Process 'die' interrupts while reading/writing from the client socket.

Up to now it was impossible to terminate a backend that was trying to
send/recv data to/from the client when the socket's buffer was already
full/empty. While the send/recv calls itself might have gotten
interrupted by signals on some platforms, we just immediately retried.

That could lead to situations where a backend couldn't be terminated ,
after a client died without the connection being closed, because it
was blocked in send/recv.

The problem was far more likely to be hit when sending data than when
reading. That's because while reading a command from the client, and
during authentication, we processed interrupts immediately . That
primarily left COPY FROM STDIN as being problematic for recv.

Change things so that that we process 'die' events immediately when
the appropriate signal arrives. We can't sensibly react to query
cancels at that point, because we might loose sync with the client as
we could be in the middle of writing a message.

We don't interrupt writes if the write buffer isn't full, as indicated
by write() returning EWOULDBLOCK, as that would lead to fewer error
messages reaching clients.

Per discussion with Kyotaro HORIGUCHI and Heikki Linnakangas

Discussion: 20140927191243.GD5423@alap3.anarazel.de

src/backend/libpq/be-secure-openssl.c

@@ -553,7 +553,7 @@ rloop:
 			if (latchret & WL_LATCH_SET)
 			{
 				ResetLatch(MyLatch);
-				ProcessClientReadInterrupt();  /* preserves errno */
+				ProcessClientReadInterrupt(true);  /* preserves errno */
 			}
 			goto rloop;
 		case SSL_ERROR_SYSCALL:
@@ -595,6 +595,7 @@ be_tls_write(Port *port, void *ptr, size_t len)
 	ssize_t		n;
 	int			err;
 	int			waitfor;
+	int			latchret;
 
 	/*
 	 * If SSL renegotiations are enabled and we're getting close to the
@@ -659,16 +660,27 @@ wloop:
 		case SSL_ERROR_WANT_READ:
 		case SSL_ERROR_WANT_WRITE:
 
+			waitfor = WL_LATCH_SET;
+
 			if (err == SSL_ERROR_WANT_READ)
-				waitfor = WL_SOCKET_READABLE;
+				waitfor |= WL_SOCKET_READABLE;
 			else
-				waitfor = WL_SOCKET_WRITEABLE;
+				waitfor |= WL_SOCKET_WRITEABLE;
+
+			latchret = WaitLatchOrSocket(MyLatch, waitfor, port->sock, 0);
 
-			WaitLatchOrSocket(MyLatch, waitfor, port->sock, 0);
 			/*
-			 * XXX: We'll, at some later point, likely want to add interrupt
-			 * processing here.
+			 * Check for interrupts here, in addition to secure_write(),
+			 * because an interrupted write in secure_raw_write() will return
+			 * here, and we cannot return to secure_write() until we've
+			 * written something.
 			 */
+			if (latchret & WL_LATCH_SET)
+			{
+				ResetLatch(MyLatch);
+				ProcessClientWriteInterrupt(true); /* preserves errno */
+			}
+
 			goto wloop;
 		case SSL_ERROR_SYSCALL:
 			/* leave it to caller to ereport the value of errno */

src/backend/libpq/be-secure.c

@@ -140,14 +140,27 @@ retry:
 		n = secure_raw_read(port, ptr, len);
 	}
 
-	/* Process interrupts that happened while (or before) receiving. */
-	ProcessClientReadInterrupt(); /* preserves errno */
-
 	/* retry after processing interrupts */
 	if (n < 0 && errno == EINTR)
 	{
+		/*
+		 * We tried to read data, the socket was empty, and we were
+		 * interrupted while waiting for readability. We only process
+		 * interrupts if we got interrupted while reading and when in blocking
+		 * mode. In other cases it's better to allow the interrupts to be
+		 * handled at higher layers.
+		 */
+		ProcessClientReadInterrupt(!port->noblock); /* preserves errno */
 		goto 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.
+	 */
+	ProcessClientReadInterrupt(false);
+
 	return n;
 }
 
@@ -224,18 +237,17 @@ retry:
 		n = secure_raw_write(port, ptr, len);
 	}
 
+	/* retry after processing interrupts */
+	if (n < 0 && errno == EINTR)
+	{
 		/*
-	 * XXX: We'll, at some later point, likely want to add interrupt
-	 * processing here.
+		 * We tried to send data, the socket was full, and we were interrupted
+		 * while waiting for writability. We only process interrupts if we got
+		 * interrupted while writing and when in blocking mode. In other cases
+		 * it's better to allow the interrupts to be handled at higher layers.
 		 */
+		ProcessClientWriteInterrupt(!port->noblock);
 
-	/*
-	 * Retry after processing interrupts. This can be triggered even though we
-	 * don't check for latch set's during writing yet, because SSL
-	 * renegotiations might have required reading from the socket.
-	 */
-	if (n < 0 && errno == EINTR)
-	{
 		goto retry;
 	}
 
@@ -262,17 +274,21 @@ wloop:
 		int		w;
 		int		save_errno = errno;
 
-		/*
-		 * We probably want to check for latches being set at some point
-		 * here. That'd allow us to handle interrupts while blocked on
-		 * writes. If set we'd not retry directly, but return. That way we
-		 * don't do anything while (possibly) inside a ssl library.
-		 */
 		w = WaitLatchOrSocket(MyLatch,
-							  WL_SOCKET_WRITEABLE,
+							  WL_LATCH_SET | WL_SOCKET_WRITEABLE,
 							  port->sock, 0);
 
-		if (w & WL_SOCKET_WRITEABLE)
+		if (w & WL_LATCH_SET)
+		{
+			ResetLatch(MyLatch);
+			/*
+			 * Force a return, so interrupts can be processed when not
+			 * (possibly) underneath a ssl library.
+			 */
+			errno = EINTR;
+			return -1;
+		}
+		else if (w & WL_SOCKET_WRITEABLE)
 		{
 			goto wloop;
 		}

src/backend/tcop/postgres.c

@@ -318,7 +318,7 @@ interactive_getc(void)
 
 	c = getc(stdin);
 
-	ProcessClientReadInterrupt();
+	ProcessClientReadInterrupt(true);
 
 	return c;
 }
@@ -529,7 +529,7 @@ ReadCommand(StringInfo inBuf)
  * Must preserve errno!
  */
 void
-ProcessClientReadInterrupt(void)
+ProcessClientReadInterrupt(bool blocked)
 {
 	int			save_errno = errno;
 
@@ -546,10 +546,56 @@ ProcessClientReadInterrupt(void)
 		if (notifyInterruptPending)
 			ProcessNotifyInterrupt();
 	}
+	else if (ProcDiePending && blocked)
+	{
+		/*
+		 * We're dying. It's safe (and sane) to handle that now.
+		 */
+		CHECK_FOR_INTERRUPTS();
+	}
 
 	errno = save_errno;
 }
 
+/*
+ * 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
+ *
+ * Must preserve errno!
+ */
+void
+ProcessClientWriteInterrupt(bool blocked)
+{
+	int			save_errno = errno;
+
+	Assert(InterruptHoldoffCount == 0 && CritSectionCount == 0);
+
+	/*
+	 * 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)
+	{
+		/*
+		 * 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.
+		 */
+		if (whereToSendOutput == DestRemote)
+			whereToSendOutput = DestNone;
+
+		CHECK_FOR_INTERRUPTS();
+	}
+
+	errno = save_errno;
+}
 
 /*
  * Do raw parsing (only).

src/include/tcop/tcopprot.h

@@ -67,7 +67,8 @@ extern void StatementCancelHandler(SIGNAL_ARGS);
 extern void FloatExceptionHandler(SIGNAL_ARGS) __attribute__((noreturn));
 extern void RecoveryConflictInterrupt(ProcSignalReason reason); /* called from SIGUSR1
 																 * handler */
-extern void ProcessClientReadInterrupt(void);
+extern void ProcessClientReadInterrupt(bool blocked);
+extern void ProcessClientWriteInterrupt(bool blocked);
 
 extern void process_postgres_switches(int argc, char *argv[],
 						  GucContext ctx, const char **dbname);