[libc] Fix exit not calling new handlers registered from a call to atexit in atexit handler

[sivachandra]

So, you do want to match the glibc behavior then? I do not know what the ideal approach though. Matching glibc behavior isn't a bad target.

[abrachet]

No link in commit message, but closed by https://github.com/llvm/llvm-project/commit/ae4b59f1798b479a71ab2a80bfe7e36812c87816

[abrachet]

In D121657#3381413, @sivachandra wrote:

So, you do want to match the glibc behavior then? I do not know what the ideal approach though. Matching glibc behavior isn't a bad target.

It was pointed out to me by @mcgrathr that not calling the newly registered functions is incorrect. glibc as you point out does this, musl and by extension Fuchsia's current libc do not.

libc/src/stdlib/atexit.cpp

 constinit ExitCallbackList exit_callbacks;
 
 } // namespace
 
 namespace internal {
 
 void call_exit_callbacks() {
   handler_list_mtx.lock();
-  for (auto callback : exit_callbacks) {
+  while (!exit_callbacks.empty()) {
     handler_list_mtx.unlock();
+    auto *callback = exit_callbacks.back();
+    exit_callbacks.pop_back();
     callback();
     handler_list_mtx.lock();
   }
   ExitCallbackList::destroy(&exit_callbacks);
 }
 
 } // namespace internal
 
 LLVM_LIBC_FUNCTION(int, atexit, (AtExitCallback * callback)) {
   handler_list_mtx.lock();
   exit_callbacks.push_back(callback);
   handler_list_mtx.unlock();
   return 0;
 }
 
 } // namespace __llvm_libc

libc/test/src/stdlib/atexit_test.cpp

 TEST(LlvmLibcAtExit, Many) {
   // In case tests ever run multiple times.
   size = 0;
 
   auto test = getTest<256>();
   EXPECT_EXITS(test, 0);
 }
 
-// POSIX doesn't specify if an atexit handler can call atexit, it only says it
-// is undefined for a handler to call exit(3). The current implementation will
-// end up invoking the newly registered function, although glibc does, other
-// libc's do not. This just tests that we don't deadlock when an exit handler
-// calls atexit.
 TEST(LlvmLibcAtExit, HandlerCallsAtExit) {
   auto test = [] {
     __llvm_libc::atexit(+[] {
-      __llvm_libc::atexit(+[] { __builtin_trap(); });
-      __llvm_libc::exit(0);
+      __llvm_libc::atexit(+[] { __llvm_libc::exit(1); });
     });
+    __llvm_libc::exit(0);
   };
-  EXPECT_EXITS(test, 0);
+  EXPECT_EXITS(test, 1);
 }