pgsql: Redesign handling of SIGTERM/control-C in parallel pg_dump/pg_re

Redesign handling of SIGTERM/control-C in parallel pg_dump/pg_restore.

Formerly, Unix builds of pg_dump/pg_restore would trap SIGINT and similar
signals and set a flag that was tested in various data-transfer loops.
This was prone to errors of omission (cf commit 3c8aa6654); and even if
the client-side response was prompt, we did nothing that would cause
long-running SQL commands (e.g. CREATE INDEX) to terminate early.
Also, the master process would effectively do nothing at all upon receipt
of SIGINT; the only reason it seemed to work was that in typical scenarios
the signal would also be delivered to the child processes. We should
support termination when a signal is delivered only to the master process,
though.

Windows builds had no console interrupt handler, so they would just fall
over immediately at control-C, again leaving long-running SQL commands to
finish unmolested.

To fix, remove the flag-checking approach altogether. Instead, allow the
Unix signal handler to send a cancel request directly and then exit(1).
In the master process, also have it forward the signal to the children.
On Windows, add a console interrupt handler that behaves approximately
the same. The main difference is that a single execution of the Windows
handler can send all the cancel requests since all the info is available
in one process, whereas on Unix each process sends a cancel only for its
own database connection.

In passing, fix an old problem that DisconnectDatabase tends to send a
cancel request before exiting a parallel worker, even if nothing went
wrong. This is at least a waste of cycles, and could lead to unexpected
log messages, or maybe even data loss if it happened in pg_restore (though
in the current code the problem seems to affect only pg_dump). The cause
was that after a COPY step, pg_dump was leaving libpq in PGASYNC_BUSY
state, causing PQtransactionStatus() to report PQTRANS_ACTIVE. That's
normally harmless because the next PQexec() will silently clear the
PGASYNC_BUSY state; but in a parallel worker we might exit without any
additional SQL commands after a COPY step. So add an extra PQgetResult()
call after a COPY to allow libpq to return to PGASYNC_IDLE state.

This is a bug fix, IMO, so back-patch to 9.3 where parallel dump/restore
were introduced.

Thanks to Kyotaro Horiguchi for Windows testing and code suggestions.

Original-Patch: <7005(dot)1464657274(at)sss(dot)pgh(dot)pa(dot)us>
Discussion: <20160602(dot)174941(dot)256342236(dot)horiguchi(dot)kyotaro(at)lab(dot)ntt(dot)co(dot)jp>

Branch
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REL9_5_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/404429038896d914f38e1ee80d6d6905be0ad261

Modified Files
--------------
src/bin/pg_dump/compress_io.c | 10 -
src/bin/pg_dump/parallel.c | 606 ++++++++++++++++++++++++++--------
src/bin/pg_dump/parallel.h | 2 +-
src/bin/pg_dump/pg_backup_archiver.c | 4 +
src/bin/pg_dump/pg_backup_archiver.h | 3 +
src/bin/pg_dump/pg_backup_db.c | 32 +-
src/bin/pg_dump/pg_backup_directory.c | 12 -
src/bin/pg_dump/pg_dump.c | 5 +
8 files changed, 505 insertions(+), 169 deletions(-)