Re: Switching XLog source from archive to streaming when primary available

On Mon, Sep 19, 2022 at 07:49:21PM +0530, Bharath Rupireddy wrote:
> SwitchFromArchiveToStreamEnabled() seemed better at this point. I'm
> attaching the v7 patch with that change. Please review it further.

As I mentioned upthread [0], I'm still a little concerned that this patch
will cause the state machine to go straight from archive recovery to
streaming replication, skipping recovery from pg_wal.  I wonder if this
could be resolved by moving the standby to the pg_wal phase instead.
Concretely, this line

+				if (switchSource)
+					break;

would instead change currentSource from XLOG_FROM_ARCHIVE to
XLOG_FROM_PG_WAL before the call to XLogFileReadAnyTLI().  I suspect the
behavior would be basically the same, but it would maintain the existing
ordering.

However, I do see the following note elsewhere in xlogrecovery.c:

 * The segment can be fetched via restore_command, or via walreceiver having
 * streamed the record, or it can already be present in pg_wal. Checking
 * pg_wal is mainly for crash recovery, but it will be polled in standby mode
 * too, in case someone copies a new segment directly to pg_wal. That is not
 * documented or recommended, though.

Given this information, the present behavior might not be too important,
but I don't see a point in changing it without good reason.

[0] https://postgr.es/m/20220906215704.GA2084086%40nathanxps13

-- 
Nathan Bossart
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