0a7d771f0f
Make the nbtree page split REDO routine consistent with original execution in its approach to acquiring and releasing buffer locks (at least for pages on the tree level of the page being split). This brings btree_xlog_split() in line with btree_xlog_unlink_page(), which was taught to couple buffer locks by commit 9a9db08a. Note that the precise order in which we both acquire and release sibling buffer locks in btree_xlog_split() now matches original execution exactly (the precise order in which the locks are released probably doesn't matter much, but we might as well be consistent about it). The rule for nbtree REDO routines from here on is that same-level locks should be acquired in an order that's consistent with original execution. It's not practical to have a similar rule for cross-level page locks, since for the most part original execution holds those locks for a period that spans multiple atomic actions/WAL records. It's also not necessary, because clearly the cross-level lock coupling is only truly needed during original execution because of the presence of concurrent inserters. This is not a bug fix (unlike the similar aforementioned commit, commit 9a9db08a). The immediate reason to tighten things up in this area is to enable an upcoming enhancement to contrib/amcheck that allows it to verify that sibling links are in agreement with only an AccessShareLock (this check produced false positives when run on a replica server on account of the inconsistency fixed by this commit). But that's not the only reason to be stricter here. It is generally useful to make locking on replicas be as close to what happens during original execution as practically possible. It makes it less likely that hard to catch bugs will slip in in the future. The previous state of affairs seems to be a holdover from before the introduction of Hot Standby, when buffer lock acquisitions during recovery were totally unnecessary. See also: commit 3bbf668d, which tightened things up in this area a few years after the introduction of Hot Standby. Discussion: https://postgr.es/m/CAH2-Wz=465cJj11YXD9RKH8z=nhQa2dofOZ_23h67EXUGOJ00Q@mail.gmail.com
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