Use SIGURG rather than SIGUSR1 for latches.

Traditionally, SIGUSR1 has been overloaded for ad-hoc signals,
procsignal.c signals and latch.c wakeups.  Move that last use over to a
new dedicated signal.  SIGURG is normally used to report out-of-band
socket data, but PostgreSQL doesn't use that facility.

The signal handler is now installed in all postmaster children by
InitializeLatchSupport().  Those wishing to disconnect from it should
call ShutdownLatchSupport().

Future patches will use this separation of signals to avoid the need for
a signal handler on some operating systems.

Discussion: https://postgr.es/m/CA+hUKGJjxPDpzBE0a3hyUywBvaZuC89yx3jK9RFZgfv_KHU7gg@mail.gmail.com

1 files changed, 33 insertions, 17 deletions

diff --git a/src/backend/storage/ipc/latch.c b/src/backend/storage/ipc/latch.c
index eacf8d51ea..0f274280e6 100644
--- a/src/backend/storage/ipc/latch.c
+++ b/src/backend/storage/ipc/latch.c
@@ -16,7 +16,7 @@
  *
  * When SetLatch is called from the same process that owns the latch,
  * SetLatch writes the byte directly to the pipe. If it's owned by another
- * process, SIGUSR1 is sent and the signal handler in the waiting process
+ * process, SIGURG is sent and the signal handler in the waiting process
  * writes the byte to the pipe on behalf of the signaling process.
  *
  * The Windows implementation uses Windows events that are inherited by all
@@ -148,6 +148,7 @@ static int	selfpipe_writefd = -1;
 static int	selfpipe_owner_pid = 0;
 
 /* Private function prototypes */
+static void latch_sigurg_handler(SIGNAL_ARGS);
 static void sendSelfPipeByte(void);
 static void drainSelfPipe(void);
 #endif							/* WIN32 */
@@ -244,6 +245,8 @@ InitializeLatchSupport(void)
 	/* Tell fd.c about these two long-lived FDs */
 	ReserveExternalFD();
 	ReserveExternalFD();
+
+	pqsignal(SIGURG, latch_sigurg_handler);
 #else
 	/* currently, nothing to do here for Windows */
 #endif
@@ -267,6 +270,24 @@ InitializeLatchWaitSet(void)
 	Assert(latch_pos == LatchWaitSetLatchPos);
 }
 
+void
+ShutdownLatchSupport(void)
+{
+	pqsignal(SIGURG, SIG_IGN);
+
+	if (LatchWaitSet)
+	{
+		FreeWaitEventSet(LatchWaitSet);
+		LatchWaitSet = NULL;
+	}
+
+	close(selfpipe_readfd);
+	close(selfpipe_writefd);
+	selfpipe_readfd = -1;
+	selfpipe_writefd = -1;
+	selfpipe_owner_pid = InvalidPid;
+}
+
 /*
  * Initialize a process-local latch.
  */
@@ -335,10 +356,6 @@ InitSharedLatch(Latch *latch)
  * any sort of locking here, meaning that we could fail to detect the error
  * if two processes try to own the same latch at about the same time.  If
  * there is any risk of that, caller must provide an interlock to prevent it.
- *
- * In any process that calls OwnLatch(), make sure that
- * latch_sigusr1_handler() is called from the SIGUSR1 signal handler,
- * as shared latches use SIGUSR1 for inter-process communication.
  */
 void
 OwnLatch(Latch *latch)
@@ -562,7 +579,7 @@ SetLatch(Latch *latch)
 			sendSelfPipeByte();
 	}
 	else
-		kill(owner_pid, SIGUSR1);
+		kill(owner_pid, SIGURG);
 #else
 
 	/*
@@ -1266,7 +1283,7 @@ WaitEventSetWait(WaitEventSet *set, long timeout,
 		 * the pipe-buffer fill up we're still ok, because the pipe is in
 		 * nonblocking mode. It's unlikely for that to happen, because the
 		 * self pipe isn't filled unless we're blocking (waiting = true), or
-		 * from inside a signal handler in latch_sigusr1_handler().
+		 * from inside a signal handler in latch_sigurg_handler().
 		 *
 		 * On windows, we'll also notice if there's a pending event for the
 		 * latch when blocking, but there's no danger of anything filling up,
@@ -1934,22 +1951,21 @@ WaitEventSetWaitBlock(WaitEventSet *set, int cur_timeout,
 }
 #endif
 
+#ifndef WIN32
 /*
- * SetLatch uses SIGUSR1 to wake up the process waiting on the latch.
- *
- * Wake up WaitLatch, if we're waiting.  (We might not be, since SIGUSR1 is
- * overloaded for multiple purposes; or we might not have reached WaitLatch
- * yet, in which case we don't need to fill the pipe either.)
+ * SetLatch uses SIGURG to wake up the process waiting on the latch.
  *
- * NB: when calling this in a signal handler, be sure to save and restore
- * errno around it.
+ * Wake up WaitLatch, if we're waiting.
  */
-#ifndef WIN32
-void
-latch_sigusr1_handler(void)
+static void
+latch_sigurg_handler(SIGNAL_ARGS)
 {
+	int			save_errno = errno;
+
 	if (waiting)
 		sendSelfPipeByte();
+
+	errno = save_errno;
 }
 #endif							/* !WIN32 */