hgweb: inline caching() and port to modern mechanisms

hgweb: inline caching() and port to modern mechanisms

We only had one consumer of this simple function. While it could be
a generic function, let's not over abstract the code.

As part of inlining, we port it off wsgirequest, fix some Python 3
issues, and set a response header on our new response object so it
is ready once we start using it to send responses.

Differential Revision: https://phab.mercurial-scm.org/D2785

--- a/mercurial/hgweb/common.py	Sat Mar 10 14:06:58 2018 -0800
+++ b/mercurial/hgweb/common.py	Sat Mar 10 14:19:27 2018 -0800
@@ -214,12 +214,6 @@
             config("ui", "username") or
             encoding.environ.get("EMAIL") or "")
 
-def caching(web, req):
-    tag = r'W/"%d"' % web.mtime
-    if req.env.get('HTTP_IF_NONE_MATCH') == tag:
-        raise ErrorResponse(HTTP_NOT_MODIFIED)
-    req.headers.append(('ETag', tag))
-
 def cspvalues(ui):
     """Obtain the Content-Security-Policy header and nonce value.
 

--- a/mercurial/hgweb/hgweb_mod.py	Sat Mar 10 14:06:58 2018 -0800
+++ b/mercurial/hgweb/hgweb_mod.py	Sat Mar 10 14:19:27 2018 -0800
@@ -18,7 +18,6 @@
     HTTP_NOT_MODIFIED,
     HTTP_OK,
     HTTP_SERVER_ERROR,
-    caching,
     cspvalues,
     permhooks,
 )
@@ -388,7 +387,13 @@
             # Don't enable caching if using a CSP nonce because then it wouldn't
             # be a nonce.
             if rctx.configbool('web', 'cache') and not rctx.nonce:
-                caching(self, wsgireq) # sets ETag header or raises NOT_MODIFIED
+                tag = 'W/"%d"' % self.mtime
+                if req.headers.get('If-None-Match') == tag:
+                    raise ErrorResponse(HTTP_NOT_MODIFIED)
+
+                wsgireq.headers.append((r'ETag', pycompat.sysstr(tag)))
+                res.headers['ETag'] = tag
+
             if cmd not in webcommands.__all__:
                 msg = 'no such method: %s' % cmd
                 raise ErrorResponse(HTTP_BAD_REQUEST, msg)