pgsql: Allow for error or refusal while absorbing a ProcSignalBarrier.

Allow for error or refusal while absorbing a ProcSignalBarrier.

Previously, the per-barrier-type functions tasked with absorbing
them were expected to always succeed and never throw an error.
However, that's a bit inconvenient. Further study has revealed that
there are realistic cases where it might not be possible to absorb
a ProcSignalBarrier without terminating the transaction, or even
the whole backend. Similarly, for some barrier types, there might
be other reasons where it's not reasonably possible to absorb the
barrier at certain points in the code, so provide a way for a
per-barrier-type function to reject absorbing the barrier.

Unfortunately, there's still no committed code making use of this
infrastructure; hopefully, we'll get there. :-(

Patch by me, reviewed by Andres Freund and Amul Sul.

Discussion: http://postgr.es/m/20200908182005.xya7wetdh3pndzim@alap3.anarazel.de
Discussion: http://postgr.es/m/CA+Tgmob56Pk1-5aTJdVPCWFHon7me4M96ENpGe9n_R4JUjjhZA@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/a3ed4d1efe9f3a1765c504ba4a224f1f393b5b14

Modified Files
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src/backend/storage/ipc/procsignal.c | 125 +++++++++++++++++++++++++++++++----
1 file changed, 113 insertions(+), 12 deletions(-)