Re: Refactor pg_rewind code and make it work against a standby

Not sure if you noticed, but piculet has twice failed the
007_standby_source.pl test that was added by 9c4f5192f:

https://buildfarm.postgresql.org/cgi-bin/show_log.pl?nm=piculet&dt=2020-11-15%2006%3A00%3A11
https://buildfarm.postgresql.org/cgi-bin/show_log.pl?nm=piculet&dt=2020-11-13%2011%3A20%3A10

and francolin failed once:

https://buildfarm.postgresql.org/cgi-bin/show_log.pl?nm=francolin&dt=2020-11-12%2018%3A57%3A33

These failures look the same:

#   Failed test 'table content after rewind and insert: query result matches'
#   at t/007_standby_source.pl line 160.
#          got: 'in A
# in A, before promotion
# in A, after C was promoted
# '
#     expected: 'in A
# in A, before promotion
# in A, after C was promoted
# in A, after rewind
# '
# Looks like you failed 1 test of 3.
[11:27:01] t/007_standby_source.pl ...
Dubious, test returned 1 (wstat 256, 0x100)
Failed 1/3 subtests

Now, I'm not sure what to make of that, but I can't help noticing that
piculet uses --disable-atomics while francolin uses --disable-spinlocks.
That leads the mind towards some kind of low-level synchronization
bug ...

            regards, tom lane