Refs #28593 -- Made URLResolver._populate() more resilient to signal interrupts.

_populate() sets the populating attribute to prevent infinite recursion in
case a urlconf includes itself. The flag is a threadlocal to avoid a race
condition [1] where one thread sets the flag and another checks it, then
proceeds to access data that's supposed to be populated (e.g. _reverse_dict)
but isn't yet.

The potential still exists for a thread to set the threadlocal, then be
interrupted by a signal such as SIGALRM and raise before resetting the
threadlocal flag. In this scenario, subsequent calls to _populate() in the
same thread will short-circuit erroneously.

The bulk of the method was already wrapped in a try/finally in df41b5a, but
since a signal interrupt can occur at any line executed by the interpreter,
this moves up the try to ensure threadlocal gets reset.

[1]: https://groups.google.com/d/msg/django-developers/D_bIeinKHjE/4NmVQUJqAgAJ

django/urls/resolvers.py

@@ -398,12 +398,12 @@ class URLResolver:
         # thread-local variable.
         if getattr(self._local, 'populating', False):
             return
+        try:
             self._local.populating = True
             lookups = MultiValueDict()
             namespaces = {}
             apps = {}
             language_code = get_language()
-        try:
             for url_pattern in reversed(self.url_patterns):
                 p_pattern = url_pattern.pattern.regex.pattern
                 if p_pattern.startswith('^'):