[builtins] Add __emutls_unregister_key function

[kongyi]

Summary

This is called by Bionic on dlclose to delete the emutls pthread key.

The return value of pthread_key_delete is unchecked and behaviour of multiple calls to the method is dependent on the implementation of pthread_key_delete.

[chh]

lib/builtins/emutls.c

396

I think this function should be useful to other non-bionic platform too.
Did you get any link error with compiler-rt tests?

399

Shouldn't this be called only after the pthread key was created?
gcc's version has a flag emutls_key_created set only after emutls_init.

[kongyi]

lib/builtins/emutls.c

396

No link error.

Other platforms don't need this for dlclose; it is Bionic specific.

399

We don't need to check this, since emutls_key_delete is not expensive to call and we can safely ignore failure from it.

[srhines]

+enh and danalbert - in case they have any other concerns about this (Ryan's already on the review). From my perspective, everything here makes sense for resolving this issue.

[rprichard]

lib/builtins/emutls.c

399

This code assumes that a zeroed pthread_key_t is never valid. I suspect that's true for Bionic? I confirmed it on aosp-master and KitKat. For the NDK's purposes, it needs to be true for android-16 and up.

It's apparently not generally true? http://www.tekkotsu.org/cvslog2web/2009/03/commit-1236808823.html

[enh]

yeah, we always used the low numbers internally.

[chh]

lib/builtins/emutls.c

399

My concern is run time error or crash in calls to pthread_key_delete(0).
That might not happen with bionic, but i think the behavior is undefined in POSIX and could happen in other platforms.
I know some other platforms already depend on this emutls.c.
Maybe they have not seen the dlclose issue yet, but it would be nice to make this function work for general cases.

[kongyi]

lib/builtins/emutls.c

399

Changed to only call pthread_key_delete if key is created.

lib/builtins/emutls.c

 static void emutls_shutdown(emutls_address_array *array);
 
 #ifndef _WIN32
 
 #include <pthread.h>
 
 static pthread_mutex_t emutls_mutex = PTHREAD_MUTEX_INITIALIZER;
 static pthread_key_t emutls_pthread_key;
+static bool emutls_key_created = false;
 
 typedef unsigned int gcc_word __attribute__((mode(word)));
 typedef unsigned int gcc_pointer __attribute__((mode(pointer)));
 
 /* Default is not to use posix_memalign, so systems like Android
  * can use thread local data without heavier POSIX memory allocators.
  */
 #ifndef EMUTLS_USE_POSIX_MEMALIGN
if (array->skip_destructor_rounds > 0) {
         emutls_shutdown(array);
         free(ptr);
     }
 }
 
 static __inline void emutls_init(void) {
     if (pthread_key_create(&emutls_pthread_key, emutls_key_destructor) != 0)
         abort();
+    emutls_key_created = true;
 }
 
 static __inline void emutls_init_once(void) {
     static pthread_once_t once = PTHREAD_ONCE_INIT;
     pthread_once(&once, emutls_init);
 }
 
 static __inline void emutls_lock() {

 
 void* __emutls_get_address(__emutls_control* control) {
     uintptr_t index = emutls_get_index(control);
     emutls_address_array* array = emutls_get_address_array(index--);
     if (array->data[index] == NULL)
         array->data[index] = emutls_allocate_object(control);
     return array->data[index];
 }
+
+#ifdef __BIONIC__
+/* Called by Bionic on dlclose to delete the emutls pthread key. */
+__attribute__((visibility("hidden")))
+void __emutls_unregister_key(void) {
+    if (emutls_key_created) {
+        pthread_key_delete(emutls_pthread_key);
+        emutls_key_created = false;
+    }
+}
+#endif