Refactor CHECK_FOR_INTERRUPTS() to add flexibility.

Split up CHECK_FOR_INTERRUPTS() to provide an additional macro
INTERRUPTS_PENDING_CONDITION(), which just tests whether an
interrupt is pending without attempting to service it.  This is
useful in situations where the caller knows that interrupts are
blocked, and would like to find out if it's worth the trouble
to unblock them.

Also add INTERRUPTS_CAN_BE_PROCESSED(), which indicates whether
CHECK_FOR_INTERRUPTS() can be relied on to clear the pending interrupt.

This commit doesn't actually add any uses of the new macros,
but a follow-on bug fix will do so.  Back-patch to all supported
branches to provide infrastructure for that fix.

Alvaro Herrera and Tom Lane

Discussion: https://postgr.es/m/20210513155351.GA7848@alvherre.pgsql

1 files changed, 23 insertions, 11 deletions

diff --git a/src/include/miscadmin.h b/src/include/miscadmin.h
index 95202d37af5..4dc343cbc59 100644
--- a/src/include/miscadmin.h
+++ b/src/include/miscadmin.h
@@ -57,6 +57,15 @@
  * allowing die interrupts: HOLD_CANCEL_INTERRUPTS() and
  * RESUME_CANCEL_INTERRUPTS().
  *
+ * Note that ProcessInterrupts() has also acquired a number of tasks that
+ * do not necessarily cause a query-cancel-or-die response.  Hence, it's
+ * possible that it will just clear InterruptPending and return.
+ *
+ * INTERRUPTS_PENDING_CONDITION() can be checked to see whether an
+ * interrupt needs to be serviced, without trying to do so immediately.
+ * Some callers are also interested in INTERRUPTS_CAN_BE_PROCESSED(),
+ * which tells whether ProcessInterrupts is sure to clear the interrupt.
+ *
  * Special mechanisms are used to let an interrupt be accepted when we are
  * waiting for a lock or when we are waiting for command input (but, of
  * course, only if the interrupt holdoff counter is zero).  See the
@@ -97,24 +106,27 @@ extern PGDLLIMPORT volatile uint32 CritSectionCount;
 /* in tcop/postgres.c */
 extern void ProcessInterrupts(void);
 
+/* Test whether an interrupt is pending */
 #ifndef WIN32
+#define INTERRUPTS_PENDING_CONDITION() \
+	(unlikely(InterruptPending))
+#else
+#define INTERRUPTS_PENDING_CONDITION() \
+	(unlikely(UNBLOCKED_SIGNAL_QUEUE()) ? pgwin32_dispatch_queued_signals() : 0, \
+	 unlikely(InterruptPending))
+#endif
 
+/* Service interrupt, if one is pending and it's safe to service it now */
 #define CHECK_FOR_INTERRUPTS() \
 do { \
-	if (unlikely(InterruptPending)) \
-		ProcessInterrupts(); \
-} while(0)
-#else							/* WIN32 */
-
-#define CHECK_FOR_INTERRUPTS() \
-do { \
-	if (unlikely(UNBLOCKED_SIGNAL_QUEUE())) \
-		pgwin32_dispatch_queued_signals(); \
-	if (unlikely(InterruptPending)) \
+	if (INTERRUPTS_PENDING_CONDITION()) \
 		ProcessInterrupts(); \
 } while(0)
-#endif							/* WIN32 */
 
+/* Is ProcessInterrupts() guaranteed to clear InterruptPending? */
+#define INTERRUPTS_CAN_BE_PROCESSED() \
+	(InterruptHoldoffCount == 0 && CritSectionCount == 0 && \
+	 QueryCancelHoldoffCount == 0)
 
 #define HOLD_INTERRUPTS()  (InterruptHoldoffCount++)