hgweb: consume generator inside context manager (issue4756)

If code inside a context manager returns a generator, the context
manager exits before the generator is iterated.

hgweb was using a context manager to control thread safe access to a
localrepository instance. But it was returning a generator, so there was
a race condition between a previous request streaming a response to the
client and a new request obtaining the released but in use repository.
By iterating the generator inside the context manager, we ensure we
don't release the repo instance until after the response has finished.

With this change, hgweb finally appears to have full localrepository
isolation between threads. I can no longer reproduce the 2 exceptions
reported in issue4756.

test-hgweb-non-interactive.t has been modified to consume the output
of calling into a WSGI application. Without this, execution of the WSGI
application stalls because of the added yield statement.

mercurial/hgweb/hgweb_mod.py

@@ -275,7 +275,8 b' class hgweb(object):'
         should be using instances of this class as the WSGI application.
         """
         with self._obtainrepo() as repo:
-            return self._runwsgi(req, repo)
+            for r in self._runwsgi(req, repo):
+                yield r
 
     def _runwsgi(self, req, repo):
         rctx = requestcontext(self, repo)

tests/test-hgweb-non-interactive.t

@@ -58,7 +58,8 b' by the WSGI standard and strictly implem'
   > }
   > 
   > i = hgweb('.')
-  > i(env, startrsp)
+  > for c in i(env, startrsp):
+  >     pass
   > print '---- ERRORS'
   > print errors.getvalue()
   > print '---- OS.ENVIRON wsgi variables'