Make isolationtester more robust on locked commands

Noah Misch diagnosed the buildfarm problems in the isolation tests
partly as failure to differentiate backends properly; the old code was
using backend IDs, which is not good enough because a new backend might
use an already used ID.  Use PIDs instead.

Also, the code was purposely careless about other concurrent activity,
because it isn't expected; and in fact, it doesn't affect the vast
majority of the time.  However, it can be observed that autovacuum can
block tables for long enough to cause sporadic failures.  The new code
accounts for that by ignoring locks held by processes not explicitly
declared in our spec file.

Author: Noah Misch

src/test/isolation/isolationtester.c

@@ -21,6 +21,7 @@
 #endif
 
 #include "libpq-fe.h"
+#include "pqexpbuffer.h"
 
 #include "isolationtester.h"
 
@@ -31,7 +32,7 @@
  * connections represent spec-defined sessions.
  */
 static PGconn **conns = NULL;
-static const char **backend_ids = NULL;
+static const char **backend_pids = NULL;
 static int	nconns = 0;
 
 static void run_all_permutations(TestSpec * testspec);
@@ -67,6 +68,7 @@ main(int argc, char **argv)
 	TestSpec   *testspec;
 	int			i;
 	PGresult   *res;
+	PQExpBufferData wait_query;
 
 	/*
 	 * If the user supplies a parameter on the command line, use it as the
@@ -89,7 +91,7 @@ main(int argc, char **argv)
 	 */
 	nconns = 1 + testspec->nsessions;
 	conns = calloc(nconns, sizeof(PGconn *));
-	backend_ids = calloc(nconns, sizeof(*backend_ids));
+	backend_pids = calloc(nconns, sizeof(*backend_pids));
 	for (i = 0; i < nconns; i++)
 	{
 		conns[i] = PQconnectdb(conninfo);
@@ -112,23 +114,22 @@ main(int argc, char **argv)
 		}
 		PQclear(res);
 
-		/* Get the backend ID for lock wait checking. */
-		res = PQexec(conns[i], "SELECT i FROM pg_stat_get_backend_idset() t(i) "
-					 "WHERE pg_stat_get_backend_pid(i) = pg_backend_pid()");
+		/* Get the backend pid for lock wait checking. */
+		res = PQexec(conns[i], "SELECT pg_backend_pid()");
 		if (PQresultStatus(res) == PGRES_TUPLES_OK)
 		{
 			if (PQntuples(res) == 1 && PQnfields(res) == 1)
-				backend_ids[i] = strdup(PQgetvalue(res, 0, 0));
+				backend_pids[i] = strdup(PQgetvalue(res, 0, 0));
 			else
 			{
-				fprintf(stderr, "backend id query returned %d rows and %d columns, expected 1 row and 1 column",
+				fprintf(stderr, "backend pid query returned %d rows and %d columns, expected 1 row and 1 column",
 						PQntuples(res), PQnfields(res));
 				exit_nicely();
 			}
 		}
 		else
 		{
-			fprintf(stderr, "backend id query failed: %s",
+			fprintf(stderr, "backend pid query failed: %s",
 					PQerrorMessage(conns[i]));
 			exit_nicely();
 		}
@@ -145,8 +146,87 @@ main(int argc, char **argv)
 			session->steps[stepindex]->session = i;
 	}
 
-	res = PQprepare(conns[0], PREP_WAITING,
-					"SELECT 1 WHERE pg_stat_get_backend_waiting($1)", 0, NULL);
+	/*
+	 * Build the query we'll use to detect lock contention among sessions in
+	 * the test specification.  Most of the time, we could get away with
+	 * simply checking whether a session is waiting for *any* lock: we don't
+	 * exactly expect concurrent use of test tables.  However, autovacuum will
+	 * occasionally take AccessExclusiveLock to truncate a table, and we must
+	 * ignore that transient wait.
+	 */
+	initPQExpBuffer(&wait_query);
+	appendPQExpBufferStr(&wait_query,
+						 "SELECT 1 FROM pg_locks holder, pg_locks waiter "
+						 "WHERE NOT waiter.granted AND waiter.pid = $1 "
+						 "AND holder.granted "
+						 "AND holder.pid <> $1 AND holder.pid IN (");
+	/* The spec syntax requires at least one session; assume that here. */
+	appendPQExpBuffer(&wait_query, "%s", backend_pids[1]);
+	for (i = 2; i < nconns; i++)
+		appendPQExpBuffer(&wait_query, ", %s", backend_pids[i]);
+	appendPQExpBufferStr(&wait_query,
+						 ") "
+
+						 "AND holder.mode = ANY (CASE waiter.mode "
+						 "WHEN 'AccessShareLock' THEN ARRAY["
+						 "'AccessExclusiveLock'] "
+						 "WHEN 'RowShareLock' THEN ARRAY["
+						 "'ExclusiveLock',"
+						 "'AccessExclusiveLock'] "
+						 "WHEN 'RowExclusiveLock' THEN ARRAY["
+						 "'ShareLock',"
+						 "'ShareRowExclusiveLock',"
+						 "'ExclusiveLock',"
+						 "'AccessExclusiveLock'] "
+						 "WHEN 'ShareUpdateExclusiveLock' THEN ARRAY["
+						 "'ShareUpdateExclusiveLock',"
+						 "'ShareLock',"
+						 "'ShareRowExclusiveLock',"
+						 "'ExclusiveLock',"
+						 "'AccessExclusiveLock'] "
+						 "WHEN 'ShareLock' THEN ARRAY["
+						 "'RowExclusiveLock',"
+						 "'ShareUpdateExclusiveLock',"
+						 "'ShareRowExclusiveLock',"
+						 "'ExclusiveLock',"
+						 "'AccessExclusiveLock'] "
+						 "WHEN 'ShareRowExclusiveLock' THEN ARRAY["
+						 "'RowExclusiveLock',"
+						 "'ShareUpdateExclusiveLock',"
+						 "'ShareLock',"
+						 "'ShareRowExclusiveLock',"
+						 "'ExclusiveLock',"
+						 "'AccessExclusiveLock'] "
+						 "WHEN 'ExclusiveLock' THEN ARRAY["
+						 "'RowShareLock',"
+						 "'RowExclusiveLock',"
+						 "'ShareUpdateExclusiveLock',"
+						 "'ShareLock',"
+						 "'ShareRowExclusiveLock',"
+						 "'ExclusiveLock',"
+						 "'AccessExclusiveLock'] "
+						 "WHEN 'AccessExclusiveLock' THEN ARRAY["
+						 "'AccessShareLock',"
+						 "'RowShareLock',"
+						 "'RowExclusiveLock',"
+						 "'ShareUpdateExclusiveLock',"
+						 "'ShareLock',"
+						 "'ShareRowExclusiveLock',"
+						 "'ExclusiveLock',"
+						 "'AccessExclusiveLock'] END) "
+
+						 "AND holder.locktype IS NOT DISTINCT FROM waiter.locktype "
+						 "AND holder.database IS NOT DISTINCT FROM waiter.database "
+						 "AND holder.relation IS NOT DISTINCT FROM waiter.relation "
+						 "AND holder.page IS NOT DISTINCT FROM waiter.page "
+						 "AND holder.tuple IS NOT DISTINCT FROM waiter.tuple "
+						 "AND holder.virtualxid IS NOT DISTINCT FROM waiter.virtualxid "
+						 "AND holder.transactionid IS NOT DISTINCT FROM waiter.transactionid "
+						 "AND holder.classid IS NOT DISTINCT FROM waiter.classid "
+						 "AND holder.objid IS NOT DISTINCT FROM waiter.objid "
+						 "AND holder.objsubid IS NOT DISTINCT FROM waiter.objsubid ");
+
+	res = PQprepare(conns[0], PREP_WAITING, wait_query.data, 0, NULL);
 	if (PQresultStatus(res) != PGRES_COMMAND_OK)
 	{
 		fprintf(stderr, "prepare of lock wait query failed: %s",
@@ -154,6 +234,7 @@ main(int argc, char **argv)
 		exit_nicely();
 	}
 	PQclear(res);
+	termPQExpBuffer(&wait_query);
 
 	/*
 	 * Run the permutations specified in the spec, or all if none were
@@ -411,9 +492,7 @@ run_permutation(TestSpec * testspec, int nsteps, Step ** steps)
  * Our caller already sent the query associated with this step.  Wait for it
  * to either complete or (if given the STEP_NONBLOCK flag) to block while
  * waiting for a lock.  We assume that any lock wait will persist until we
- * have executed additional steps in the permutation.  This is not fully
- * robust -- a concurrent autovacuum could briefly take a lock with which we
- * conflict.  The risk may be low enough to discount.
+ * have executed additional steps in the permutation.
  *
  * When calling this function on behalf of a given step for a second or later
  * time, pass the STEP_RETRY flag.  This only affects the messages printed.
@@ -450,7 +529,7 @@ try_complete_step(Step *step, int flags)
 			int			ntuples;
 
 			res = PQexecPrepared(conns[0], PREP_WAITING, 1,
-								 &backend_ids[step->session + 1],
+								 &backend_pids[step->session + 1],
 								 NULL, NULL, 0);
 			if (PQresultStatus(res) != PGRES_TUPLES_OK)
 			{