Fix a couple of bugs with replication slot advancing feature

Enterprise / PostgreSQL - Michael Paquier [paquier.xyz] - 11 June 2018 00:26 EDT

A review of the code has showed up a couple of issues fixed by this commit:- Physical slots have been using the confirmed LSN position as a start comparison point which is always 0/0, instead use the restart LSN position (logical slots need to use the confirmed LSN position, which was correct).- The actual slot update was incorrect for both physical and logical slots. Physical slots need to use their restart_lsn as base comparison point (confirmed_flush was used because of previous point), and logical slots need to begin reading WAL from restart_lsn (confirmed_flush was used as well), while confirmed_flush is compiled depending on the decoding context and record read, and is the LSN position returned back to the caller.- Never return 0/0 if a slot cannot be advanced. This way, if a slot is advanced while the activity is idle, then the same position is returned to the caller over and over without raising an error. Instead return the LSN the slot has been advanced to. With repetitive calls, the same position is returned hence caller can directly monitor the difference in progress in bytes by doing simply LSN difference calculations, which should be monotonic.

Note that as the slot is owned by the backend advancing it, then the read of those fields is fine lock-less, while updates need to happen while the slot mutex is held, so fix that on the way as well. Other locks for in-memory data of replication slots have been already fixed previously.

Some of those issues have been pointed out by Petr and Simon during the patch, while I noticed some of them after looking at the code. This also visibly takes of a recently-discovered bug causing assertion failures which can be triggered by a two-step slot forwarding which first advanced the slot to a WAL page boundary and secondly advanced it to the latest position, say 'FF/FFFFFFF' to make sure that the newest LSN is used as forward point. It would have been nice to drop a test for that, but the set of operators working on pg_lsn limits it, so this is left for a future exercise.

Author: Michael Paquier

f731cfa94c Fix a couple of bugs with replication slot advancing feature
src/backend/replication/slotfuncs.c | 50 ++++++++++++++++++++++++++-----------
1 file changed, 36 insertions(+), 14 deletions(-)

Upstream: git.postgresql.org


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