Fix race condition in XLogLogicalInfo and ProcSignal initialization.

Hi all,

I found a race condition issue between XLogLogicalInfo and ProcSignal
initialization while reviewing another issue[1]. I'm starting a
separate thread for the subject as it's not related to the issue
reported on that thread.

The issue is that child processes could miss the
PROCSIGNAL_BARRIER_UPDATE_XLOG_LOGICAL_INFO signal during the
initialization and end up in an inconsistent state because
InitializeProcessXLogLogicalInfo() is called (in BaseInit()) before
ProcSignalInit(). If the startup emits the signal to a process who is
between two steps, the process would not reflect the latest
XLogLogicalInfo state. I think we should move
InitializeProcessXLogLogicalInfo() after ProcSignalInit() like we do
so for InitLocalDataChecksumState().

I've attached the patch to fix this issue. Feedback is very welcome.

Regards,

[1] https://www.postgresql.org/message-id/CAD21AoBj%2BzKvgw_Q8gjr4YbKccW_uMe3OFQ5%2BKT246FHUuNXSQ%40mail.gmail.com
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Masahiko Sawada
Amazon Web Services: https://aws.amazon.com