Re: [Patch] ALTER SYSTEM READ ONLY

On Wed, Jul 28, 2021 at 7:33 AM Amul Sul <sulamul@gmail.com> wrote:
> I was too worried about how I could miss that & after thinking more
> about that, I realized that the operation for ArchiveRecoveryRequested
> is never going to be skipped in the startup process and that never
> left for the checkpoint process to do that later. That is the reason
> that assert was added there.
>
> When ArchiveRecoveryRequested, the server will no longer be in
> the wal prohibited mode, we implicitly change the state to
> wal-permitted. Here is the snip from the 0003 patch:

Ugh, OK. That makes sense, but I'm still not sure that I like it. I've
kind of been wondering: why not have XLogAcceptWrites() be the
responsibility of the checkpointer all the time, in every case? That
would require fixing some more things, and this is one of them, but
then it would be consistent, which means that any bugs would be likely
to get found and fixed. If calling XLogAcceptWrites() from the
checkpointer is some funny case that only happens when the system
crashes while WAL is prohibited, then we might fail to notice that we
have a bug.

This is especially true given that we have very little test coverage
in this area. Andres was ranting to me about this earlier this week,
and I wasn't sure he was right, but then I noticed that we have
exactly zero tests in the entire source tree that make use of
recovery_end_command. We really need a TAP test for that, I think.
It's too scary to do much reorganization of the code without having
any tests at all for the stuff we're moving around. Likewise, we're
going to need TAP tests for the stuff that is specific to this patch.
For example, we should have a test that crashes the server while it's
read only, brings it back up, checks that we still can't write WAL,
then re-enables WAL, and checks that we now can write WAL. There are
probably a bunch of other things that we should test, too.