98f58a30c1
These bugs can cause data loss on standbys started with hot_standby=on at the moment they start to accept read only queries, by marking committed transactions as uncommited. The likelihood of such corruptions is small unless the primary has a high transaction rate. 5a031a5556ff83b8a9646892715d7fef415b83c3 fixed bugs in HS's startup logic by maintaining less state until at least STANDBY_SNAPSHOT_PENDING state was reached, missing the fact that both clog and subtrans are written to before that. This only failed to fail in common cases because the usage of ExtendCLOG in procarray.c was superflous since clog extensions are actually WAL logged. f44eedc3f0f347a856eea8590730769125964597/I then tried to fix the missing extensions of pg_subtrans due to the former commit's changes - which are not WAL logged - by performing the extensions when switching to a state > STANDBY_INITIALIZED and not performing xid assignments before that - again missing the fact that ExtendCLOG is unneccessary - but screwed up twice: Once because latestObservedXid wasn't updated anymore in that state due to the earlier commit and once by having an off-by-one error in the loop performing extensions. This means that whenever a CLOG_XACTS_PER_PAGE (32768 with default settings) boundary was crossed between the start of the checkpoint recovery started from and the first xl_running_xact record old transactions commit bits in pg_clog could be overwritten if they started and committed in that window. Fix this mess by not performing ExtendCLOG() in HS at all anymore since it's unneeded and evidently dangerous and by performing subtrans extensions even before reaching STANDBY_SNAPSHOT_PENDING. Analysis and patch by Andres Freund. Reported by Christophe Pettus. Backpatch down to 9.0, like the previous commit that caused this.
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