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: <7005.1464657274@sss.pgh.pa.us>
Discussion: <20160602.174941.256342236.horiguchi.kyotaro@lab.ntt.co.jp>

1 files changed, 25 insertions, 7 deletions

diff --git a/src/bin/pg_dump/pg_backup_db.c b/src/bin/pg_dump/pg_backup_db.c
index 818bc9efe14..352595e49fa 100644
--- a/src/bin/pg_dump/pg_backup_db.c
+++ b/src/bin/pg_dump/pg_backup_db.c
@@ -12,6 +12,7 @@
 #include "postgres_fe.h"
 
 #include "dumputils.h"
+#include "parallel.h"
 #include "pg_backup_archiver.h"
 #include "pg_backup_db.h"
 #include "pg_backup_utils.h"
@@ -106,6 +107,9 @@ ReconnectToServer(ArchiveHandle *AH, const char *dbname, const char *username)
 
 	newConn = _connectDB(AH, newdbname, newusername);
 
+	/* Update ArchiveHandle's connCancel before closing old connection */
+	set_archive_cancel_info(AH, newConn);
+
 	PQfinish(AH->connection);
 	AH->connection = newConn;
 
@@ -327,6 +331,9 @@ ConnectDatabase(Archive *AHX,
 	_check_database_version(AH);
 
 	PQsetNoticeProcessor(AH->connection, notice_processor, NULL);
+
+	/* arrange for SIGINT to issue a query cancel on this connection */
+	set_archive_cancel_info(AH, AH->connection);
 }
 
 /*
@@ -337,19 +344,25 @@ void
 DisconnectDatabase(Archive *AHX)
 {
 	ArchiveHandle *AH = (ArchiveHandle *) AHX;
-	PGcancel   *cancel;
 	char		errbuf[1];
 
 	if (!AH->connection)
 		return;
 
-	if (PQtransactionStatus(AH->connection) == PQTRANS_ACTIVE)
+	if (AH->connCancel)
 	{
-		if ((cancel = PQgetCancel(AH->connection)))
-		{
-			PQcancel(cancel, errbuf, sizeof(errbuf));
-			PQfreeCancel(cancel);
-		}
+		/*
+		 * If we have an active query, send a cancel before closing.  This is
+		 * of no use for a normal exit, but might be helpful during
+		 * exit_horribly().
+		 */
+		if (PQtransactionStatus(AH->connection) == PQTRANS_ACTIVE)
+			PQcancel(AH->connCancel, errbuf, sizeof(errbuf));
+
+		/*
+		 * Prevent signal handler from sending a cancel after this.
+		 */
+		set_archive_cancel_info(AH, NULL);
 	}
 
 	PQfinish(AH->connection);
@@ -631,6 +644,11 @@ EndDBCopyMode(Archive *AHX, const char *tocEntryTag)
 								tocEntryTag, PQerrorMessage(AH->connection));
 		PQclear(res);
 
+		/* Do this to ensure we've pumped libpq back to idle state */
+		if (PQgetResult(AH->connection) != NULL)
+			write_msg(NULL, "WARNING: unexpected extra results during COPY of table \"%s\"\n",
+					  tocEntryTag);
+
 		AH->pgCopyIn = false;
 	}
 }