that its xid argument has already added itself
to the lock table.  That assumption led to another assumption that if
locking the xid has succeeded but the xid is reported as still in
progress, then the input xid must have been a subtransaction.

These assumptions hold true for the original uses of this code in
locking related to on-disk tuples, but they break down in logical
replication slot snapshot building -- in particular, when a standby
snapshot logged contains an xid that's already in ProcArray but not yet
in the lock table.  This leads to assertion failures that can be
reproduced all the way back to 9.4, when logical decoding was
introduced.

To fix, change SubTransGetParent to SubTransGetTopmostTransaction which
has a slightly different API: it returns the argument Xid if there is no
parent, and it goes all the way to the top instead of moving up the
levels one by one.  Also, to avoid busy-waiting, add a 1ms sleep to give
the other process time to register itself in the lock table.

For consistency, change ConditionalXactLockTableWait the same way.

Author: Petr Jelínek
Discussion: https://postgr.es/m/1B3E32D8-FCF4-40B4-AEF9-5C0E3AC57969@postgrespro.ru
Reported-by: Konstantin Knizhnik
Diagnosed-by: Stas Kelvich, Petr Jelínek
Reviewed-by: Andres Freund, Robert Haas

diff --git a/src/backend/storage/lmgr/lmgr.c b/src/backend/storage/lmgr/lmgr.c
index 7b08555b071cc0fe5384604c7e537a2adb2aa994..9a2f94beaf760ea5c6534e113228f933bb47fe6f 100644 (file)
--- a/src/backend/storage/lmgr/lmgr.c
+++ b/src/backend/storage/lmgr/lmgr.c
@@ -557,6 +557,7 @@ XactLockTableWait(TransactionId xid, Relation rel, ItemPointer ctid,
        LOCKTAG         tag;
        XactLockTableWaitInfo info;
        ErrorContextCallback callback;
+       bool            first = true;
 
        /*
         * If an operation is specified, set up our verbose error context
@@ -590,7 +591,26 @@ XactLockTableWait(TransactionId xid, Relation rel, ItemPointer ctid,
 
                if (!TransactionIdIsInProgress(xid))
                        break;
-               xid = SubTransGetParent(xid);
+
+               /*
+                * If the Xid belonged to a subtransaction, then the lock would have
+                * gone away as soon as it was finished; for correct tuple visibility,
+                * the right action is to wait on its parent transaction to go away.
+                * But instead of going levels up one by one, we can just wait for the
+                * topmost transaction to finish with the same end result, which also
+                * incurs less locktable traffic.
+                *
+                * Some uses of this function don't involve tuple visibility -- such
+                * as when building snapshots for logical decoding.  It is possible to
+                * see a transaction in ProcArray before it registers itself in the
+                * locktable.  The topmost transaction in that case is the same xid,
+                * so we try again after a short sleep.  (Don't sleep the first time
+                * through, to avoid slowing down the normal case.)
+                */
+               if (!first)
+                       pg_usleep(1000L);
+               first = false;
+               xid = SubTransGetTopmostTransaction(xid);
        }
 
        if (oper != XLTW_None)
@@ -607,6 +627,7 @@ bool
 ConditionalXactLockTableWait(TransactionId xid)
 {
        LOCKTAG         tag;
+       bool            first = true;
 
        for (;;)
        {
@@ -622,7 +643,12 @@ ConditionalXactLockTableWait(TransactionId xid)
 
                if (!TransactionIdIsInProgress(xid))
                        break;
-               xid = SubTransGetParent(xid);
+
+               /* See XactLockTableWait about this case */
+               if (!first)
+                       pg_usleep(1000L);
+               first = false;
+               xid = SubTransGetTopmostTransaction(xid);
        }
 
        return true;