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In ordinary operation, VACUUM must be careful to take a cleanup lock on each leaf page of a btree index; this ensures that no indexscans could still be "in flight" to heap tuples due to be deleted. (Because of possible index-tuple motion due to concurrent page splits, it's not enough to lock only the pages we're deleting index tuples from.) In Hot Standby, the WAL replay process must likewise lock every leaf page. There were several bugs in the code for that: * The replay scan might come across unused, all-zero pages in the index. While btree_xlog_vacuum itself did the right thing (ie, nothing) with such pages, xlogutils.c supposed that such pages must be corrupt and would throw an error. This accounts for various reports of replication failures with "PANIC: WAL contains references to invalid pages". To fix, add a ReadBufferMode value that instructs XLogReadBufferExtended not to complain when we're doing this. * btree_xlog_vacuum performed the extra locking if standbyState == STANDBY_SNAPSHOT_READY, but that's not the correct test: we won't open up for hot standby queries until the database has reached consistency, and we don't want to do the extra locking till then either, for fear of reading corrupted pages (which bufmgr.c would complain about). Fix by exporting a new function from xlog.c that will report whether we're actually in hot standby replay mode. * To ensure full coverage of the index in the replay scan, btvacuumscan would emit a dummy WAL record for the last page of the index, if no vacuuming work had been done on that page. However, if the last page of the index is all-zero, that would result in corruption of said page, since the functions called on it weren't prepared to handle that case. There's no need to lock any such pages, so change the logic to target the last normal leaf page instead. The first two of these bugs were diagnosed by Andres Freund, the other one by me. Fixes based on ideas from Heikki Linnakangas and myself. This has been wrong since Hot Standby was introduced, so back-patch to 9.0. |
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