Fix Race Condition (#637)

Related: MONGOCRYPT-526

Fix race condition by moving critical section into the mutex protected scope.

See the following repo for a tool to test the race condition fixed in this PR: https://github.com/kkloberdanz/libmongocrypt_stresstest

src/mongocrypt.c

@@ -597,36 +597,31 @@ static bool _validate_csfle_singleton(mongocrypt_t *crypt, _loaded_csfle found)
 static void _csfle_drop_global_ref(void) {
     mlib_call_once(&g_csfle_init_flag, init_csfle_state);
 
-    bool dropped_last_ref = false;
-    csfle_global_lib_state old_state = {.refcount = 0};
     MONGOCRYPT_WITH_MUTEX(g_csfle_state.mtx) {
         assert(g_csfle_state.refcount > 0);
         int new_rc = --g_csfle_state.refcount;
         if (new_rc == 0) {
-            old_state = g_csfle_state;
-            dropped_last_ref = true;
-        }
-    }
-
-    if (dropped_last_ref) {
-        mongo_crypt_v1_status *status = old_state.vtable.status_create();
-        const int destroy_rc = old_state.vtable.lib_destroy(old_state.csfle_lib, status);
-        if (destroy_rc != MONGO_CRYPT_V1_SUCCESS && status) {
-            fprintf(stderr,
-                    "csfle lib_destroy() failed: %s [Error %d, code %d]\n",
-                    old_state.vtable.status_get_explanation(status),
-                    old_state.vtable.status_get_error(status),
-                    old_state.vtable.status_get_code(status));
-        }
-        old_state.vtable.status_destroy(status);
-
+            mongo_crypt_v1_status *status = g_csfle_state.vtable.status_create();
+            const int destroy_rc = g_csfle_state.vtable.lib_destroy(g_csfle_state.csfle_lib, status);
+            if (destroy_rc != MONGO_CRYPT_V1_SUCCESS && status) {
+                fprintf(stderr,
+                        "csfle lib_destroy() failed: %s [Error %d, code %d]\n",
+                        g_csfle_state.vtable.status_get_explanation(status),
+                        g_csfle_state.vtable.status_get_error(status),
+                        g_csfle_state.vtable.status_get_code(status));
+            }
+            g_csfle_state.vtable.status_destroy(status);
 #ifndef __linux__
-        mcr_dll_close(old_state.dll);
+            mcr_dll_close(g_csfle_state.dll);
+#else
+            /// NOTE: On Linux, skip closing the CSFLE library itself, since a bug in
+            /// the way ld-linux and GCC interact causes static destructors to not run
+            /// during dlclose(). Still, free the error string:
+            ///
+            /// Please see: https://jira.mongodb.org/browse/SERVER-63710
+            mstr_free(g_csfle_state.dll.error_string);
 #endif
-        /// NOTE: On Linux, skip closing the CSFLE library itself, since a bug in
-        /// the way ld-linux and GCC interact causes static destructors to not run
-        /// during dlclose(). Still, free the error string:
-        mstr_free(old_state.dll.error_string);
+        }
     }
 }