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Commit3c27944

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Make XactLockTableWait work for transactions that are not yet self-locked
XactLockTableWait assumed that its xid argument has already added itselfto the lock table. That assumption led to another assumption that iflocking the xid has succeeded but the xid is reported as still inprogress, then the input xid must have been a subtransaction.These assumptions hold true for the original uses of this code inlocking related to on-disk tuples, but they break down in logicalreplication slot snapshot building -- in particular, when a standbysnapshot logged contains an xid that's already in ProcArray but not yetin the lock table. This leads to assertion failures that can bereproduced all the way back to 9.4, when logical decoding wasintroduced.To fix, change SubTransGetParent to SubTransGetTopmostTransaction whichhas a slightly different API: it returns the argument Xid if there is noparent, and it goes all the way to the top instead of moving up thelevels one by one. Also, to avoid busy-waiting, add a 1ms sleep to givethe other process time to register itself in the lock table.For consistency, change ConditionalXactLockTableWait the same way.Author: Petr JelínekDiscussion:https://postgr.es/m/1B3E32D8-FCF4-40B4-AEF9-5C0E3AC57969@postgrespro.ruReported-by: Konstantin KnizhnikDiagnosed-by: Stas Kelvich, Petr JelínekReviewed-by: Andres Freund, Robert Haas
1 parent6fcde24 commit3c27944

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  • src/backend/storage/lmgr

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+28
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‎src/backend/storage/lmgr/lmgr.c

Lines changed: 28 additions & 2 deletions
Original file line numberDiff line numberDiff line change
@@ -557,6 +557,7 @@ XactLockTableWait(TransactionId xid, Relation rel, ItemPointer ctid,
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LOCKTAGtag;
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XactLockTableWaitInfoinfo;
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ErrorContextCallbackcallback;
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boolfirst= true;
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/*
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* If an operation is specified, set up our verbose error context
@@ -590,7 +591,26 @@ XactLockTableWait(TransactionId xid, Relation rel, ItemPointer ctid,
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if (!TransactionIdIsInProgress(xid))
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break;
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xid=SubTransGetParent(xid);
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/*
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* If the Xid belonged to a subtransaction, then the lock would have
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* gone away as soon as it was finished; for correct tuple visibility,
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* the right action is to wait on its parent transaction to go away.
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* But instead of going levels up one by one, we can just wait for the
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* topmost transaction to finish with the same end result, which also
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* incurs less locktable traffic.
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*
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* Some uses of this function don't involve tuple visibility -- such
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* as when building snapshots for logical decoding. It is possible to
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* see a transaction in ProcArray before it registers itself in the
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* locktable. The topmost transaction in that case is the same xid,
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* so we try again after a short sleep. (Don't sleep the first time
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* through, to avoid slowing down the normal case.)
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*/
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if (!first)
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pg_usleep(1000L);
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first= false;
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xid=SubTransGetTopmostTransaction(xid);
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}
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if (oper!=XLTW_None)
@@ -607,6 +627,7 @@ bool
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ConditionalXactLockTableWait(TransactionIdxid)
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{
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LOCKTAGtag;
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boolfirst= true;
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for (;;)
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{
@@ -622,7 +643,12 @@ ConditionalXactLockTableWait(TransactionId xid)
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if (!TransactionIdIsInProgress(xid))
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break;
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xid=SubTransGetParent(xid);
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/* See XactLockTableWait about this case */
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if (!first)
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pg_usleep(1000L);
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first= false;
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xid=SubTransGetTopmostTransaction(xid);
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}
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return true;

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