Make checkpoint requests more robust

Enterprise / PostgreSQL - Tom Lane [sss.pgh.pa.us] - 19 March 2019 16:49 EDT

Commit 6f6a6d8b1 introduced a delay of up to 2 seconds if we're trying to request a checkpoint but the checkpointer hasn't started yet (or, much less likely, our kill() call fails). However buildfarm experience shows that that's not quite enough for slow or heavily-loaded machines. There's no good reason to assume that the checkpointer won't start eventually, so we may as well make the timeout much longer, say 60 sec.

However, if the caller didn't say CHECKPOINT_WAIT, it seems like a bad idea to be waiting at all, much less for as long as 60 sec. We can remove the need for that, and make this whole thing more robust, by adjusting the code so that the existence of a pending checkpoint request is clear from the contents of shared memory, and making sure that the checkpointer process will notice it at startup even if it did not get a signal. In this way there's no need for a non-CHECKPOINT_WAIT call to wait at all; if it can't send the signal, it can nonetheless assume that the checkpointer will eventually service the request.

A potential downside of this change is that "kill -INT" on the checkpointer process is no longer enough to trigger a checkpoint, should anyone be relying on something so hacky. But there's no obvious reason to do it like that rather than issuing a plain old CHECKPOINT command, so we'll assume that nobody is. There doesn't seem to be a way to preserve this undocumented quasi-feature without introducing race conditions.

Since a principal reason for messing with this is to prevent intermittent buildfarm failures, back-patch to all supported branches.

Discussion: https://postgr.es/m/27830.1552752475@sss.pgh.pa.us

0dfe3d0ef5 Make checkpoint requests more robust.
src/backend/postmaster/checkpointer.c | 58 ++++++++++++++++++++---------------
src/include/access/xlog.h | 5 +--
2 files changed, 36 insertions(+), 27 deletions(-)

Upstream: git.postgresql.org


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