pgsql: Minor cleanup for access/transam/parallel.c.

Minor cleanup for access/transam/parallel.c.

ParallelMessagePending *must* be marked volatile, because it's set
by a signal handler.  On the other hand, it's pointless for
HandleParallelMessageInterrupt to save/restore errno; that must be,
and is, done at the outer level of the SIGUSR1 signal handler.

Calling CHECK_FOR_INTERRUPTS() inside HandleParallelMessages, which itself
is called from CHECK_FOR_INTERRUPTS(), seems both useless and hazardous.
The comment claiming that this is needed to handle the error queue going
away is certainly misguided, in any case.

Improve a couple of error message texts, and use
ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE to report loss of parallel worker
connection, since that's what's used in e.g. tqueue.c.  (Maybe it would be
worth inventing a dedicated ERRCODE for this type of failure?  But I do not
think ERRCODE_INTERNAL_ERROR is appropriate.)

Minor stylistic cleanups.

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/a5fe473ad79d8d2c85a625621c165e8c601274e4

Modified Files
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src/backend/access/transam/parallel.c | 25 ++++++++++++-------------
src/include/access/parallel.h         | 13 ++++++-------
2 files changed, 18 insertions(+), 20 deletions(-)