Protect access to the external_snapshot_blob global with a lock.

The external_snapshot_blob is a global and might be accessed from multiple
threads.  Protect it with a lock.

BUG=457656
LOG=N
R=vogelheim@chromium.org

Review URL: https://codereview.chromium.org/918213002

Cr-Commit-Position: refs/heads/master@{#26638}

src/snapshot-external.cc

@@ -6,6 +6,7 @@
 
 #include "src/snapshot.h"
 
+#include "src/base/platform/mutex.h"
 #include "src/serialize.h"
 #include "src/snapshot-source-sink.h"
 #include "src/v8.h"  // for V8::Initialize
@@ -19,9 +20,12 @@
 namespace v8 {
 namespace internal {
 
+static base::LazyMutex external_startup_data_mutex = LAZY_MUTEX_INITIALIZER;
 static v8::StartupData external_startup_blob = {NULL, 0};
 
 void SetSnapshotFromFile(StartupData* snapshot_blob) {
+  base::LockGuard<base::Mutex> lock_guard(
+      external_startup_data_mutex.Pointer());
   DCHECK(snapshot_blob);
   DCHECK(snapshot_blob->data);
   DCHECK(snapshot_blob->raw_size > 0);
@@ -31,5 +35,9 @@ void SetSnapshotFromFile(StartupData* snapshot_blob) {
 }
 
 
-const v8::StartupData Snapshot::SnapshotBlob() { return external_startup_blob; }
+const v8::StartupData Snapshot::SnapshotBlob() {
+  base::LockGuard<base::Mutex> lock_guard(
+      external_startup_data_mutex.Pointer());
+  return external_startup_blob;
+}
 } }  // namespace v8::internal