Amend r19196 with a few extra bells and whistles:

* use "warn_on_fork" instead of "do_not_warn_on_fork" -- i.e.,
   use positive logic instead of negative logic
 * ensure that pthread_atfork() is only called once
 * amended the error message to include the hostname, PID, and
   MPI_COMM_WORLD rank of the offender
 * ensure that the warn_fork_cb() function is only defined if
   HAVE_PTHREAD_H so that we don't get a compiler warning if it isn't
   used

This commit was SVN r19204.

The following SVN revision numbers were found above:
  r19196 --> open-mpi/ompi@277e4ac292

ompi/runtime/help-mpi-runtime.txt

@@ -77,8 +77,18 @@ In order to operate in a heterogeneous environment, please reconfigure
 Open MPI with --enable-heterogeneous.
 #
 [mpi_init:warn-fork]
-The program has executed an operation involving a call to "fork". Open MPI
-is currently operating in a condition that could result in memory corruption
-issues in this situation - for example, use of the OpenIB BTL. Use of fork
-is therefore not recommended in this situation.
+An MPI process has executed an operation involving a call to the
+"fork()" system call to create a child process.  Open MPI is currently
+operating in a condition that could result in memory corruption or
+other system errors; your MPI job may hang, crash, or produce silent
+data corruption.  The use of fork() (or system() or other calls that
+create child processes) is strongly discouraged.  
 
+The process that invoked fork was:
+
+  Local host:          %s (PID %d)
+  MPI_COMM_WORLD rank: %d
+
+If you are *absolutely sure* that your application will successfully
+and correctly survive a call to fork(), you may disable this warning
+by setting the mpi_warn_on_fork MCA parameter to 0.

ompi/runtime/mpiruntime.h

@@ -61,7 +61,7 @@ OMPI_DECLSPEC extern struct opal_thread_t *ompi_mpi_main_thread;
 OMPI_DECLSPEC extern bool ompi_mpi_maffinity_setup;
 
 /** Do we want to be warned on fork or not? */
-OMPI_DECLSPEC extern bool ompi_do_not_warn_on_fork;
+OMPI_DECLSPEC extern bool ompi_warn_on_fork;
 
 /** In ompi_mpi_init: a list of all memory associated with calling
     MPI_REGISTER_DATAREP so that we can free it during

ompi/runtime/ompi_mpi_init.c

@@ -118,23 +118,29 @@ opal_thread_t *ompi_mpi_main_thread = NULL;
 
 bool ompi_mpi_maffinity_setup = false;
 
-bool ompi_do_not_warn_on_fork;
+bool ompi_warn_on_fork;
 
 static bool fork_warning_issued = false;
+static bool atfork_called = false;
 
-static void warn_fork(void)
+#if HAVE_PTHREAD_H
+static void warn_fork_cb(void)
 {
     if (ompi_mpi_initialized && !ompi_mpi_finalized && !fork_warning_issued) {
-        orte_show_help("help-mpi-runtime.txt", "mpi_init:warn-fork", true);
+        orte_show_help("help-mpi-runtime.txt", "mpi_init:warn-fork", true,
+                       orte_process_info.nodename, getpid(),
+                       ompi_mpi_comm_world.c_my_rank);
         fork_warning_issued = true;
     }
 }
+#endif
 
 void ompi_warn_fork(void)
 {
 #if HAVE_PTHREAD_H
-    if (!ompi_do_not_warn_on_fork) {
-        pthread_atfork(warn_fork, NULL, NULL);
+    if (ompi_warn_on_fork && !atfork_called) {
+        pthread_atfork(warn_fork_cb, NULL, NULL);
+        atfork_called = true;
     }
 #endif
 }

ompi/runtime/ompi_mpi_params.c

@@ -275,11 +275,11 @@ int ompi_mpi_register_params(void)
                                 (int) ompi_mpi_paffinity_alone, &value);
     ompi_mpi_paffinity_alone = OPAL_INT_TO_BOOL(value);
 
-    mca_base_param_reg_int_name("mpi", "do_not_warn_on_fork",
-                                "If nonzero, do not warn if program forks under conditions that could cause memory corruption issues",
+    mca_base_param_reg_int_name("mpi", "warn_on_fork",
+                                "If nonzero, issue a warning if program forks under conditions that could cause system errors",
                                 false, false, 
-                                (int) false, &value);
-    ompi_do_not_warn_on_fork = OPAL_INT_TO_BOOL(value);
+                                (int) true, &value);
+    ompi_warn_on_fork = OPAL_INT_TO_BOOL(value);
     
     /* Sparse group storage support */