Fix undefined behaviour in pulseaudio --start.

Don't call pthread_join() to join a thread from a different
process than the thread was created in. Doing so can lead to
undefined behaviour.

On OpenBSD, the symptom was a pulseaudio process with a single
thread waiting forever for other threads to join. Since that
process also held the autospawn lock, starting new pulseaudio
processes with --start kept failing. The problem was analyzed
with help from Philip Guenther.

This patch adds a pa_thread_free_nojoin() function which can
be used to free resources for a thread without a join, as
suggested by Tanu Kaskinen.

See https://bugs.freedesktop.org/show_bug.cgi?id=71738

src/pulsecore/lock-autospawn.c

@@ -114,8 +114,15 @@ static void unref(bool after_fork) {
     if (n_ref > 0)
         return;
 
+    /* Join threads only in the process the new thread was created in
+     * to avoid undefined behaviour.
+     * POSIX.1-2008 XSH 2.9.2 Thread IDs: "applications should only assume
+     * that thread IDs are usable and unique within a single process." */
     if (thread) {
-        pa_thread_free(thread);
+        if (after_fork)
+            pa_thread_free_nojoin(thread);
+	else
+            pa_thread_free(thread);
         thread = NULL;
     }
 

src/pulsecore/thread-posix.c

@@ -127,6 +127,13 @@ void pa_thread_free(pa_thread *t) {
     pa_xfree(t);
 }
 
+void pa_thread_free_nojoin(pa_thread *t) {
+    pa_assert(t);
+
+    pa_xfree(t->name);
+    pa_xfree(t);
+}
+
 int pa_thread_join(pa_thread *t) {
     pa_assert(t);
     pa_assert(t->thread_func);

src/pulsecore/thread.h

@@ -39,6 +39,7 @@ typedef void (*pa_thread_func_t) (void *userdata);
 
 pa_thread* pa_thread_new(const char *name, pa_thread_func_t thread_func, void *userdata);
 void pa_thread_free(pa_thread *t);
+void pa_thread_free_nojoin(pa_thread *t);
 int pa_thread_join(pa_thread *t);
 int pa_thread_is_running(pa_thread *t);
 pa_thread *pa_thread_self(void);