libpq-be-fe-helpers.h: wrap new cancel APIs

Commit 61461a300c1c introduced new functions to libpq for cancelling
queries.  This commit introduces a helper function that backend-side
libraries and extensions can use to invoke those.  This function takes a
timeout and can itself be interrupted while it is waiting for a cancel
request to be sent and processed, instead of being blocked.

This replaces the usage of the old functions in postgres_fdw and dblink.

Finally, it also adds some test coverage for the cancel support in
postgres_fdw.

Author: Jelte Fennema-Nio <postgres@jeltef.nl>
Discussion: https://postgr.es/m/CAGECzQT_VgOWWENUqvUV9xQmbaCyXjtRRAYO8W07oqashk_N+g@mail.gmail.com

3 files changed, 45 insertions, 24 deletions

diff --git a/contrib/postgres_fdw/connection.c b/contrib/postgres_fdw/connection.c
index 4931ebf5915..2532e453c4e 100644
--- a/contrib/postgres_fdw/connection.c
+++ b/contrib/postgres_fdw/connection.c
@@ -133,7 +133,7 @@ static void pgfdw_inval_callback(Datum arg, int cacheid, uint32 hashvalue);
 static void pgfdw_reject_incomplete_xact_state_change(ConnCacheEntry *entry);
 static void pgfdw_reset_xact_state(ConnCacheEntry *entry, bool toplevel);
 static bool pgfdw_cancel_query(PGconn *conn);
-static bool pgfdw_cancel_query_begin(PGconn *conn);
+static bool pgfdw_cancel_query_begin(PGconn *conn, TimestampTz endtime);
 static bool pgfdw_cancel_query_end(PGconn *conn, TimestampTz endtime,
 								   bool consume_input);
 static bool pgfdw_exec_cleanup_query(PGconn *conn, const char *query,
@@ -1315,36 +1315,31 @@ pgfdw_cancel_query(PGconn *conn)
 	endtime = TimestampTzPlusMilliseconds(GetCurrentTimestamp(),
 										  CONNECTION_CLEANUP_TIMEOUT);
 
-	if (!pgfdw_cancel_query_begin(conn))
+	if (!pgfdw_cancel_query_begin(conn, endtime))
 		return false;
 	return pgfdw_cancel_query_end(conn, endtime, false);
 }
 
+/*
+ * Submit a cancel request to the given connection, waiting only until
+ * the given time.
+ *
+ * We sleep interruptibly until we receive confirmation that the cancel
+ * request has been accepted, and if it is, return true; if the timeout
+ * lapses without that, or the request fails for whatever reason, return
+ * false.
+ */
 static bool
-pgfdw_cancel_query_begin(PGconn *conn)
+pgfdw_cancel_query_begin(PGconn *conn, TimestampTz endtime)
 {
-	PGcancel   *cancel;
-	char		errbuf[256];
+	char	   *errormsg = libpqsrv_cancel(conn, endtime);
 
-	/*
-	 * Issue cancel request.  Unfortunately, there's no good way to limit the
-	 * amount of time that we might block inside PQgetCancel().
-	 */
-	if ((cancel = PQgetCancel(conn)))
-	{
-		if (!PQcancel(cancel, errbuf, sizeof(errbuf)))
-		{
-			ereport(WARNING,
-					(errcode(ERRCODE_CONNECTION_FAILURE),
-					 errmsg("could not send cancel request: %s",
-							errbuf)));
-			PQfreeCancel(cancel);
-			return false;
-		}
-		PQfreeCancel(cancel);
-	}
+	if (errormsg != NULL)
+		ereport(WARNING,
+				errcode(ERRCODE_CONNECTION_FAILURE),
+				errmsg("could not send cancel request: %s", errormsg));
 
-	return true;
+	return errormsg == NULL;
 }
 
 static bool
@@ -1685,7 +1680,11 @@ pgfdw_abort_cleanup_begin(ConnCacheEntry *entry, bool toplevel,
 	 */
 	if (PQtransactionStatus(entry->conn) == PQTRANS_ACTIVE)
 	{
-		if (!pgfdw_cancel_query_begin(entry->conn))
+		TimestampTz endtime;
+
+		endtime = TimestampTzPlusMilliseconds(GetCurrentTimestamp(),
+											  CONNECTION_CLEANUP_TIMEOUT);
+		if (!pgfdw_cancel_query_begin(entry->conn, endtime))
 			return false;		/* Unable to cancel running query */
 		*cancel_requested = lappend(*cancel_requested, entry);
 	}
diff --git a/contrib/postgres_fdw/expected/postgres_fdw.out b/contrib/postgres_fdw/expected/postgres_fdw.out
index 3f0110c52b9..b7af86d3511 100644
--- a/contrib/postgres_fdw/expected/postgres_fdw.out
+++ b/contrib/postgres_fdw/expected/postgres_fdw.out
@@ -2739,6 +2739,21 @@ SELECT t1.c1, t2.c2 FROM v4 t1 LEFT JOIN ft5 t2 ON (t1.c1 = t2.c1) ORDER BY t1.c
 (10 rows)
 
 ALTER VIEW v4 OWNER TO regress_view_owner;
+-- Make sure this big CROSS JOIN query is pushed down
+EXPLAIN (VERBOSE, COSTS OFF) SELECT count(*) FROM ft1 CROSS JOIN ft2 CROSS JOIN ft4 CROSS JOIN ft5;
+                                                                             QUERY PLAN                                                                              
+---------------------------------------------------------------------------------------------------------------------------------------------------------------------
+ Foreign Scan
+   Output: (count(*))
+   Relations: Aggregate on ((((public.ft1) INNER JOIN (public.ft2)) INNER JOIN (public.ft4)) INNER JOIN (public.ft5))
+   Remote SQL: SELECT count(*) FROM ((("S 1"."T 1" r1 INNER JOIN "S 1"."T 1" r2 ON (TRUE)) INNER JOIN "S 1"."T 3" r4 ON (TRUE)) INNER JOIN "S 1"."T 4" r6 ON (TRUE))
+(4 rows)
+
+-- Make sure query cancellation works
+SET statement_timeout = '10ms';
+select count(*) from ft1 CROSS JOIN ft2 CROSS JOIN ft4 CROSS JOIN ft5; -- this takes very long
+ERROR:  canceling statement due to statement timeout
+RESET statement_timeout;
 -- ====================================================================
 -- Check that userid to use when querying the remote table is correctly
 -- propagated into foreign rels present in subqueries under an UNION ALL
diff --git a/contrib/postgres_fdw/sql/postgres_fdw.sql b/contrib/postgres_fdw/sql/postgres_fdw.sql
index 5fffc4c53bd..6e1c819159c 100644
--- a/contrib/postgres_fdw/sql/postgres_fdw.sql
+++ b/contrib/postgres_fdw/sql/postgres_fdw.sql
@@ -737,6 +737,13 @@ SELECT t1.c1, t2.c2 FROM v4 t1 LEFT JOIN ft5 t2 ON (t1.c1 = t2.c1) ORDER BY t1.c
 SELECT t1.c1, t2.c2 FROM v4 t1 LEFT JOIN ft5 t2 ON (t1.c1 = t2.c1) ORDER BY t1.c1, t2.c1 OFFSET 10 LIMIT 10;
 ALTER VIEW v4 OWNER TO regress_view_owner;
 
+-- Make sure this big CROSS JOIN query is pushed down
+EXPLAIN (VERBOSE, COSTS OFF) SELECT count(*) FROM ft1 CROSS JOIN ft2 CROSS JOIN ft4 CROSS JOIN ft5;
+-- Make sure query cancellation works
+SET statement_timeout = '10ms';
+select count(*) from ft1 CROSS JOIN ft2 CROSS JOIN ft4 CROSS JOIN ft5; -- this takes very long
+RESET statement_timeout;
+
 -- ====================================================================
 -- Check that userid to use when querying the remote table is correctly
 -- propagated into foreign rels present in subqueries under an UNION ALL