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

django/core/management/__init__.py

@@ -13,7 +13,7 @@ from django.core.exceptions import ImproperlyConfigured
 from django.core.management.base import (BaseCommand, CommandError,
     CommandParser, handle_default_options)
 from django.core.management.color import color_style
-from django.utils import lru_cache, six
+from django.utils import autoreload, lru_cache, six
 from django.utils._os import npath, upath
 
 
@@ -309,6 +309,19 @@ class ManagementUtility(object):
                 settings.configure()
 
         if settings.configured:
+            # Start the auto-reloading dev server even if the code is broken.
+            # The hardcoded condition is a code smell but we can't rely on a
+            # flag on the command class because we haven't located it yet.
+            if subcommand == 'runserver' and '--noreload' not in self.argv:
+                try:
+                    autoreload.check_errors(django.setup)()
+                except Exception:
+                    # The exception will be raised later in the child process
+                    # started by the autoreloader.
+                    pass
+
+            # In all other cases, django.setup() is required to succeed.
+            else:
                 django.setup()
 
         self.autocomplete()

django/core/management/commands/runserver.py

@@ -102,6 +102,10 @@ class Command(BaseCommand):
         from django.conf import settings
         from django.utils import translation
 
+        # If an exception was silenced in ManagementUtility.execute in order
+        # to be raised in the child process, raise it now.
+        autoreload.raise_last_exception()
+
         threading = options.get('use_threading')
         shutdown_message = options.get('shutdown_message', '')
         quit_command = 'CTRL-BREAK' if sys.platform == 'win32' else 'CONTROL-C'

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

docs/releases/1.8.5.txt

@@ -9,6 +9,8 @@ Django 1.8.5 fixes several bugs in 1.8.4.
 Bugfixes
 ========
 
+* Made the development server's autoreload more robust (:ticket:`24704`).
+
 * Fixed ``AssertionError`` in some delete queries with a model containing a
   field that is both a foreign and primary key (:ticket:`24951`).