Use WaitEventSet API for postmaster's event loop.

Switch to a design similar to regular backends, instead of the previous
arrangement where signal handlers did non-trivial state management and
called fork().  The main changes are:

* The postmaster now has its own local latch to wait on.  (For now, we
  don't want other backends setting its latch directly, but that could
  probably be made to work with more research on robustness.)

* The existing signal handlers are cut in two: a handle_pm_XXX() part
  that just sets pending_pm_XXX flags and the latch, and a
  process_pm_XXX() part that runs later when the latch is seen.

* Signal handlers are now installed with the regular pqsignal()
  function rather than the special pqsignal_pm() function; historical
  portability concerns about the effect of SA_RESTART on select() are no
  longer relevant, and we don't need to block signals anymore.

Reviewed-by: Andres Freund <andres@anarazel.de>
Discussion: https://postgr.es/m/CA%2BhUKG%2BZ-HpOj1JsO9eWUP%2Bar7npSVinsC_npxSy%2BjdOMsx%3DGg%40mail.gmail.com

1 files changed, 17 insertions, 1 deletions

diff --git a/src/backend/postmaster/fork_process.c b/src/backend/postmaster/fork_process.c
index 569b52e8495..509587636e2 100644
--- a/src/backend/postmaster/fork_process.c
+++ b/src/backend/postmaster/fork_process.c
@@ -12,24 +12,28 @@
 #include "postgres.h"
 
 #include <fcntl.h>
+#include <signal.h>
 #include <time.h>
 #include <sys/stat.h>
 #include <sys/time.h>
 #include <unistd.h>
 
+#include "libpq/pqsignal.h"
 #include "postmaster/fork_process.h"
 
 #ifndef WIN32
 /*
  * Wrapper for fork(). Return values are the same as those for fork():
  * -1 if the fork failed, 0 in the child process, and the PID of the
- * child in the parent process.
+ * child in the parent process.  Signals are blocked while forking, so
+ * the child must unblock.
  */
 pid_t
 fork_process(void)
 {
 	pid_t		result;
 	const char *oomfilename;
+	sigset_t	save_mask;
 
 #ifdef LINUX_PROFILE
 	struct itimerval prof_itimer;
@@ -51,6 +55,13 @@ fork_process(void)
 	getitimer(ITIMER_PROF, &prof_itimer);
 #endif
 
+	/*
+	 * We start postmaster children with signals blocked.  This allows them to
+	 * install their own handlers before unblocking, to avoid races where they
+	 * might run the postmaster's handler and miss an important control signal.
+	 * With more analysis this could potentially be relaxed.
+	 */
+	sigprocmask(SIG_SETMASK, &BlockSig, &save_mask);
 	result = fork();
 	if (result == 0)
 	{
@@ -103,6 +114,11 @@ fork_process(void)
 		/* do post-fork initialization for random number generation */
 		pg_strong_random_init();
 	}
+	else
+	{
+		/* in parent, restore signal mask */
+		sigprocmask(SIG_SETMASK, &save_mask, NULL);
+	}
 
 	return result;
 }