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Commitda668a5

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Fix xmin/xmax horizon computation during logical decoding initialization.
When building the initial historic catalog snapshot there werescenarios where snapbuild.c would use incorrect xmin/xmax values whenstarting from a xl_running_xacts record. The values used were always abit suspect, but happened to be correct in the easy to testcases. Notably the values used when the the initial snapshot wascomputed while no other transactions were running were correct.This is likely to be the cause of the occasional buildfarm failures onanimals markhor and tick; but it's quite possible to reproduceproblems without CLOBBER_CACHE_ALWAYS.Backpatch to 9.4, where logical decoding was introduced.
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‎src/backend/replication/logical/snapbuild.c

Lines changed: 13 additions & 7 deletions
Original file line numberDiff line numberDiff line change
@@ -28,7 +28,7 @@
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*
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* As the percentage of transactions modifying the catalog normally is fairly
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* small in comparisons to ones only manipulating user data, we keep track of
31-
* the committed catalog modifying ones inside(xmin, xmax) instead of keeping
31+
* the committed catalog modifying ones inside[xmin, xmax) instead of keeping
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* track of all running transactions like it's done in a normal snapshot. Note
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* that we're generally only looking at transactions that have acquired an
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* xid. That is we keep a list of transactions between snapshot->(xmin, xmax)
@@ -1250,10 +1250,11 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn
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/* can decode everything after this */
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builder->start_decoding_at=lsn+1;
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1253-
builder->xmin=running->oldestRunningXid;
1254-
builder->xmax=running->latestCompletedXid;
1255-
TransactionIdAdvance(builder->xmax);
1253+
/* As no transactions wererunning xmin/xmax can be trivially set. */
1254+
builder->xmin=running->nextXid;/* < are finished */
1255+
builder->xmax=running->nextXid;/* >= are running */
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1257+
/* so we can safely use the faster comparisons */
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Assert(TransactionIdIsNormal(builder->xmin));
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Assert(TransactionIdIsNormal(builder->xmax));
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@@ -1294,9 +1295,14 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn
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* instead of running transactions we don't need to know anything
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* about uncommitted subtransactions.
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*/
1297-
builder->xmin=running->oldestRunningXid;
1298-
builder->xmax=running->latestCompletedXid;
1299-
TransactionIdAdvance(builder->xmax);
1298+
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/*
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* Start with an xmin/xmax that's correct for future, when all the
1301+
* currently running transactions have finished. We'll update both
1302+
* while waiting for the pending transactions to finish.
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*/
1304+
builder->xmin=running->nextXid;/* < are finished */
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builder->xmax=running->nextXid;/* >= are running */
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/* so we can safely use the faster comparisons */
13021308
Assert(TransactionIdIsNormal(builder->xmin));

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