Fix latent portability issue in pgwin32_dispatch_queued_signals().

tglsfdc · tglsfdc · commit 58666ed28ab5 · 2016-04-04T11:13:17.000-04:00
The first iteration of the signal-checking loop would compute sigmask(0)
which expands to 1&lt;&lt;(-1) which is undefined behavior according to the
C standard.  The lack of field reports of trouble suggest that it
evaluates to 0 on all existing Windows compilers, but that's hardly
something to rely on.  Since signal 0 isn't a queueable signal anyway,
we can just make the loop iterate from 1 instead, and save a few cycles
as well as avoiding the undefined behavior.

In passing, avoid evaluating the volatile expression UNBLOCKED_SIGNAL_QUEUE
twice in a row; there's no reason to waste cycles like that.

Noted by Aleksander Alekseev, though this isn't his proposed fix.
Back-patch to all supported branches.
diff --git a/src/backend/port/win32/signal.c b/src/backend/port/win32/signal.c
@@ -33,6 +33,7 @@ HANDLE		pgwin32_initial_signal_pipe = INVALID_HANDLE_VALUE;
  */
 static CRITICAL_SECTION pg_signal_crit_sec;
 
+/* Note that array elements 0 are unused since they correspond to signal 0 */
 static pqsigfunc pg_signal_array[PG_SIGNAL_COUNT];
 static pqsigfunc pg_signal_defaults[PG_SIGNAL_COUNT];
 
@@ -105,15 +106,15 @@ pgwin32_signal_initialize(void)
 void
 pgwin32_dispatch_queued_signals(void)
 {
-	int			i;
+	int			exec_mask;
 
 	EnterCriticalSection(&pg_signal_crit_sec);
-	while (UNBLOCKED_SIGNAL_QUEUE())
+	while ((exec_mask = UNBLOCKED_SIGNAL_QUEUE()) != 0)
 	{
 		/* One or more unblocked signals queued for execution */
-		int			exec_mask = UNBLOCKED_SIGNAL_QUEUE();
+		int			i;
 
-		for (i = 0; i < PG_SIGNAL_COUNT; i++)
+		for (i = 1; i < PG_SIGNAL_COUNT; i++)
 		{
 			if (exec_mask & sigmask(i))
 			{

Original file line number	Diff line number	Diff line change
`@@ -33,6 +33,7 @@ HANDLE pgwin32_initial_signal_pipe = INVALID_HANDLE_VALUE;`
`33`	`33`	`*/`
`34`	`34`	`static CRITICAL_SECTION pg_signal_crit_sec;`
`35`	`35`
	`36`	`+/* Note that array elements 0 are unused since they correspond to signal 0 */`
`36`	`37`	`static pqsigfunc pg_signal_array[PG_SIGNAL_COUNT];`
`37`	`38`	`static pqsigfunc pg_signal_defaults[PG_SIGNAL_COUNT];`
`38`	`39`
`@@ -105,15 +106,15 @@ pgwin32_signal_initialize(void)`
`105`	`106`	`void`
`106`	`107`	`pgwin32_dispatch_queued_signals(void)`
`107`	`108`	`{`
`108`		`- int i;`
	`109`	`+ int exec_mask;`
`109`	`110`
`110`	`111`	`EnterCriticalSection(&pg_signal_crit_sec);`
`111`		`- while (UNBLOCKED_SIGNAL_QUEUE())`
	`112`	`+ while ((exec_mask = UNBLOCKED_SIGNAL_QUEUE()) != 0)`
`112`	`113`	`{`
`113`	`114`	`/* One or more unblocked signals queued for execution */`
`114`		`- int exec_mask = UNBLOCKED_SIGNAL_QUEUE();`
	`115`	`+ int i;`
`115`	`116`
`116`		`- for (i = 0; i < PG_SIGNAL_COUNT; i++)`
	`117`	`+ for (i = 1; i < PG_SIGNAL_COUNT; i++)`
`117`	`118`	`{`
`118`	`119`	`if (exec_mask & sigmask(i))`
`119`	`120`	`{`