Provide windows notification of thread termination

base/win/dllmain.cc

+// Copyright (c) 2011 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+// Windows doesn't support pthread_key_create's destr_function, and in fact
+// it's a bit tricky to get code to run when a thread exits.  This is
+// cargo-cult magic from http://www.codeproject.com/threads/tls.asp.
+// We are trying to be compatible with both a LoadLibrary style invocation, as
+// well as static linking. This code only needs to be included if we use
+// LoadLibrary, but it hooks into the "standard" set of TLS callbacks that are
+// provided for static linking.
+
+// This code is deliberately written to match the style of calls seen in
+// base/threading/thread_local_storage_win.cc.  Please keep the two in sync if
+// coding conventions are changed.
+
+// WARNING: Do *NOT* try to include this in the construction of the base
+// library, even though it potentially drives code in
+// base/threading/thread_local_storage_win.cc.  If you do, some users will end
+// up getting duplicate definition of DllMain() in some of their later links.
+
+// Force a reference to _tls_used to make the linker create the TLS directory
+// if it's not already there (that is, even if __declspec(thread) is not used).
+// Force a reference to p_thread_callback_dllmain_typical_entry to prevent whole
+// program optimization from discarding the variables.
+
+#include <windows.h>
+
+#include "base/logging.h"
+
+// Indicate if another service is scanning the callbacks.  When this becomes
+// set to true, then DllMain() will stop supporting the callback service. This
+// value is set to true the first time any of our callbacks are called, as that
+// shows that some other service is handling callbacks.
+static bool linker_notifications_are_active = false;
+
+// This will be our mostly no-op callback that we'll list.  We won't
+// deliberately call it, and if it is called, that means we don't need to do any
+// of the callbacks anymore.  We expect such a call to arrive via a
+// THREAD_ATTACH message, long before we'd have to perform our THREAD_DETACH
+// callbacks.
+static void NTAPI on_callback(PVOID h, DWORD reason, PVOID reserved);
+
+#ifdef _WIN64
+
+#pragma comment(linker, "/INCLUDE:_tls_used")
+#pragma comment(linker, "/INCLUDE:p_thread_callback_dllmain_typical_entry")
+
+#else  // _WIN64
+
+#pragma comment(linker, "/INCLUDE:__tls_used")
+#pragma comment(linker, "/INCLUDE:_p_thread_callback_dllmain_typical_entry")
+
+#endif  // _WIN64
+
+// Explicitly depend on tlssup.cc variable to bracket the list of TLS callbacks.
+extern "C" PIMAGE_TLS_CALLBACK __xl_a;
+extern "C" PIMAGE_TLS_CALLBACK __xl_z;
+
+// extern "C" suppresses C++ name mangling so we know the symbol names for the
+// linker /INCLUDE:symbol pragmas above.
+extern "C" {
+#ifdef _WIN64
+
+// .CRT section is merged with .rdata on x64 so it must be constant data.
+#pragma data_seg(push, old_seg)
+// Use a typical possible name in the .CRT$XL? list of segments.
+#pragma const_seg(".CRT$XLB")
+// When defining a const variable, it must have external linkage to be sure the
+// linker doesn't discard it.
+extern const PIMAGE_TLS_CALLBACK p_thread_callback_dllmain_typical_entry;
+const PIMAGE_TLS_CALLBACK p_thread_callback_dllmain_typical_entry = on_callback;
+#pragma data_seg(pop, old_seg)
+
+#else  // _WIN64
+
+#pragma data_seg(push, old_seg)
+// Use a typical possible name in the .CRT$XL? list of segments.
+#pragma data_seg(".CRT$XLB")
+PIMAGE_TLS_CALLBACK p_thread_callback_dllmain_typical_entry = on_callback;
+#pragma data_seg(pop, old_seg)
+
+#endif  // _WIN64
+}  // extern "C"
+
+
+// Make DllMain call the listed callbacks.  This way any third parties that are
+// linked in will also be called.
+BOOL WINAPI DllMain(PVOID h, DWORD reason, PVOID reserved) {
+  if (DLL_THREAD_DETACH != reason && DLL_PROCESS_DETACH != reason)
+    return true;  // We won't service THREAD_ATTACH calls.
+
+  if (linker_notifications_are_active)
+    return true;  // Some other service is doing this work.
+
+  for (PIMAGE_TLS_CALLBACK* it = &__xl_a; it < &__xl_z; ++it) {
+    if (*it == NULL || *it == on_callback)
+      continue;  // Don't bother to call our own callback.
+    (*it)(h, reason, reserved);
+  }
+  return true;
+}
+
+static void NTAPI on_callback(PVOID h, DWORD reason, PVOID reserved) {
+  // Do nothing.  We were just a place holder in the list used to test that we
+  // call all items.
+  // If we are called, it means that some other system is scanning the callbacks
+  // and we don't need to do so in DllMain().
+  linker_notifications_are_active = true;
+  // Note: If some other routine some how plays this same game... we could both
+  // decide not to do the scanning <sigh>, but this trick should suppress
+  // duplicate calls on Vista, where the runtime takes care of the callbacks,
+  // and allow us to do the callbacks on XP, where we are currently devoid of
+  // callbacks (due to an explicit LoadLibrary call).
+}