Reinitialize IO mutexes in systhreads5 (#2160)

ocaml/otherlibs/systhreads/st_stubs.c

@@ -463,6 +463,7 @@ static void caml_thread_remove_and_free(caml_thread_t th)
 
 static void caml_thread_reinitialize(void)
 {
+  struct channel * chan;
   caml_thread_t th, next;
 
   th = Active_thread->next;
@@ -484,6 +485,16 @@ static void caml_thread_reinitialize(void)
      s->lock (busy = 1) */
   struct caml_locking_scheme *s = atomic_load(&Locking_scheme(Caml_state->id));
   s->reinitialize_after_fork(s->context);
+
+  /* Reinitialize IO mutexes, in case the fork happened while another thread
+     had locked the channel. If so, we're likely in an inconsistent state,
+     but we may be able to proceed anyway. */
+  caml_plat_mutex_init(&caml_all_opened_channels_mutex);
+  for (chan = caml_all_opened_channels;
+       chan != NULL;
+       chan = chan->next) {
+    caml_plat_mutex_init(&chan->mutex);
+  }
 }
 
 CAMLprim value caml_thread_join(value th);

ocaml/runtime/caml/io.h

@@ -105,6 +105,7 @@ CAMLextern void (*caml_channel_mutex_unlock) (struct channel *);
 CAMLextern void (*caml_channel_mutex_unlock_exn) (void);
 
 CAMLextern struct channel * caml_all_opened_channels;
+CAMLextern caml_plat_mutex caml_all_opened_channels_mutex;
 
 #define Lock(channel) \
   if (caml_channel_mutex_lock != NULL) (*caml_channel_mutex_lock)(channel)