Server-side fix for delayed NOTIFY and SIGTERM processing.

Commit 4f85fde8 introduced some code that was meant to ensure that we'd
process cancel, die, sinval catchup, and notify interrupts while waiting
for client input.  But there was a flaw: it supposed that the process
latch would be set upon arrival at secure_read() if any such interrupt
was pending.  In reality, we might well have cleared the process latch
at some earlier point while those flags remained set -- particularly
notifyInterruptPending, which can't be handled as long as we're within
a transaction.

To fix the NOTIFY case, also attempt to process signals (except
ProcDiePending) before trying to read.

Also, if we see that ProcDiePending is set before we read, forcibly set the
process latch to ensure that we will handle that signal promptly if no data
is available.  I also made it set the process latch on the way out, in case
there is similar logic elsewhere.  (It remains true that we won't service
ProcDiePending here unless we need to wait for input.)

The code for handling ProcDiePending during a write needs those changes,
too.

Also be a little more careful about when to reset whereToSendOutput,
and improve related comments.

Back-patch to 9.5 where this code was added.  I'm not entirely convinced
that older branches don't have similar issues, but the complaint at hand
is just about the >= 9.5 code.

Jeff Janes and Tom Lane

Discussion: https://postgr.es/m/CAOYf6ec-TmRYjKBXLLaGaB-jrd=mjG1Hzn1a1wufUAR39PQYhw@mail.gmail.com

src/backend/libpq/be-secure.c

@@ -145,6 +145,9 @@ secure_read(Port *port, void *ptr, size_t len)
 	ssize_t		n;
 	int			waitfor;
 
+	/* Deal with any already-pending interrupt condition. */
+	ProcessClientReadInterrupt(false);
+
 retry:
 #ifdef USE_SSL
 	waitfor = 0;
@@ -209,9 +212,8 @@ retry:
 	}
 
 	/*
-	 * Process interrupts that happened while (or before) receiving. Note that
-	 * we signal that we're not blocking, which will prevent some types of
-	 * interrupts from being processed.
+	 * Process interrupts that happened during a successful (or non-blocking,
+	 * or hard-failed) read.
 	 */
 	ProcessClientReadInterrupt(false);
 
@@ -248,6 +250,9 @@ secure_write(Port *port, void *ptr, size_t len)
 	ssize_t		n;
 	int			waitfor;
 
+	/* Deal with any already-pending interrupt condition. */
+	ProcessClientWriteInterrupt(false);
+
 retry:
 	waitfor = 0;
 #ifdef USE_SSL
@@ -287,17 +292,16 @@ retry:
 
 			/*
 			 * We'll retry the write. Most likely it will return immediately
-			 * because there's still no data available, and we'll wait for the
-			 * socket to become ready again.
+			 * because there's still no buffer space available, and we'll wait
+			 * for the socket to become ready again.
 			 */
 		}
 		goto retry;
 	}
 
 	/*
-	 * Process interrupts that happened while (or before) sending. Note that
-	 * we signal that we're not blocking, which will prevent some types of
-	 * interrupts from being processed.
+	 * Process interrupts that happened during a successful (or non-blocking,
+	 * or hard-failed) write.
 	 */
 	ProcessClientWriteInterrupt(false);
 

src/backend/tcop/postgres.c

@@ -315,7 +315,7 @@ interactive_getc(void)
 
 	c = getc(stdin);
 
-	ProcessClientReadInterrupt(true);
+	ProcessClientReadInterrupt(false);
 
 	return c;
 }
@@ -520,8 +520,9 @@ ReadCommand(StringInfo inBuf)
 /*
  * ProcessClientReadInterrupt() - Process interrupts specific to client reads
  *
- * This is called just after low-level reads. That might be after the read
- * finished successfully, or it was interrupted via interrupt.
+ * This is called just before and after low-level reads.
+ * 'blocked' is true if no data was available to read and we plan to retry,
+ * false if about to read or done reading.
  *
  * Must preserve errno!
  */
@@ -532,23 +533,31 @@ ProcessClientReadInterrupt(bool blocked)
 
 	if (DoingCommandRead)
 	{
-		/* Check for general interrupts that arrived while reading */
+		/* Check for general interrupts that arrived before/while reading */
 		CHECK_FOR_INTERRUPTS();
 
-		/* Process sinval catchup interrupts that happened while reading */
+		/* Process sinval catchup interrupts, if any */
 		if (catchupInterruptPending)
 			ProcessCatchupInterrupt();
 
-		/* Process sinval catchup interrupts that happened while reading */
+		/* Process notify interrupts, if any */
 		if (notifyInterruptPending)
 			ProcessNotifyInterrupt();
 	}
-	else if (ProcDiePending && blocked)
+	else if (ProcDiePending)
 	{
 		/*
-		 * We're dying. It's safe (and sane) to handle that now.
+		 * We're dying.  If there is no data available to read, then it's safe
+		 * (and sane) to handle that now.  If we haven't tried to read yet,
+		 * make sure the process latch is set, so that if there is no data
+		 * then we'll come back here and die.  If we're done reading, also
+		 * make sure the process latch is set, as we might've undesirably
+		 * cleared it while reading.
 		 */
-		CHECK_FOR_INTERRUPTS();
+		if (blocked)
+			CHECK_FOR_INTERRUPTS();
+		else
+			SetLatch(MyLatch);
 	}
 
 	errno = save_errno;
@@ -557,9 +566,9 @@ ProcessClientReadInterrupt(bool blocked)
 /*
  * ProcessClientWriteInterrupt() - Process interrupts specific to client writes
  *
- * This is called just after low-level writes. That might be after the read
- * finished successfully, or it was interrupted via interrupt. 'blocked' tells
- * us whether the
+ * This is called just before and after low-level writes.
+ * 'blocked' is true if no data could be written and we plan to retry,
+ * false if about to write or done writing.
  *
  * Must preserve errno!
  */
@@ -568,25 +577,39 @@ ProcessClientWriteInterrupt(bool blocked)
 {
 	int			save_errno = errno;
 
-	/*
-	 * We only want to process the interrupt here if socket writes are
-	 * blocking to increase the chance to get an error message to the client.
-	 * If we're not blocked there'll soon be a CHECK_FOR_INTERRUPTS(). But if
-	 * we're blocked we'll never get out of that situation if the client has
-	 * died.
-	 */
-	if (ProcDiePending && blocked)
+	if (ProcDiePending)
 	{
 		/*
-		 * We're dying. It's safe (and sane) to handle that now. But we don't
-		 * want to send the client the error message as that a) would possibly
-		 * block again b) would possibly lead to sending an error message to
-		 * the client, while we already started to send something else.
+		 * We're dying.  If it's not possible to write, then we should handle
+		 * that immediately, else a stuck client could indefinitely delay our
+		 * response to the signal.  If we haven't tried to write yet, make
+		 * sure the process latch is set, so that if the write would block
+		 * then we'll come back here and die.  If we're done writing, also
+		 * make sure the process latch is set, as we might've undesirably
+		 * cleared it while writing.
 		 */
-		if (whereToSendOutput == DestRemote)
-			whereToSendOutput = DestNone;
+		if (blocked)
+		{
+			/*
+			 * Don't mess with whereToSendOutput if ProcessInterrupts wouldn't
+			 * do anything.
+			 */
+			if (InterruptHoldoffCount == 0 && CritSectionCount == 0)
+			{
+				/*
+				 * We don't want to send the client the error message, as a)
+				 * that would possibly block again, and b) it would likely
+				 * lead to loss of protocol sync because we may have already
+				 * sent a partial protocol message.
+				 */
+				if (whereToSendOutput == DestRemote)
+					whereToSendOutput = DestNone;
 
-		CHECK_FOR_INTERRUPTS();
+				CHECK_FOR_INTERRUPTS();
+			}
+		}
+		else
+			SetLatch(MyLatch);
 	}
 
 	errno = save_errno;