1 files changed, 96 insertions, 0 deletions
diff --git a/browser/app/winlauncher/freestanding/SafeThreadLocal.h b/browser/app/winlauncher/freestanding/SafeThreadLocal.h
new file mode 100644
index 0000000000..e4b869f649
--- /dev/null
+++ b/browser/app/winlauncher/freestanding/SafeThreadLocal.h
@@ -0,0 +1,96 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at https://mozilla.org/MPL/2.0/. */
+
+#ifndef mozilla_freestanding_SafeThreadLocal_h
+#define mozilla_freestanding_SafeThreadLocal_h
+
+#include <type_traits>
+
+#include "mozilla/NativeNt.h"
+#include "mozilla/ThreadLocal.h"
+
+namespace mozilla {
+namespace freestanding {
+
+// We cannot fall back to the Tls* APIs because kernel32 might not have been
+// loaded yet.
+#if defined(__MINGW32__) && !defined(HAVE_THREAD_TLS_KEYWORD)
+#  error "This code requires the compiler to have native TLS support"
+#endif  // defined(__MINGW32__) && !defined(HAVE_THREAD_TLS_KEYWORD)
+
+/**
+ * This class holds data as a thread-local variable, or as a global variable
+ * if the thread local storage is not initialized yet.  It should be safe
+ * because in that early stage we assume there is no more than a single thread.
+ */
+template <typename T>
+class SafeThreadLocal final {
+  static MOZ_THREAD_LOCAL(T) sThreadLocal;
+  static T sGlobal;
+  static bool sIsTlsUsed;
+
+  // In normal cases, TLS is always available and the class uses sThreadLocal
+  // without changing sMainThreadId.  So sMainThreadId is likely to be 0.
+  //
+  // If TLS is not available, we use sGlobal instead and update sMainThreadId
+  // so that that thread keeps using sGlobal even after TLS is initialized
+  // later.
+  static DWORD sMainThreadId;
+
+  // Need non-inline accessors to prevent the compiler from generating code
+  // accessing sThreadLocal before checking a condition.
+  MOZ_NEVER_INLINE static void SetGlobalValue(T aValue) { sGlobal = aValue; }
+  MOZ_NEVER_INLINE static T GetGlobalValue() { return sGlobal; }
+
+ public:
+  static void set(T aValue) {
+    static_assert(std::is_pointer_v<T>,
+                  "SafeThreadLocal must be used with a pointer");
+
+    if (sMainThreadId == mozilla::nt::RtlGetCurrentThreadId()) {
+      SetGlobalValue(aValue);
+    } else if (sIsTlsUsed) {
+      MOZ_ASSERT(mozilla::nt::RtlGetThreadLocalStoragePointer(),
+                 "Once TLS is used, TLS should be available till the end.");
+      sThreadLocal.set(aValue);
+    } else if (mozilla::nt::RtlGetThreadLocalStoragePointer()) {
+      sIsTlsUsed = true;
+      sThreadLocal.set(aValue);
+    } else {
+      MOZ_ASSERT(sMainThreadId == 0,
+                 "A second thread cannot be created before TLS is available.");
+      sMainThreadId = mozilla::nt::RtlGetCurrentThreadId();
+      SetGlobalValue(aValue);
+    }
+  }
+
+  static T get() {
+    if (sMainThreadId == mozilla::nt::RtlGetCurrentThreadId()) {
+      return GetGlobalValue();
+    } else if (sIsTlsUsed) {
+      return sThreadLocal.get();
+    }
+    return GetGlobalValue();
+  }
+};
+
+template <typename T>
+MOZ_THREAD_LOCAL(T)
+SafeThreadLocal<T>::sThreadLocal;
+
+template <typename T>
+T SafeThreadLocal<T>::sGlobal = nullptr;
+
+template <typename T>
+bool SafeThreadLocal<T>::sIsTlsUsed = false;
+
+template <typename T>
+DWORD SafeThreadLocal<T>::sMainThreadId = 0;
+
+}  // namespace freestanding
+}  // namespace mozilla
+
+#endif  // mozilla_freestanding_SafeThreadLocal_h