Move isolationtester's is-blocked query into C code for speed.

Commit 4deb41381 modified isolationtester's query to see whether a
session is blocked to also check for waits occurring in GetSafeSnapshot.
However, it did that in a way that enormously increased the query's
runtime under CLOBBER_CACHE_ALWAYS, causing the buildfarm members
that use that to run about four times slower than before, and in some
cases fail entirely.  To fix, push the entire logic into a dedicated
backend function.  This should actually reduce the CLOBBER_CACHE_ALWAYS
runtime from what it was previously, though I've not checked that.

In passing, expose a SQL function to check for safe-snapshot blockage,
comparable to pg_blocking_pids.  This is more or less free given the
infrastructure built to solve the other problem, so we might as well.

Thomas Munro

Discussion: https://postgr.es/m/20170407165749.pstcakbc637opkax@alap3.anarazel.de

1 files changed, 119 insertions, 0 deletions

diff --git a/src/backend/utils/adt/lockfuncs.c b/src/backend/utils/adt/lockfuncs.c
index 63f956e6708..ef4824f79cc 100644
--- a/src/backend/utils/adt/lockfuncs.c
+++ b/src/backend/utils/adt/lockfuncs.c
@@ -518,6 +518,125 @@ pg_blocking_pids(PG_FUNCTION_ARGS)
 
 
 /*
+ * pg_safe_snapshot_blocking_pids - produce an array of the PIDs blocking
+ * given PID from getting a safe snapshot
+ *
+ * XXX this does not consider parallel-query cases; not clear how big a
+ * problem that is in practice
+ */
+Datum
+pg_safe_snapshot_blocking_pids(PG_FUNCTION_ARGS)
+{
+	int			blocked_pid = PG_GETARG_INT32(0);
+	int		   *blockers;
+	int			num_blockers;
+	Datum	   *blocker_datums;
+
+	/* A buffer big enough for any possible blocker list without truncation */
+	blockers = (int *) palloc(MaxBackends * sizeof(int));
+
+	/* Collect a snapshot of processes waited for by GetSafeSnapshot */
+	num_blockers =
+		GetSafeSnapshotBlockingPids(blocked_pid, blockers, MaxBackends);
+
+	/* Convert int array to Datum array */
+	if (num_blockers > 0)
+	{
+		int			i;
+
+		blocker_datums = (Datum *) palloc(num_blockers * sizeof(Datum));
+		for (i = 0; i < num_blockers; ++i)
+			blocker_datums[i] = Int32GetDatum(blockers[i]);
+	}
+	else
+		blocker_datums = NULL;
+
+	/* Construct array, using hardwired knowledge about int4 type */
+	PG_RETURN_ARRAYTYPE_P(construct_array(blocker_datums, num_blockers,
+										  INT4OID,
+										  sizeof(int32), true, 'i'));
+}
+
+
+/*
+ * pg_isolation_test_session_is_blocked - support function for isolationtester
+ *
+ * Check if specified PID is blocked by any of the PIDs listed in the second
+ * argument.  Currently, this looks for blocking caused by waiting for
+ * heavyweight locks or safe snapshots.  We ignore blockage caused by PIDs
+ * not directly under the isolationtester's control, eg autovacuum.
+ *
+ * This is an undocumented function intended for use by the isolation tester,
+ * and may change in future releases as required for testing purposes.
+ */
+Datum
+pg_isolation_test_session_is_blocked(PG_FUNCTION_ARGS)
+{
+	int			blocked_pid = PG_GETARG_INT32(0);
+	ArrayType  *interesting_pids_a = PG_GETARG_ARRAYTYPE_P(1);
+	ArrayType  *blocking_pids_a;
+	int32	   *interesting_pids;
+	int32	   *blocking_pids;
+	int			num_interesting_pids;
+	int			num_blocking_pids;
+	int			dummy;
+	int			i,
+				j;
+
+	/* Validate the passed-in array */
+	Assert(ARR_ELEMTYPE(interesting_pids_a) == INT4OID);
+	if (array_contains_nulls(interesting_pids_a))
+		elog(ERROR, "array must not contain nulls");
+	interesting_pids = (int32 *) ARR_DATA_PTR(interesting_pids_a);
+	num_interesting_pids = ArrayGetNItems(ARR_NDIM(interesting_pids_a),
+										  ARR_DIMS(interesting_pids_a));
+
+	/*
+	 * Get the PIDs of all sessions blocking the given session's attempt to
+	 * acquire heavyweight locks.
+	 */
+	blocking_pids_a =
+		DatumGetArrayTypeP(DirectFunctionCall1(pg_blocking_pids, blocked_pid));
+
+	Assert(ARR_ELEMTYPE(blocking_pids_a) == INT4OID);
+	Assert(!array_contains_nulls(blocking_pids_a));
+	blocking_pids = (int32 *) ARR_DATA_PTR(blocking_pids_a);
+	num_blocking_pids = ArrayGetNItems(ARR_NDIM(blocking_pids_a),
+									   ARR_DIMS(blocking_pids_a));
+
+	/*
+	 * Check if any of these are in the list of interesting PIDs, that being
+	 * the sessions that the isolation tester is running.  We don't use
+	 * "arrayoverlaps" here, because it would lead to cache lookups and one of
+	 * our goals is to run quickly under CLOBBER_CACHE_ALWAYS.  We expect
+	 * blocking_pids to be usually empty and otherwise a very small number in
+	 * isolation tester cases, so make that the outer loop of a naive search
+	 * for a match.
+	 */
+	for (i = 0; i < num_blocking_pids; i++)
+		for (j = 0; j < num_interesting_pids; j++)
+		{
+			if (blocking_pids[i] == interesting_pids[j])
+				PG_RETURN_BOOL(true);
+		}
+
+	/*
+	 * Check if blocked_pid is waiting for a safe snapshot.  We could in
+	 * theory check the resulting array of blocker PIDs against the
+	 * interesting PIDs whitelist, but since there is no danger of autovacuum
+	 * blocking GetSafeSnapshot there seems to be no point in expending cycles
+	 * on allocating a buffer and searching for overlap; so it's presently
+	 * sufficient for the isolation tester's purposes to use a single element
+	 * buffer and check if the number of safe snapshot blockers is non-zero.
+	 */
+	if (GetSafeSnapshotBlockingPids(blocked_pid, &dummy, 1) > 0)
+		PG_RETURN_BOOL(true);
+
+	PG_RETURN_BOOL(false);
+}
+
+
+/*
  * Functions for manipulating advisory locks
  *
  * We make use of the locktag fields as follows: