Only call thread-local destructors on DLL_THREAD_DETACH.

In VS2015's debug runtime, the C runtime has been unloaded by the time
DLL_PROCESS_DETACH is called and things crash. Instead, don't run destructors
at that point.

This means we do *not* free memory associated with any remaining thread-locals
on application shutdown, only shutdown of individual threads. This is actually
desirable since it's consistent with pthreads. If an individual thread calls
pthread_exit, destructors are run. If the entire process exits, they are not.

(It's also consistent with thread_none.c which never bothers to free
anything.)

BUG=chromium:595795

Change-Id: I3e64d46ea03158fefff583c1e3e12dfa0c0e172d
Reviewed-on: https://boringssl-review.googlesource.com/7601
Reviewed-by: Steven Valdez <svaldez@google.com>
Reviewed-by: David Benjamin <davidben@google.com>

crypto/thread_win.c

diff --git a/crypto/thread_win.c b/crypto/thread_win.c
index e48ab5f..8e231eb 100644
--- a/crypto/thread_win.c
+++ b/crypto/thread_win.c
@@ -150,9 +150,14 @@
   g_thread_local_failed = (g_thread_local_key == TLS_OUT_OF_INDEXES);
 }
 
-static void NTAPI thread_local_destructor(PVOID module,
-                                          DWORD reason, PVOID reserved) {
-  if (DLL_THREAD_DETACH != reason && DLL_PROCESS_DETACH != reason) {
+static void NTAPI thread_local_destructor(PVOID module, DWORD reason,
+                                          PVOID reserved) {
+  /* Only free memory on |DLL_THREAD_DETACH|, not |DLL_PROCESS_DETACH|. In
+   * VS2015's debug runtime, the C runtime has been unloaded by the time
+   * |DLL_PROCESS_DETACH| runs. See https://crbug.com/575795. This is consistent
+   * with |pthread_key_create| which does not call destructors on process exit,
+   * only thread exit. */
+  if (reason != DLL_THREAD_DETACH) {
     return;
   }