[libc] ensure tls dtors are called in main thread (#133641)

This is a part of allocator patch since I want to make sure the TLS for
allocators are correctly handled.

TLS dtors are not invoked on exit previously. This departures from major
libc implementations.

This PR fixes the issue by aligning the behavior with bionic.


https://android.googlesource.com/platform/bionic/+/refs/heads/main/libc/bionic/exit.cpp

I believe the finalization order is also consistent with glibc now:

On main thread exiting:

- pthread_key dtors are not called (unless exiting with `pthread_exit`)
- `__cxa` based tls dtors are called
- `::__cxa_atexit` and `::atexit` dtors are called
- `.fini` dtors are called


![image](https://github.com/user-attachments/assets/737c4845-cab6-47a9-aa00-32997be141bd)

Author: SchrodingerZhu

libc/src/__support/threads/thread.cpp

@@ -163,6 +163,8 @@ void call_atexit_callbacks(ThreadAttributes *attrib) {
   }
 }
 
+extern "C" void __cxa_thread_finalize() { call_atexit_callbacks(self.attrib); }
+
 } // namespace internal
 
 cpp::optional<unsigned int> new_tss_key(TSSDtor *dtor) {

libc/src/__support/threads/thread.h

@@ -110,7 +110,7 @@ struct alignas(STACK_ALIGNMENT) ThreadAttributes {
   ThreadAtExitCallbackMgr *atexit_callback_mgr;
   void *platform_data;
 
-  constexpr ThreadAttributes()
+  LIBC_INLINE constexpr ThreadAttributes()
       : detach_state(uint32_t(DetachState::DETACHED)), stack(nullptr),
         stacksize(0), guardsize(0), tls(0), tls_size(0), owned_stack(false),
         tid(-1), style(ThreadStyle::POSIX), retval(),

libc/src/stdlib/exit.cpp

@@ -15,7 +15,17 @@ namespace LIBC_NAMESPACE_DECL {
 
 extern "C" void __cxa_finalize(void *);
 
+// exit needs to clean up TLS and call associated destructors.
+// TODO: Strictly speaking, it is not valid to call exit in overlay mode
+//       as we have no way to ensure system libc will call the TLS destructors.
+//       We should run exit related tests in hermetic mode but this is currently
+//       blocked by https://github.com/llvm/llvm-project/issues/133925.
+extern "C" [[gnu::weak]] void __cxa_thread_finalize();
+
+// TODO: use recursive mutex to protect this routine.
 [[noreturn]] LLVM_LIBC_FUNCTION(void, exit, (int status)) {
+  if (__cxa_thread_finalize)
+    __cxa_thread_finalize();
   __cxa_finalize(nullptr);
   internal::exit(status);
 }

libc/test/integration/src/__support/threads/CMakeLists.txt

@@ -25,3 +25,24 @@ add_integration_test(
   DEPENDS
     libc.src.__support.threads.thread
 )
+
+add_integration_test(
+  main_exit_test
+  SUITE
+    libc-support-threads-integration-tests
+  SRCS
+    main_exit_test.cpp
+  DEPENDS
+    libc.src.__support.threads.thread
+)
+
+add_integration_test(
+  double_exit_test
+  SUITE
+    libc-support-threads-integration-tests
+  SRCS
+    double_exit_test.cpp
+  DEPENDS
+    libc.src.__support.threads.thread
+    libc.src.stdlib.exit
+)

libc/test/integration/src/__support/threads/double_exit_test.cpp

@@ -0,0 +1,33 @@
+//===-- Test handling of thread local data --------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#include "src/__support/threads/thread.h"
+#include "src/stdlib/exit.h"
+#include "test/IntegrationTest/test.h"
+
+extern "C" {
+[[gnu::weak]]
+void *__dso_handle = nullptr;
+int __cxa_thread_atexit_impl(void (*func)(void *), void *arg, void *dso);
+}
+
+int call_num = 0;
+
+[[gnu::destructor]]
+void check() {
+  // This destructor should be called only once.
+  if (call_num != 1)
+    __builtin_trap();
+}
+
+TEST_MAIN() {
+  __cxa_thread_atexit_impl([](void *) { LIBC_NAMESPACE::exit(0); }, nullptr,
+                           __dso_handle);
+  __cxa_thread_atexit_impl([](void *) { ++call_num; }, nullptr, __dso_handle);
+  LIBC_NAMESPACE::exit(1);
+}

libc/test/integration/src/__support/threads/main_exit_test.cpp

@@ -0,0 +1,30 @@
+//===-- Test handling of thread local data --------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#include "src/__support/threads/thread.h"
+#include "test/IntegrationTest/test.h"
+
+bool called = false;
+
+extern "C" {
+[[gnu::weak]]
+void *__dso_handle = nullptr;
+int __cxa_thread_atexit_impl(void (*func)(void *), void *arg, void *dso);
+}
+
+[[gnu::destructor]]
+void destructor() {
+  if (!called)
+    __builtin_trap();
+}
+
+TEST_MAIN() {
+  __cxa_thread_atexit_impl([](void *) { called = true; }, nullptr,
+                           __dso_handle);
+  return 0;
+}