Issue #22290: Fix error handling in the _posixsubprocess module.

* Don't call the garbage collector with an exception set: it causes an
  assertion to fail in debug mode.
* Enhance also error handling if allocating an array for the executable list
  failed.
* Add an unit test for 4 different errors in the _posixsubprocess module.

Lib/test/test_subprocess.py

@@ -2216,6 +2216,39 @@ class POSIXProcessTestCase(BaseTestCase):
 
         self.assertNotIn(fd, remaining_fds)
 
+    @support.cpython_only
+    def test_fork_exec(self):
+        # Issue #22290: fork_exec() must not crash on memory allocation failure
+        # or other errors
+        import _posixsubprocess
+        gc_enabled = gc.isenabled()
+        try:
+            # Use a preexec function and enable the garbage collector
+            # to force fork_exec() to re-enable the garbage collector
+            # on error.
+            func = lambda: None
+            gc.enable()
+
+            executable_list = "exec"   # error: must be a sequence
+
+            for args, exe_list, cwd, env_list in (
+                (123,      [b"exe"], None, [b"env"]),
+                ([b"arg"], 123,      None, [b"env"]),
+                ([b"arg"], [b"exe"], 123,  [b"env"]),
+                ([b"arg"], [b"exe"], None, 123),
+            ):
+                with self.assertRaises(TypeError):
+                    _posixsubprocess.fork_exec(
+                        args, exe_list,
+                        True, [], cwd, env_list,
+                        -1, -1, -1, -1,
+                        1, 2, 3, 4,
+                        True, True, func)
+        finally:
+            if not gc_enabled:
+                gc.disable()
+
+
 
 @unittest.skipUnless(mswindows, "Windows specific tests")
 class Win32ProcessTestCase(BaseTestCase):

Modules/_posixsubprocess.c

@@ -538,6 +538,7 @@ subprocess_fork_exec(PyObject* self, PyObject *args)
     int need_to_reenable_gc = 0;
     char *const *exec_array, *const *argv = NULL, *const *envp = NULL;
     Py_ssize_t arg_num;
+    int import_lock_held = 0;
 
     if (!PyArg_ParseTuple(
             args, "OOpOOOiiiiiiiiiiO:fork_exec",
@@ -590,10 +591,8 @@ subprocess_fork_exec(PyObject* self, PyObject *args)
     }
 
     exec_array = _PySequence_BytesToCharpArray(executable_list);
-    if (!exec_array) {
-        Py_XDECREF(gc_module);
-        return NULL;
-    }
+    if (!exec_array)
+        goto cleanup;
 
     /* Convert args and env into appropriate arguments for exec() */
     /* These conversions are done in the parent process to avoid allocating
@@ -635,6 +634,7 @@ subprocess_fork_exec(PyObject* self, PyObject *args)
         if (!preexec_fn_args_tuple)
             goto cleanup;
         _PyImport_AcquireLock();
+        import_lock_held = 1;
     }
 
     if (cwd_obj != Py_None) {
@@ -682,6 +682,7 @@ subprocess_fork_exec(PyObject* self, PyObject *args)
         PyErr_SetString(PyExc_RuntimeError,
                         "not holding the import lock");
     }
+    import_lock_held = 0;
 
     /* Parent process */
     if (envp)
@@ -704,18 +705,25 @@ subprocess_fork_exec(PyObject* self, PyObject *args)
     return PyLong_FromPid(pid);
 
 cleanup:
+    if (import_lock_held)
+        _PyImport_ReleaseLock();
     if (envp)
         _Py_FreeCharPArray(envp);
     if (argv)
         _Py_FreeCharPArray(argv);
+    if (exec_array)
         _Py_FreeCharPArray(exec_array);
     Py_XDECREF(converted_args);
     Py_XDECREF(fast_args);
     Py_XDECREF(preexec_fn_args_tuple);
 
     /* Reenable gc if it was disabled. */
-    if (need_to_reenable_gc)
+    if (need_to_reenable_gc) {
+        PyObject *exctype, *val, *tb;
+        PyErr_Fetch(&exctype, &val, &tb);
         _enable_gc(gc_module);
+        PyErr_Restore(exctype, val, tb);
+    }
     Py_XDECREF(gc_module);
     return NULL;
 }