Fixed #24704 -- Made the autoreloader survive SyntaxErrors.

With this change, it's expected to survive anything except errors
that make it impossible to import the settings. It's too complex
to fallback to a sensible behavior with a broken settings module.

Harcoding things about runserver in ManagementUtility.execute is
atrocious but it's the only way out of the chicken'n'egg problem:
the current implementation of the autoreloader primarily watches
imported Python modules -- and then a few other things that were
bolted on top of this design -- but we want it to kick in even if
the project contains import-time errors and django.setup() fails.

At some point we should throw away this code and replace it by an
off-the-shelf autoreloader that watches the working directory and
re-runs `django-admin runserver` whenever something changes.

Backport of fe6ddb837d from master

1 files changed, 12 insertions, 1 deletions

diff --git a/django/utils/autoreload.py b/django/utils/autoreload.py
index 869020359a..a07fb04abf 100644
--- a/django/utils/autoreload.py
+++ b/django/utils/autoreload.py
@@ -40,6 +40,7 @@ import traceback
 from django.apps import apps
 from django.conf import settings
 from django.core.signals import request_finished
+from django.utils import six
 from django.utils._os import npath
 from django.utils.six.moves import _thread as thread
 
@@ -75,6 +76,7 @@ I18N_MODIFIED = 2
 _mtimes = {}
 _win = (sys.platform == "win32")
 
+_exception = None
 _error_files = []
 _cached_modules = set()
 _cached_filenames = []
@@ -222,11 +224,14 @@ def code_changed():
 
 def check_errors(fn):
     def wrapper(*args, **kwargs):
+        global _exception
         try:
             fn(*args, **kwargs)
         except (ImportError, IndentationError, NameError, SyntaxError,
                 TypeError, AttributeError):
-            et, ev, tb = sys.exc_info()
+            _exception = sys.exc_info()
+
+            et, ev, tb = _exception
 
             if getattr(ev, 'filename', None) is None:
                 # get the filename from the last item in the stack
@@ -242,6 +247,12 @@ def check_errors(fn):
     return wrapper
 
 
+def raise_last_exception():
+    global _exception
+    if _exception is not None:
+        six.reraise(*_exception)
+
+
 def ensure_echo_on():
     if termios:
         fd = sys.stdin