gh-100227: Lock Around Use of the Global "atexit" State (gh-105514)

The risk of a race with this state is relatively low, but we play it safe anyway.

Author: ericsnowcurrently

Include/internal/pycore_atexit.h

@@ -15,6 +15,7 @@ extern "C" {
 typedef void (*atexit_callbackfunc)(void);
 
 struct _atexit_runtime_state {
+    PyThread_type_lock mutex;
 #define NEXITFUNCS 32
     atexit_callbackfunc callbacks[NEXITFUNCS];
     int ncallbacks;

Python/pylifecycle.c

@@ -2973,24 +2973,35 @@ wait_for_thread_shutdown(PyThreadState *tstate)
 
 int Py_AtExit(void (*func)(void))
 {
-    if (_PyRuntime.atexit.ncallbacks >= NEXITFUNCS)
+    struct _atexit_runtime_state *state = &_PyRuntime.atexit;
+    PyThread_acquire_lock(state->mutex, WAIT_LOCK);
+    if (state->ncallbacks >= NEXITFUNCS) {
+        PyThread_release_lock(state->mutex);
         return -1;
-    _PyRuntime.atexit.callbacks[_PyRuntime.atexit.ncallbacks++] = func;
+    }
+    state->callbacks[state->ncallbacks++] = func;
+    PyThread_release_lock(state->mutex);
     return 0;
 }
 
 static void
 call_ll_exitfuncs(_PyRuntimeState *runtime)
 {
+    atexit_callbackfunc exitfunc;
     struct _atexit_runtime_state *state = &runtime->atexit;
+
+    PyThread_acquire_lock(state->mutex, WAIT_LOCK);
     while (state->ncallbacks > 0) {
         /* pop last function from the list */
         state->ncallbacks--;
-        atexit_callbackfunc exitfunc = state->callbacks[state->ncallbacks];
+        exitfunc = state->callbacks[state->ncallbacks];
         state->callbacks[state->ncallbacks] = NULL;
 
+        PyThread_release_lock(state->mutex);
         exitfunc();
+        PyThread_acquire_lock(state->mutex, WAIT_LOCK);
     }
+    PyThread_release_lock(state->mutex);
 
     fflush(stdout);
     fflush(stderr);

Python/pystate.c

@@ -380,7 +380,16 @@ _Py_COMP_DIAG_IGNORE_DEPR_DECLS
 static const _PyRuntimeState initial = _PyRuntimeState_INIT(_PyRuntime);
 _Py_COMP_DIAG_POP
 
-#define NUMLOCKS 5
+#define NUMLOCKS 6
+#define LOCKS_INIT(runtime) \
+    { \
+        &(runtime)->interpreters.mutex, \
+        &(runtime)->xidregistry.mutex, \
+        &(runtime)->getargs.mutex, \
+        &(runtime)->unicode_state.ids.lock, \
+        &(runtime)->imports.extensions.mutex, \
+        &(runtime)->atexit.mutex, \
+    }
 
 static int
 alloc_for_runtime(PyThread_type_lock locks[NUMLOCKS])
@@ -427,13 +436,7 @@ init_runtime(_PyRuntimeState *runtime,
 
     PyPreConfig_InitPythonConfig(&runtime->preconfig);
 
-    PyThread_type_lock *lockptrs[NUMLOCKS] = {
-        &runtime->interpreters.mutex,
-        &runtime->xidregistry.mutex,
-        &runtime->getargs.mutex,
-        &runtime->unicode_state.ids.lock,
-        &runtime->imports.extensions.mutex,
-    };
+    PyThread_type_lock *lockptrs[NUMLOCKS] = LOCKS_INIT(runtime);
     for (int i = 0; i < NUMLOCKS; i++) {
         assert(locks[i] != NULL);
         *lockptrs[i] = locks[i];
@@ -512,13 +515,7 @@ _PyRuntimeState_Fini(_PyRuntimeState *runtime)
         LOCK = NULL; \
     }
 
-    PyThread_type_lock *lockptrs[NUMLOCKS] = {
-        &runtime->interpreters.mutex,
-        &runtime->xidregistry.mutex,
-        &runtime->getargs.mutex,
-        &runtime->unicode_state.ids.lock,
-        &runtime->imports.extensions.mutex,
-    };
+    PyThread_type_lock *lockptrs[NUMLOCKS] = LOCKS_INIT(runtime);
     for (int i = 0; i < NUMLOCKS; i++) {
         FREE_LOCK(*lockptrs[i]);
     }
@@ -541,13 +538,7 @@ _PyRuntimeState_ReInitThreads(_PyRuntimeState *runtime)
     PyMemAllocatorEx old_alloc;
     _PyMem_SetDefaultAllocator(PYMEM_DOMAIN_RAW, &old_alloc);
 
-    PyThread_type_lock *lockptrs[NUMLOCKS] = {
-        &runtime->interpreters.mutex,
-        &runtime->xidregistry.mutex,
-        &runtime->getargs.mutex,
-        &runtime->unicode_state.ids.lock,
-        &runtime->imports.extensions.mutex,
-    };
+    PyThread_type_lock *lockptrs[NUMLOCKS] = LOCKS_INIT(runtime);
     int reinit_err = 0;
     for (int i = 0; i < NUMLOCKS; i++) {
         reinit_err += _PyThread_at_fork_reinit(lockptrs[i]);