malloc: Run fork handler as late as possible [BZ #19431]

Previously, a thread M invoking fork would acquire locks in this order:

  (M1) malloc arena locks (in the registered fork handler)
  (M2) libio list lock

A thread F invoking flush (NULL) would acquire locks in this order:

  (F1) libio list lock
  (F2) individual _IO_FILE locks

A thread G running getdelim would use this order:

  (G1) _IO_FILE lock
  (G2) malloc arena lock

After executing (M1), (F1), (G1), none of the threads can make progress.

This commit changes the fork lock order to:

  (M'1) libio list lock
  (M'2) malloc arena locks

It explicitly encodes the lock order in the implementations of fork,
and does not rely on the registration order, thus avoiding the deadlock.

sysdeps/mach/hurd/fork.c

@@ -26,6 +26,7 @@
 #include <assert.h>
 #include "hurdmalloc.h"		/* XXX */
 #include <tls.h>
+#include <malloc/malloc-internal.h>
 
 #undef __fork
 
@@ -107,6 +108,12 @@ __fork (void)
       /* Run things that prepare for forking before we create the task.  */
       RUN_HOOK (_hurd_fork_prepare_hook, ());
 
+      /* Acquire malloc locks.  This needs to come last because fork
+	 handlers may use malloc, and the libio list lock has an
+	 indirect malloc dependency as well (via the getdelim
+	 function).  */
+      __malloc_fork_lock_parent ();
+
       /* Lock things that want to be locked before we fork.  */
       {
 	void *const *p;
@@ -604,6 +611,9 @@ __fork (void)
 			   nthreads * sizeof (*threads));
 	}
 
+      /* Release malloc locks.  */
+      __malloc_fork_unlock_parent ();
+
       /* Run things that want to run in the parent to restore it to
 	 normality.  Usually prepare hooks and parent hooks are
 	 symmetrical: the prepare hook arrests state in some way for the
@@ -655,6 +665,9 @@ __fork (void)
       /* Forking clears the trace flag.  */
       __sigemptyset (&_hurdsig_traced);
 
+      /* Release malloc locks.  */
+      __malloc_fork_unlock_child ();
+
       /* Run things that want to run in the child task to set up.  */
       RUN_HOOK (_hurd_fork_child_hook, ());