Re: Infinite loop in XLogPageRead() on standby

Wow.  Have you seen that in an actual production environment?

my $start_page = start_of_page($end_lsn);
my $wal_file = write_wal($primary, $TLI, $start_page,
                         "\x00" x $WAL_BLOCK_SIZE);
# copy the file we just "hacked" to the archive
copy($wal_file, $primary->archive_dir);

So you are emulating a failure by filling with zeros the second page
where the last emit_message() generated a record, and the page before
that includes the continuation record.  Then abuse of WAL archiving to
force the replay of the last record.  That's kind of cool.

> To be honest, I don't know yet how to fix it nicely. I am thinking about
> returning XLREAD_FAIL from XLogPageRead() if it suddenly switched to a new
> timeline while trying to read a page and if this page is invalid.

Hmm.  I suspect that you may be right on a TLI change when reading a
page.  There are a bunch of side cases with continuation records and
header validation around XLogReaderValidatePageHeader().  Perhaps you
have an idea of patch to show your point?

Nit.  In your test, it seems to me that you should not call directly
set_standby_mode and enable_restoring, just rely on has_restoring with
the standby option included.