# HG changeset patch
# Parent  c852c7d8d681aed89c11fc8e8ea6f66b61f9591d
Issue #25764: Preserve subprocess fork exception when preexec_fn used

diff -r c852c7d8d681 Lib/test/test_subprocess.py
--- a/Lib/test/test_subprocess.py	Sun Nov 29 13:14:00 2015 +0200
+++ b/Lib/test/test_subprocess.py	Mon Nov 30 01:59:53 2015 +0000
@@ -1512,6 +1512,22 @@
             if not enabled:
                 gc.disable()
 
+    def test_preexec_fork_failure(self):
+        # The internal code did not preserve the previous exception when
+        # re-enabling garbage collection
+        try:
+            from resource import getrlimit, setrlimit, RLIMIT_NPROC
+        except ImportError as err:
+            self.skipTest(err)  # RLIMIT_NPROC is specific to Linux and BSD
+        limits = getrlimit(RLIMIT_NPROC)
+        [_, hard] = limits
+        setrlimit(RLIMIT_NPROC, (0, hard))
+        self.addCleanup(setrlimit, RLIMIT_NPROC, limits)
+        # Forking should raise EAGAIN, translated to BlockingIOError
+        with self.assertRaises(BlockingIOError):
+            subprocess.call([sys.executable, '-c', ''],
+                            preexec_fn=lambda: None)
+
     def test_args_string(self):
         # args is a string
         fd, fname = tempfile.mkstemp()
diff -r c852c7d8d681 Misc/NEWS
--- a/Misc/NEWS	Sun Nov 29 13:14:00 2015 +0200
+++ b/Misc/NEWS	Mon Nov 30 01:59:53 2015 +0000
@@ -104,6 +104,9 @@
 Library
 -------
 
+- Issue #25764: In the subprocess module, preserve any exception caused by
+  fork() failure when preexec_fn is used.
+
 - Issue #10131: Fixed deep copying of minidom documents.  Based on patch
   by Marian Ganisin.
 
diff -r c852c7d8d681 Modules/_posixsubprocess.c
--- a/Modules/_posixsubprocess.c	Sun Nov 29 13:14:00 2015 +0200
+++ b/Modules/_posixsubprocess.c	Mon Nov 30 01:59:53 2015 +0000
@@ -47,17 +47,28 @@
 #define POSIX_CALL(call)   do { if ((call) == -1) goto error; } while (0)
 
 
-/* Given the gc module call gc.enable() and return 0 on success. */
+/* If gc was disabled, call gc.enable().  Release the gc module.  Return 0 on
+success. */
 static int
-_enable_gc(PyObject *gc_module)
+_cleanup_gc(int need_to_reenable_gc, PyObject *gc_module)
 {
     PyObject *result;
     _Py_IDENTIFIER(enable);
+    PyObject *exctype, *val, *tb;
 
-    result = _PyObject_CallMethodId(gc_module, &PyId_enable, NULL);
-    if (result == NULL)
-        return 1;
-    Py_DECREF(result);
+    if (need_to_reenable_gc) {
+        PyErr_Fetch(&exctype, &val, &tb);
+        result = _PyObject_CallMethodId(gc_module, &PyId_enable, NULL);
+        if (exctype != NULL) {
+            PyErr_Restore(exctype, val, tb);
+        }
+        if (result == NULL) {
+            Py_DECREF(gc_module);
+            return 1;
+        }
+        Py_DECREF(result);
+    }
+    Py_XDECREF(gc_module);
     return 0;
 }
 
@@ -708,14 +719,12 @@
     if (argv)
         _Py_FreeCharPArray(argv);
     _Py_FreeCharPArray(exec_array);
+    Py_XDECREF(preexec_fn_args_tuple);
 
     /* Reenable gc in the parent process (or if fork failed). */
-    if (need_to_reenable_gc && _enable_gc(gc_module)) {
-        Py_XDECREF(gc_module);
+    if (_cleanup_gc(need_to_reenable_gc, gc_module)) {
         return NULL;
     }
-    Py_XDECREF(preexec_fn_args_tuple);
-    Py_XDECREF(gc_module);
 
     if (pid == -1)
         return NULL;  /* fork() failed.  Exception set earlier. */
@@ -736,15 +745,7 @@
     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) {
-        PyObject *exctype, *val, *tb;
-        PyErr_Fetch(&exctype, &val, &tb);
-        _enable_gc(gc_module);
-        PyErr_Restore(exctype, val, tb);
-    }
-    Py_XDECREF(gc_module);
+    _cleanup_gc(need_to_reenable_gc, gc_module);
     return NULL;
 }