Detect invalid permutations in isolationtester

isolationtester is now able to continue running other permutations when
it detects that one of them is invalid, which is useful during initial
development of spec files.

Author: Alexander Shulgin

src/test/isolation/isolationtester.c

@@ -550,8 +550,53 @@ run_permutation(TestSpec * testspec, int nsteps, Step ** steps)
 	for (i = 0; i < nsteps; i++)
 	{
 		Step *step = steps[i];
+		PGconn *conn = conns[1 + step->session];
 
-		if (!PQsendQuery(conns[1 + step->session], step->sql))
+		if (waiting != NULL && step->session == waiting->session)
+		{
+			PGcancel *cancel;
+			PGresult *res;
+			int j;
+
+			/*
+			 * This permutation is invalid: it can never happen in real life.
+			 *
+			 * A session is blocked on an earlier step (waiting) and no further
+			 * steps from this session can run until it is unblocked, but it
+			 * can only be unblocked by running steps from other sessions.
+			 */
+			fprintf(stderr, "invalid permutation detected\n");
+
+			/* Cancel the waiting statement from this session. */
+			cancel = PQgetCancel(conn);
+			if (cancel != NULL)
+			{
+				char buf[256];
+
+				PQcancel(cancel, buf, sizeof(buf));
+
+				/* Be sure to consume the error message. */
+				while ((res = PQgetResult(conn)) != NULL)
+					PQclear(res);
+
+				PQfreeCancel(cancel);
+			}
+
+			/*
+			 * Now we really have to complete all the running transactions to
+			 * make sure teardown doesn't block.
+			 */
+			for (j = 1; j < nconns; j++)
+			{
+				res = PQexec(conns[j], "ROLLBACK");
+				if (res != NULL)
+					PQclear(res);
+			}
+
+			goto teardown;
+		}
+
+		if (!PQsendQuery(conn, step->sql))
 		{
 			fprintf(stdout, "failed to send query for step %s: %s\n",
 					step->name, PQerrorMessage(conns[1 + step->session]));
@@ -590,6 +635,7 @@ run_permutation(TestSpec * testspec, int nsteps, Step ** steps)
 		report_error_message(waiting);
 	}
 
+teardown:
 	/* Perform per-session teardown */
 	for (i = 0; i < testspec->nsessions; i++)
 	{