Fix off-by-one possibly leading to skipped XLOG_RUNNING_XACTS records.
Since 6ef2eba3f5
("Skip checkpoints, archiving on idle systems."),
GetLastImportantRecPtr() is used to avoid performing superfluous
checkpoints, xlog switches, running-xact records when the system is
idle. Unfortunately the check concerning running-xact records had a
off-by-one error, leading to such records being potentially skipped
when only a single record has been inserted since the last
running-xact record.
An alternative approach would have been to change
GetLastImportantRecPtr()'s definition to point to the end of records,
but that would make the checkpoint code more complicated.
Author: Andres Freund
Discussion: https://postgr.es/m/20170505012447.wsrympaxnfis6ojt@alap3.anarazel.de
Backpatch: no, code only present in master
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parent
334b82cd56
commit
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@ -325,10 +325,13 @@ BackgroundWriterMain(void)
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/*
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* Only log if enough time has passed and interesting records have
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* been inserted since the last snapshot.
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* been inserted since the last snapshot. Have to compare with <=
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* instead of < because GetLastImportantRecPtr() points at the
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* start of a record, whereas last_snapshot_lsn points just past
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* the end of the record.
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*/
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if (now >= timeout &&
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last_snapshot_lsn < GetLastImportantRecPtr())
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last_snapshot_lsn <= GetLastImportantRecPtr())
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{
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last_snapshot_lsn = LogStandbySnapshot();
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last_snapshot_ts = now;
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@ -611,7 +611,8 @@ CheckArchiveTimeout(void)
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{
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/*
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* Switch segment only when "important" WAL has been logged since the
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* last segment switch.
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* last segment switch (last_switch_lsn points to end of segment
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* switch occurred in).
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*/
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if (GetLastImportantRecPtr() > last_switch_lsn)
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{
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