Issue #15001: fix segfault on "del sys.modules['__main__']"

Patch by Victor Stinner.
diff --git a/Lib/test/test_cmd_line.py b/Lib/test/test_cmd_line.py
index 05e29d4..d12caeb 100644
--- a/Lib/test/test_cmd_line.py
+++ b/Lib/test/test_cmd_line.py
@@ -2,9 +2,12 @@
 # All tests are executed with environment variables ignored
 # See test_cmd_line_script.py for testing of script execution
 
-import test.test_support, unittest
+import test.test_support
 import sys
-from test.script_helper import spawn_python, kill_python, python_exit_code
+import unittest
+from test.script_helper import (
+    assert_python_ok, spawn_python, kill_python, python_exit_code
+)
 
 
 class CmdLineTest(unittest.TestCase):
@@ -101,6 +104,18 @@
         data = self.start_python('-R', '-c', code)
         self.assertTrue('hash_randomization=1' in data)
 
+    def test_del___main__(self):
+        # Issue #15001: PyRun_SimpleFileExFlags() did crash because it kept a
+        # borrowed reference to the dict of __main__ module and later modify
+        # the dict whereas the module was destroyed
+        filename = test.test_support.TESTFN
+        self.addCleanup(test.test_support.unlink, filename)
+        with open(filename, "w") as script:
+            print >>script, "import sys"
+            print >>script, "del sys.modules['__main__']"
+        assert_python_ok(filename)
+
+
 def test_main():
     test.test_support.run_unittest(CmdLineTest)
     test.test_support.reap_children()
diff --git a/Misc/NEWS b/Misc/NEWS
index b2ad14b..16ddf41 100644
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -9,6 +9,9 @@
 Core and Builtins
 -----------------
 
+- Issue #15001: fix segfault on "del sys.modules['__main__']". Patch by Victor
+  Stinner.
+
 - Issue #5057: the peepholer no longer optimizes subscription on unicode
   literals (e.g. u'foo'[0]) in order to produce compatible pyc files between
   narrow and wide builds.
diff --git a/Python/pythonrun.c b/Python/pythonrun.c
index afb4c51..a2e1e74 100644
--- a/Python/pythonrun.c
+++ b/Python/pythonrun.c
@@ -907,19 +907,20 @@
 {
     PyObject *m, *d, *v;
     const char *ext;
-    int set_file_name = 0, ret, len;
+    int set_file_name = 0, len, ret = -1;
 
     m = PyImport_AddModule("__main__");
     if (m == NULL)
         return -1;
+    Py_INCREF(m);
     d = PyModule_GetDict(m);
     if (PyDict_GetItemString(d, "__file__") == NULL) {
         PyObject *f = PyString_FromString(filename);
         if (f == NULL)
-            return -1;
+            goto done;
         if (PyDict_SetItemString(d, "__file__", f) < 0) {
             Py_DECREF(f);
-            return -1;
+            goto done;
         }
         set_file_name = 1;
         Py_DECREF(f);
@@ -932,7 +933,6 @@
             fclose(fp);
         if ((fp = fopen(filename, "rb")) == NULL) {
             fprintf(stderr, "python: Can't reopen .pyc file\n");
-            ret = -1;
             goto done;
         }
         /* Turn on optimization if a .pyo file is given */
@@ -945,7 +945,6 @@
     }
     if (v == NULL) {
         PyErr_Print();
-        ret = -1;
         goto done;
     }
     Py_DECREF(v);
@@ -955,6 +954,7 @@
   done:
     if (set_file_name && PyDict_DelItemString(d, "__file__"))
         PyErr_Clear();
+    Py_DECREF(m);
     return ret;
 }