warn on multithreaded fork

* libguile/posix.c (scm_fork): Issue a warning on a multithreaded fork.
* doc/ref/posix.texi (Processes): Add note about multithreaded fork.

* test-suite/tests/00-socket.test: Moved here, from socket.test, so as
  to run before any threads are created.
* test-suite/Makefile.am: Adapt.

diff --git a/doc/ref/posix.texi b/doc/ref/posix.texi
index 0a688e9..18aadca 100644 (file)
--- a/doc/ref/posix.texi
+++ b/doc/ref/posix.texi
@@ -1,7 +1,7 @@
 @c -*-texinfo-*-
 @c This is part of the GNU Guile Reference Manual.
 @c Copyright (C)  1996, 1997, 2000, 2001, 2002, 2003, 2004, 2006, 2007,
-@c   2008, 2009, 2010, 2011, 2012 Free Software Foundation, Inc.
+@c   2008, 2009, 2010, 2011, 2012, 2013 Free Software Foundation, Inc.
 @c See the file guile.texi for copying conditions.
 
 @node POSIX
@@ -1785,6 +1785,19 @@ Creates a new ``child'' process by duplicating the current ``parent'' process.
 In the child the return value is 0.  In the parent the return value is
 the integer process ID of the child.
 
+Note that it is unsafe to fork a process that has multiple threads
+running, as only the thread that calls @code{primitive-fork} will
+persist in the child.  Any resources that other threads held, such as
+locked mutexes or open file descriptors, are lost.  Indeed, @acronym{POSIX}
+specifies that only async-signal-safe procedures are safe to call after
+a multithreaded fork, which is a very limited set.  Guile issues a
+warning if it detects a fork from a multi-threaded program.
+
+If you are going to @code{exec} soon after forking, the procedures in
+@code{(ice-9 popen)} may be useful to you, as they fork and exec within
+an async-signal-safe function carefully written to ensure robust program
+behavior, even in the presence of threads.  @xref{Pipes}, for more.
+
 This procedure has been renamed from @code{fork} to avoid a naming conflict
 with the scsh fork.
 @end deffn

diff --git a/libguile/posix.c b/libguile/posix.c
index 4d5fdcf..324f21b 100644 (file)
--- a/libguile/posix.c
+++ b/libguile/posix.c
@@ -1249,6 +1249,22 @@ SCM_DEFINE (scm_fork, "primitive-fork", 0, 0, 0,
 #define FUNC_NAME s_scm_fork
 {
   int pid;
+  if (scm_ilength (scm_all_threads ()) != 1)
+    /* Other threads may be holding on to resources that Guile needs --
+       it is not safe to permit one thread to fork while others are
+       running.
+
+       In addition, POSIX clearly specifies that if a multi-threaded
+       program forks, the child must only call functions that are
+       async-signal-safe.  We can't guarantee that in general.  The best
+       we can do is to allow forking only very early, before any call to
+       sigaction spawns the signal-handling thread.  */
+    scm_display
+      (scm_from_latin1_string
+       ("warning: call to primitive-fork while multiple threads are running;\n"
+        "         further behavior unspecified.  See \"Processes\" in the\n"
+        "         manual, for more information.\n"),
+       scm_current_warning_port ());
   pid = fork ();
   if (pid == -1)
     SCM_SYSERROR;

diff --git a/test-suite/Makefile.am b/test-suite/Makefile.am
index 880e1e2..9fba7b8 100644 (file)
--- a/test-suite/Makefile.am
+++ b/test-suite/Makefile.am
@@ -23,6 +23,7 @@
 SUBDIRS = standalone vm
 
 SCM_TESTS = tests/00-initial-env.test          \
+           tests/00-socket.test                \
             tests/alist.test                   \
            tests/and-let-star.test             \
            tests/arbiters.test                 \
@@ -114,7 +115,6 @@ SCM_TESTS = tests/00-initial-env.test               \
            tests/regexp.test                   \
            tests/session.test                  \
            tests/signals.test                  \
-           tests/socket.test                   \
            tests/srcprop.test                  \
            tests/srfi-1.test                   \
            tests/srfi-6.test                   \

diff --git a/test-suite/tests/socket.test b/test-suite/tests/00-socket.test
similarity index 100%
rename from test-suite/tests/socket.test
rename to test-suite/tests/00-socket.test