Replace the last CRITICAL_SECTION with SRWLOCK.

We don't support Windows XP, so we can rely on SRWLOCK. Per
https://crbug.com/592752, SRWLOCKs are more efficient and less of a
hassle to use. We'd previously converted CRYPTO_MUTEX to SRWLOCK, but I
missed this one. Not that this one lock matters much, may as well. It's
less initialization code.

Change-Id: I7ae435be5202b0a19f42015c9abff932dc04dbc7
Reviewed-on: https://boringssl-review.googlesource.com/c/33445
Commit-Queue: David Benjamin <davidben@google.com>
Commit-Queue: Adam Langley <agl@google.com>
Reviewed-by: Adam Langley <agl@google.com>

crypto/thread_win.c

diff --git a/crypto/thread_win.c b/crypto/thread_win.c
index 4501165..c8e19f5 100644
--- a/crypto/thread_win.c
+++ b/crypto/thread_win.c
@@ -82,7 +82,7 @@
   ReleaseSRWLockExclusive(&lock->lock);
 }
 
-static CRITICAL_SECTION g_destructors_lock;
+static SRWLOCK g_destructors_lock = SRWLOCK_INIT;
 static thread_local_destructor_t g_destructors[NUM_OPENSSL_THREAD_LOCALS];
 
 static CRYPTO_once_t g_thread_local_init_once = CRYPTO_ONCE_INIT;
@@ -90,10 +90,6 @@
 static int g_thread_local_failed;
 
 static void thread_local_init(void) {
-  if (!InitializeCriticalSectionAndSpinCount(&g_destructors_lock, 0x400)) {
-    g_thread_local_failed = 1;
-    return;
-  }
   g_thread_local_key = TlsAlloc();
   g_thread_local_failed = (g_thread_local_key == TLS_OUT_OF_INDEXES);
 }
@@ -121,12 +117,11 @@
 
   thread_local_destructor_t destructors[NUM_OPENSSL_THREAD_LOCALS];
 
-  EnterCriticalSection(&g_destructors_lock);
+  AcquireSRWLockExclusive(&g_destructors_lock);
   OPENSSL_memcpy(destructors, g_destructors, sizeof(destructors));
-  LeaveCriticalSection(&g_destructors_lock);
+  ReleaseSRWLockExclusive(&g_destructors_lock);
 
-  unsigned i;
-  for (i = 0; i < NUM_OPENSSL_THREAD_LOCALS; i++) {
+  for (unsigned i = 0; i < NUM_OPENSSL_THREAD_LOCALS; i++) {
     if (destructors[i] != NULL) {
       destructors[i](pointers[i]);
     }
@@ -250,9 +245,9 @@
     }
   }
 
-  EnterCriticalSection(&g_destructors_lock);
+  AcquireSRWLockExclusive(&g_destructors_lock);
   g_destructors[index] = destructor;
-  LeaveCriticalSection(&g_destructors_lock);
+  ReleaseSRWLockExclusive(&g_destructors_lock);
 
   pointers[index] = value;
   return 1;