isolationtester: don't repeat the is-it-waiting query when retrying a step.

If we're retrying a step, then we already decided it was blocked on a lock,
and there's no need to recheck that.  The original coding of commit
38f8bdc resulted in a large number of
is-it-waiting queries when dealing with multiple concurrently-blocked
sessions, which is fairly pointless and also results in test failures in
CLOBBER_CACHE_ALWAYS builds, where the is-it-waiting query is quite slow.

This definition also permits appending pg_sleep() calls to steps where it's
needed to control the order of finish of concurrent steps.  Before, that
did not work nicely because we'd decide that a step performing a sleep was
not blocked and hang up waiting for it to finish, rather than noticing the
completion of the concurrent step we're supposed to notice first.

In passing, revise handling of removal of completed waiting steps
to make it a bit less messy.

Author: tglsfdc

src/test/isolation/isolationtester.c

@@ -540,8 +540,8 @@ run_permutation(TestSpec *testspec, int nsteps, Step **steps)
 	for (i = 0; i < nsteps; i++)
 	{
 		Step	   *step = steps[i];
-		Step	   *oldstep = NULL;
 		PGconn	   *conn = conns[1 + step->session];
+		Step	   *oldstep = NULL;
 		bool		mustwait;
 
 		/*
@@ -563,33 +563,30 @@ run_permutation(TestSpec *testspec, int nsteps, Step **steps)
 			if (step->session == waiting[w]->session)
 			{
 				oldstep = waiting[w];
+
+				/* Wait for previous step on this connection. */
+				try_complete_step(oldstep, STEP_RETRY);
+
+				/* Remove that step from the waiting[] array. */
+				if (w + 1 < nwaiting)
+					memcpy(&waiting[w], &waiting[w + 1],
+						   (nwaiting - (w + 1)) * sizeof(Step *));
+				nwaiting--;
+
 				break;
 			}
 		}
 		if (oldstep != NULL)
 		{
-			/* Wait for previous step on this connection. */
-			try_complete_step(oldstep, STEP_RETRY);
-
 			/*
-			 * Attempt to complete any steps that were previously waiting.
-			 * Remove any that have completed, and also remove the one for
-			 * which we just waited.
+			 * Check for completion of any steps that were previously waiting.
+			 * Remove any that have completed from waiting[], and include them
+			 * in the list for report_multiple_error_messages().
 			 */
 			w = 0;
 			nerrorstep = 0;
 			while (w < nwaiting)
 			{
-				if (waiting[w] == oldstep)
-				{
-					/* We just waited for this; remove it. */
-					if (w + 1 < nwaiting)
-						memcpy(&waiting[w], &waiting[w + 1],
-							   (nwaiting - (w + 1)) * sizeof(Step *));
-					nwaiting--;
-					continue;
-				}
-
 				if (try_complete_step(waiting[w], STEP_NONBLOCK | STEP_RETRY))
 				{
 					/* Still blocked on a lock, leave it alone. */
@@ -621,7 +618,7 @@ run_permutation(TestSpec *testspec, int nsteps, Step **steps)
 		/* Try to complete this step without blocking.  */
 		mustwait = try_complete_step(step, STEP_NONBLOCK);
 
-		/* Attempt to complete any steps that were previously waiting. */
+		/* Check for completion of any steps that were previously waiting. */
 		w = 0;
 		nerrorstep = 0;
 		while (w < nwaiting)
@@ -646,7 +643,7 @@ run_permutation(TestSpec *testspec, int nsteps, Step **steps)
 			waiting[nwaiting++] = step;
 	}
 
-	/* Finish any waiting query. */
+	/* Wait for any remaining queries. */
 	for (w = 0; w < nwaiting; ++w)
 	{
 		try_complete_step(waiting[w], STEP_RETRY);
@@ -702,9 +699,10 @@ run_permutation(TestSpec *testspec, int nsteps, Step **steps)
  * 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.
+ * time, pass the STEP_RETRY flag.  In this case we don't need to recheck
+ * whether it's waiting for a lock.
  *
- * If the connection returns an error, the message is saved in step->errormsg.
+ * If the query returns an error, the message is saved in step->errormsg.
  * Caller should call report_error_message shortly after this, to have it
  * printed and cleared.
  *
@@ -750,6 +748,14 @@ try_complete_step(Step *step, int flags)
 			{
 				int			ntuples;
 
+				/*
+				 * If this is a retry, assume without checking that the step
+				 * is still blocked.  This rule saves a lot of PREP_WAITING
+				 * queries and avoids any possible flappiness in the answer.
+				 */
+				if (flags & STEP_RETRY)
+					return true;
+
 				res = PQexecPrepared(conns[0], PREP_WAITING, 1,
 									 &backend_pids[step->session + 1],
 									 NULL, NULL, 0);
@@ -764,9 +770,8 @@ try_complete_step(Step *step, int flags)
 
 				if (ntuples >= 1)		/* waiting to acquire a lock */
 				{
-					if (!(flags & STEP_RETRY))
-						printf("step %s: %s <waiting ...>\n",
-							   step->name, step->sql);
+					printf("step %s: %s <waiting ...>\n",
+						   step->name, step->sql);
 					return true;
 				}
 				/* else, not waiting */