hgweb: establish class for holding per request context

hgweb: establish class for holding per request context

Currently, hgweb applications have many instance variables holding
mutated state. This is somewhat problematic because multiple threads
may race accessing or changing this state.

This patch starts a series that will add more thread safety to
hgweb applications. It will do this by moving mutated state out
of hgweb and into per-request instances of the newly established
"requestcontext" class.

Our new class currently behaves like a proxy to hgweb instances. This
should change once all state is captured in it instead of hgweb. The
effectiveness of this proxy is demonstrated by passing instances of
it - not hgweb instances/self - to various functions.

--- a/mercurial/hgweb/hgweb_mod.py	Sat Aug 22 14:22:40 2015 -0700
+++ b/mercurial/hgweb/hgweb_mod.py	Sat Aug 22 14:59:36 2015 -0700
@@ -61,6 +61,25 @@
     return reversed(breadcrumb)
 
 
+class requestcontext(object):
+    """Holds state/context for an individual request.
+
+    Servers can be multi-threaded. Holding state on the WSGI application
+    is prone to race conditions. Instances of this class exist to hold
+    mutable and race-free state for requests.
+    """
+    def __init__(self, app):
+        object.__setattr__(self, 'app', app)
+        object.__setattr__(self, 'repo', app.repo)
+
+    # Proxy unknown reads and writes to the application instance
+    # until everything is moved to us.
+    def __getattr__(self, name):
+        return getattr(self.app, name)
+
+    def __setattr__(self, name, value):
+        return setattr(self.app, name, value)
+
 class hgweb(object):
     """HTTP server for individual repositories.
 
@@ -193,6 +212,7 @@
         should be using instances of this class as the WSGI application.
         """
         self.refresh(req)
+        rctx = requestcontext(self)
 
         # work with CGI variables to create coherent structure
         # use SCRIPT_NAME, PATH_INFO and QUERY_STRING as well as our REPO_NAME
@@ -223,7 +243,7 @@
                 if query:
                     raise ErrorResponse(HTTP_NOT_FOUND)
                 if cmd in perms:
-                    self.check_perm(req, perms[cmd])
+                    self.check_perm(rctx, req, perms[cmd])
                 return protocol.call(self.repo, req, cmd)
             except ErrorResponse as inst:
                 # A client that sends unbundle without 100-continue will
@@ -284,7 +304,7 @@
 
             # check read permissions non-static content
             if cmd != 'static':
-                self.check_perm(req, None)
+                self.check_perm(rctx, req, None)
 
             if cmd == '':
                 req.form['cmd'] = [tmpl.cache['default']]
@@ -297,9 +317,9 @@
                 raise ErrorResponse(HTTP_BAD_REQUEST, msg)
             elif cmd == 'file' and 'raw' in req.form.get('style', []):
                 self.ctype = ctype
-                content = webcommands.rawfile(self, req, tmpl)
+                content = webcommands.rawfile(rctx, req, tmpl)
             else:
-                content = getattr(webcommands, cmd)(self, req, tmpl)
+                content = getattr(webcommands, cmd)(rctx, req, tmpl)
                 req.respond(HTTP_OK, ctype)
 
             return content
@@ -442,6 +462,6 @@
         'zip': ('application/zip', 'zip', '.zip', None),
         }
 
-    def check_perm(self, req, op):
+    def check_perm(self, rctx, req, op):
         for permhook in permhooks:
-            permhook(self, req, op)
+            permhook(rctx, req, op)