Implement better error reporting for non-existent executable and/or cwd when using posix_spawn.

Author: kulikjak

Lib/test/test_subprocess.py

@@ -2018,6 +2018,7 @@ def _get_chdir_exception(self):
                       self._nonexistent_dir)
         return desired_exception
 
+    @mock.patch("subprocess._HAVE_POSIX_SPAWN_CHDIR", new=False)
     def test_exception_cwd(self):
         """Test error in the child raised in the parent for a bad cwd."""
         desired_exception = self._get_chdir_exception()

Modules/posixmodule.c

@@ -7760,7 +7760,7 @@ parse_posix_spawn_flags(PyObject *module, const char *func_name, PyObject *setpg
 static int
 parse_file_actions(PyObject *file_actions,
                    posix_spawn_file_actions_t *file_actionsp,
-                   PyObject *temp_buffer)
+                   PyObject *temp_buffer, PyObject** cwd)
 {
     PyObject *seq;
     PyObject *file_action = NULL;
@@ -7884,7 +7884,8 @@ parse_file_actions(PyObject *file_actions,
                     Py_DECREF(path);
                     goto fail;
                 }
-                Py_DECREF(path);
+                Py_XDECREF(cwd);
+                *cwd = path;
                 break;
             }
 #endif
@@ -7924,6 +7925,7 @@ py_posix_spawn(int use_posix_spawnp, PyObject *module, path_t *path, PyObject *a
     Py_ssize_t argc, envc;
     PyObject *result = NULL;
     PyObject *temp_buffer = NULL;
+    PyObject *cwd = NULL;
     pid_t pid;
     int err_code;
 
@@ -7995,7 +7997,7 @@ py_posix_spawn(int use_posix_spawnp, PyObject *module, path_t *path, PyObject *a
         if (!temp_buffer) {
             goto exit;
         }
-        if (parse_file_actions(file_actions, &file_actions_buf, temp_buffer)) {
+        if (parse_file_actions(file_actions, &file_actions_buf, temp_buffer, &cwd)) {
             goto exit;
         }
         file_actionsp = &file_actions_buf;
@@ -8027,6 +8029,17 @@ py_posix_spawn(int use_posix_spawnp, PyObject *module, path_t *path, PyObject *a
 
     if (err_code) {
         errno = err_code;
+#ifdef HAVE_POSIX_SPAWN_FILE_ACTIONS_ADDCHDIR_NP
+        if (errno == ENOENT && cwd != NULL) {
+            /* ENOENT can occur when either the path of the executable or the
+             * cwd given via file_actions doesn't exist. Since it's not feasible
+             * to determine which of those paths caused the problem, we return
+             * an exception with both. */
+            PyErr_Format(PyExc_FileNotFoundError, "Either '%S' or '%s' doesn't exist.",
+                         path->object, PyBytes_AS_STRING(cwd));
+            goto exit;
+        }
+#endif
         PyErr_SetFromErrnoWithFilenameObject(PyExc_OSError, path->object);
         goto exit;
     }
@@ -8048,6 +8061,7 @@ py_posix_spawn(int use_posix_spawnp, PyObject *module, path_t *path, PyObject *a
     if (argvlist) {
         free_string_array(argvlist, argc);
     }
+    Py_XDECREF(cwd);
     Py_XDECREF(temp_buffer);
     return result;
 }