diff --git a/Misc/NEWS.d/next/Library/2018-09-13-03-59-43.bpo-34658.ykZ-ia.rst b/Misc/NEWS.d/next/Library/2018-09-13-03-59-43.bpo-34658.ykZ-ia.rst
new file mode 100644
index 00000000000000..35375a088300cc
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2018-09-13-03-59-43.bpo-34658.ykZ-ia.rst
@@ -0,0 +1,3 @@
+Fix a rare interpreter unhandled exception state SystemError only seen when
+using subprocess with a preexec_fn while an after_parent handler has been
+registered with os.register_at_fork and the fork system call fails.
diff --git a/Modules/_posixsubprocess.c b/Modules/_posixsubprocess.c
index aeb10f9ecfe47c..dd69b9eb1ff6c9 100644
--- a/Modules/_posixsubprocess.c
+++ b/Modules/_posixsubprocess.c
@@ -564,6 +564,7 @@ subprocess_fork_exec(PyObject* self, PyObject *args)
char *const *exec_array, *const *argv = NULL, *const *envp = NULL;
Py_ssize_t arg_num;
int need_after_fork = 0;
+ int saved_errno = 0;
if (!PyArg_ParseTuple(
args, "OOpO!OOiiiiiiiiiiO:fork_exec",
@@ -700,14 +701,14 @@ subprocess_fork_exec(PyObject* self, PyObject *args)
_exit(255);
return NULL; /* Dead code to avoid a potential compiler warning. */
}
- Py_XDECREF(cwd_obj2);
-
+ /* Parent (original) process */
if (pid == -1) {
- /* Capture the errno exception before errno can be clobbered. */
- PyErr_SetFromErrno(PyExc_OSError);
+ /* Capture errno for the exception. */
+ saved_errno = errno;
}
- /* Parent process */
+ Py_XDECREF(cwd_obj2);
+
if (need_after_fork)
PyOS_AfterFork_Parent();
if (envp)
@@ -723,8 +724,13 @@ subprocess_fork_exec(PyObject* self, PyObject *args)
Py_XDECREF(preexec_fn_args_tuple);
Py_XDECREF(gc_module);
- if (pid == -1)
- return NULL; /* fork() failed. Exception set earlier. */
+ if (pid == -1) {
+ errno = saved_errno;
+ /* We can't call this above as PyOS_AfterFork_Parent() calls back
+ * into Python code which would see the unreturned error. */
+ PyErr_SetFromErrno(PyExc_OSError);
+ return NULL; /* fork() failed. */
+ }
return PyLong_FromPid(pid);
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