Unix.fork: reinitialize IO channel mutexes in the child instead of de…

…stroying them

Some of these mutexes may be locked, so both destroying and
reinitializing are undefined behaviors.  However, destroying can cause
fatal errors in the C pthread library (ocaml#12636), while reinitializing
should not.

Fixes: ocaml#12636

otherlibs/systhreads/st_posix.h

@@ -197,31 +197,41 @@ Caml_inline void st_thread_yield(st_masterlock * m)
 
 typedef pthread_mutex_t * st_mutex;
 
-static int st_mutex_create(st_mutex * res)
+static int st_mutex_init(st_mutex m)
 {
   int rc;
   pthread_mutexattr_t attr;
-  st_mutex m;
 
   rc = pthread_mutexattr_init(&attr);
   if (rc != 0) goto error1;
   rc = pthread_mutexattr_settype(&attr, PTHREAD_MUTEX_ERRORCHECK);
   if (rc != 0) goto error2;
-  m = caml_stat_alloc_noexc(sizeof(pthread_mutex_t));
-  if (m == NULL) { rc = ENOMEM; goto error2; }
   rc = pthread_mutex_init(m, &attr);
-  if (rc != 0) goto error3;
+  if (rc != 0) goto error2;
   pthread_mutexattr_destroy(&attr);
-  *res = m;
   return 0;
-error3:
-  caml_stat_free(m);
 error2:
   pthread_mutexattr_destroy(&attr);
 error1:
   return rc;
 }
 
+static int st_mutex_create(st_mutex * res)
+{
+  int rc;
+  st_mutex m;
+
+  m = caml_stat_alloc_noexc(sizeof(pthread_mutex_t));
+  if (m == NULL) { return ENOMEM; }
+  rc = st_mutex_init(m);
+  if (rc != 0) {
+    caml_stat_free(m);
+    return rc;
+  }
+  *res = m;
+  return 0;
+}
+
 static int st_mutex_destroy(st_mutex m)
 {
   int rc;

otherlibs/systhreads/st_stubs.c

@@ -417,14 +417,11 @@ static void caml_thread_reinitialize(void)
   /* Tick thread is not currently running in child process, will be
      re-created at next Thread.create */
   caml_tick_thread_running = 0;
-  /* Destroy all IO mutexes; will be reinitialized on demand */
+  /* Reinitialize all IO mutexes */
   for (chan = caml_all_opened_channels;
        chan != NULL;
        chan = chan->next) {
-    if (chan->mutex != NULL) {
-      st_mutex_destroy(chan->mutex);
-      chan->mutex = NULL;
-    }
+    if (chan->mutex != NULL) st_mutex_init(chan->mutex);
   }
 }
 

otherlibs/systhreads/st_win32.h

@@ -222,6 +222,12 @@ static DWORD st_mutex_destroy(st_mutex m)
   return 0;
 }
 
+/* Unused under Windows because there is no fork() */
+static DWORD st_mutex_init(st_mutex m)
+{
+  return 0;
+}
+
 /* Error codes with the 29th bit set are reserved for the application */
 
 #define MUTEX_DEADLOCK (1<<29 | 1)