[libc] Fix callback type in exit_handlers.cpp not matching (#97642)

Summary:
This file is an object library, but uses the `LIBC_COPT_PUBLIC_PACKAING`
option. This will always be undefined which leads to a type mismatch
when uses actually try to link against it. This patch simply removes
this and turns it into a header only library. This means that the
implementations of the callback lists and the mutexes need to live in
their respective files. The result is that `atexit` needs to be defined
for `at_quick_exit` to be valid.

Author: jhuber6

libc/src/stdlib/CMakeLists.txt

@@ -51,6 +51,8 @@ add_entrypoint_object(
   DEPENDS
     libc.src.__support.OSUtil.osutil
     .exit_handler
+    .at_quick_exit
+    .atexit
 )
 
 add_entrypoint_object(
@@ -462,15 +464,11 @@ add_entrypoint_object(
 
 # TODO: Move all exit functions to linux specific 
 
-if (TARGET libc.src.__support.threads.mutex)
-add_object_library(
+if(TARGET libc.src.__support.threads.mutex)
+add_header_library(
   exit_handler
-  SRCS
-    exit_handler.cpp
   HDRS
     exit_handler.h
-  CXX_STANDARD
-    20 # For constinit 
   DEPENDS
     libc.src.__support.CPP.mutex
     libc.src.__support.CPP.new
@@ -487,6 +485,8 @@ add_entrypoint_object(
     atexit.cpp
   HDRS
     atexit.h
+  CXX_STANDARD
+    20 # For constinit 
   DEPENDS
     .exit_handler
 )
@@ -497,8 +497,11 @@ add_entrypoint_object(
     at_quick_exit.cpp
   HDRS
     at_quick_exit.h
+  CXX_STANDARD
+    20 # For constinit 
   DEPENDS
     .exit_handler
+    .atexit
 )
 
 list(APPEND exit_deps

libc/src/stdlib/at_quick_exit.cpp

@@ -14,6 +14,8 @@
 
 namespace LIBC_NAMESPACE_DECL {
 
+constinit ExitCallbackList at_quick_exit_callbacks;
+
 LLVM_LIBC_FUNCTION(int, at_quick_exit, (__atexithandler_t callback)) {
   return add_atexit_unit(
       at_quick_exit_callbacks,

libc/src/stdlib/atexit.cpp

@@ -14,6 +14,9 @@
 
 namespace LIBC_NAMESPACE_DECL {
 
+constinit ExitCallbackList atexit_callbacks;
+Mutex handler_list_mtx(false, false, false, false);
+
 extern "C" {
 
 int __cxa_atexit(AtExitCallback *callback, void *payload, void *) {

libc/src/stdlib/exit_handler.cpp

@@ -38,16 +38,32 @@ using ExitCallbackList = ReverseOrderBlockStore<AtExitUnit, 32>;
 using ExitCallbackList = FixedVector<AtExitUnit, CALLBACK_LIST_SIZE_FOR_TESTS>;
 #endif
 
-extern ExitCallbackList atexit_callbacks;
-extern ExitCallbackList at_quick_exit_callbacks;
-
+// This is handled by the 'atexit' implementation and shared by 'at_quick_exit'.
 extern Mutex handler_list_mtx;
 
-void stdc_at_exit_func(void *payload);
+LIBC_INLINE void stdc_at_exit_func(void *payload) {
+  reinterpret_cast<StdCAtExitCallback *>(payload)();
+}
 
-void call_exit_callbacks(ExitCallbackList &callbacks);
+LIBC_INLINE void call_exit_callbacks(ExitCallbackList &callbacks) {
+  handler_list_mtx.lock();
+  while (!callbacks.empty()) {
+    AtExitUnit &unit = callbacks.back();
+    callbacks.pop_back();
+    handler_list_mtx.unlock();
+    unit.callback(unit.payload);
+    handler_list_mtx.lock();
+  }
+  ExitCallbackList::destroy(&callbacks);
+}
 
-int add_atexit_unit(ExitCallbackList &callbacks, const AtExitUnit &unit);
+LIBC_INLINE int add_atexit_unit(ExitCallbackList &callbacks,
+                                const AtExitUnit &unit) {
+  cpp::lock_guard lock(handler_list_mtx);
+  if (callbacks.push_back(unit))
+    return 0;
+  return -1;
+}
 
 } // namespace LIBC_NAMESPACE_DECL