SIGPIPE handling, take two.

pqsecure_write tries to catch SIGPIPE signals generated by network
disconnects by setting the signal handler to SIG_IGN. The current
approach causes several problems:
- it always sets SA_RESTART when it restores the old handler.
- it's not reliable for multi threaded apps, because another thread
could change the signal handler inbetween.
- it's slow, because after setting a signal handler to SIG_IGN the
kernel must enumerate all threads and clear all pending signals (at
least FreeBSD-5.1 and linux-2.6 do that. Earlier linux kernels don't -
their signal handling is known to be broken for multithreaded apps).

Initially I proposed a new option for PQconnectdb, but Tom didn't like
that. The attached patch autodetects if it should set the signal
handler, Tom proposed that. The code doesn't try to check if the signal
is "handled" by blocking it, because I haven't figured out how to check
that: sigprocmask is undefined for multithreaded apps and calling
pthread_sigmask would force every libpq user to link against libpthread.

--
    Manfred
? src/interfaces/libpq/libpq.so.3.1
Index: src/interfaces/libpq/fe-connect.c
===================================================================
RCS file: /projects/cvsroot/pgsql-server/src/interfaces/libpq/fe-connect.c,v
retrieving revision 1.263
diff -c -r1.263 fe-connect.c
*** src/interfaces/libpq/fe-connect.c    18 Oct 2003 05:02:06 -0000    1.263
--- src/interfaces/libpq/fe-connect.c    2 Nov 2003 18:29:40 -0000
***************
*** 41,46 ****
--- 41,47 ----
  #include <netinet/tcp.h>
  #endif
  #include <arpa/inet.h>
+ #include <signal.h>
  #endif

  #include "libpq/ip.h"
***************
*** 951,956 ****
--- 952,983 ----
      else if (conn->sslmode[0] == 'a')    /* "allow" */
          conn->wait_ssl_try = true;
  #endif
+     /*
+      * Autodetect SIGPIPE signal handling:
+      * The default action per Unix spec is kill current process and
+      * that's not acceptable. If the current setting is not the default,
+      * then assume that the caller knows what he's doing and leave the
+      * signal handler unchanged. Otherwise set the signal handler to
+      * SIG_IGN around each send() syscall. Unfortunately this is both
+      * unreliable and slow for multithreaded apps.
+      */
+     conn->do_sigaction = true;
+ #if !defined(HAVE_POSIX_SIGNALS)
+     {
+         pqsigfunc old;
+          old = signal(SIGPIPE, SIG_IGN);
+         if (old != SIG_DFL)
+             conn->do_sigaction = false;
+         signal(SIGPIPE, old);
+     }
+ #else
+     {
+         struct sigaction oact;
+
+         if (sigaction(SIGPIPE, NULL, &oact) == 0 && oact.sa_handler != SIG_DFL)
+             conn->do_sigaction = false;
+     }
+ #endif   /* !HAVE_POSIX_SIGNALS */

      /*
       * Set up to try to connect, with protocol 3.0 as the first attempt.
Index: src/interfaces/libpq/fe-secure.c
===================================================================
RCS file: /projects/cvsroot/pgsql-server/src/interfaces/libpq/fe-secure.c,v
retrieving revision 1.32
diff -c -r1.32 fe-secure.c
*** src/interfaces/libpq/fe-secure.c    29 Sep 2003 16:38:04 -0000    1.32
--- src/interfaces/libpq/fe-secure.c    2 Nov 2003 18:29:41 -0000
***************
*** 348,354 ****
      ssize_t        n;

  #ifndef WIN32
!     pqsigfunc    oldsighandler = pqsignal(SIGPIPE, SIG_IGN);
  #endif

  #ifdef USE_SSL
--- 348,357 ----
      ssize_t        n;

  #ifndef WIN32
!     pqsigfunc    oldsighandler = NULL;
!
!     if (conn->do_sigaction)
!         oldsighandler = pqsignal(SIGPIPE, SIG_IGN);
  #endif

  #ifdef USE_SSL
***************
*** 408,414 ****
          n = send(conn->sock, ptr, len, 0);

  #ifndef WIN32
!     pqsignal(SIGPIPE, oldsighandler);
  #endif

      return n;
--- 411,418 ----
          n = send(conn->sock, ptr, len, 0);

  #ifndef WIN32
!     if (conn->do_sigaction)
!         pqsignal(SIGPIPE, oldsighandler);
  #endif

      return n;
Index: src/interfaces/libpq/libpq-int.h
===================================================================
RCS file: /projects/cvsroot/pgsql-server/src/interfaces/libpq/libpq-int.h,v
retrieving revision 1.82
diff -c -r1.82 libpq-int.h
*** src/interfaces/libpq/libpq-int.h    5 Sep 2003 02:08:36 -0000    1.82
--- src/interfaces/libpq/libpq-int.h    2 Nov 2003 18:29:42 -0000
***************
*** 329,334 ****
--- 329,335 ----
      char        peer_dn[256 + 1];        /* peer distinguished name */
      char        peer_cn[SM_USER + 1];    /* peer common name */
  #endif
+     bool        do_sigaction;    /* set SIGPIPE to SIG_IGN around every send() call */

      /* Buffer for current error message */
      PQExpBufferData errorMessage;        /* expansible string */