hgweb: refactor 304 handling code

We had generic code in wsgirequest for handling HTTP 304 responses.
We also had a special case for it in the catch all exception handler
in the WSGI application.

We only ever raise 304 in one place. So, we don't need to treat it
specially in the catch all exception handler.

But it is useful to validate behavior of 304 responses. We port the
code that sends a 304 to use the new response API. We then move the
code for screening 304 sanity into the new response API.

As part of doing so, we discovered that we would send
Content-Length: 0. This is not allowed. So, we fix our response code
to not emit that header for empty response bodies.

Differential Revision: https://phab.mercurial-scm.org/D2794

mercurial/hgweb/hgweb_mod.py

@@ -15,7 +15,6 b' from .common import ('
     ErrorResponse,
     HTTP_BAD_REQUEST,
     HTTP_NOT_FOUND,
-    HTTP_NOT_MODIFIED,
     HTTP_OK,
     HTTP_SERVER_ERROR,
     cspvalues,
@@ -391,7 +390,10 b' class hgweb(object):'
             if rctx.configbool('web', 'cache') and not rctx.nonce:
                 tag = 'W/"%d"' % self.mtime
                 if req.headers.get('If-None-Match') == tag:
-                    raise ErrorResponse(HTTP_NOT_MODIFIED)
+                    res.status = '304 Not Modified'
+                    # Response body not allowed on 304.
+                    res.setbodybytes('')
+                    return res.sendresponse()
 
                 wsgireq.headers.append((r'ETag', pycompat.sysstr(tag)))
                 res.headers['ETag'] = tag
@@ -426,9 +428,6 b' class hgweb(object):'
             return tmpl('error', error=pycompat.bytestr(inst))
         except ErrorResponse as inst:
             wsgireq.respond(inst, ctype)
-            if inst.code == HTTP_NOT_MODIFIED:
-                # Not allowed to return a body on a 304
-                return ['']
             return tmpl('error', error=pycompat.bytestr(inst))
 
     def check_perm(self, rctx, req, op):

mercurial/hgweb/request.py

@@ -15,7 +15,6 b' import wsgiref.headers as wsgiheaders'
 
 from .common import (
     ErrorResponse,
-    HTTP_NOT_MODIFIED,
     statusmessage,
 )
 
@@ -361,7 +360,10 b' class wsgiresponse(object):'
             raise error.ProgrammingError('cannot define body multiple times')
 
     def setbodybytes(self, b):
-        """Define the response body as static bytes."""
+        """Define the response body as static bytes.
+
+        The empty string signals that there is no response body.
+        """
         self._verifybody()
         self._bodybytes = b
         self.headers['Content-Length'] = '%d' % len(b)
@@ -408,6 +410,35 b' class wsgiresponse(object):'
             and not self._bodywillwrite):
             raise error.ProgrammingError('response body not defined')
 
+        # RFC 7232 Section 4.1 states that a 304 MUST generate one of
+        # {Cache-Control, Content-Location, Date, ETag, Expires, Vary}
+        # and SHOULD NOT generate other headers unless they could be used
+        # to guide cache updates. Furthermore, RFC 7230 Section 3.3.2
+        # states that no response body can be issued. Content-Length can
+        # be sent. But if it is present, it should be the size of the response
+        # that wasn't transferred.
+        if self.status.startswith('304 '):
+            # setbodybytes('') will set C-L to 0. This doesn't conform with the
+            # spec. So remove it.
+            if self.headers.get('Content-Length') == '0':
+                del self.headers['Content-Length']
+
+            # Strictly speaking, this is too strict. But until it causes
+            # problems, let's be strict.
+            badheaders = {k for k in self.headers.keys()
+                          if k.lower() not in ('date', 'etag', 'expires',
+                                               'cache-control',
+                                               'content-location',
+                                               'vary')}
+            if badheaders:
+                raise error.ProgrammingError(
+                    'illegal header on 304 response: %s' %
+                    ', '.join(sorted(badheaders)))
+
+            if self._bodygen is not None or self._bodywillwrite:
+                raise error.ProgrammingError("must use setbodybytes('') with "
+                                             "304 responses")
+
         # Various HTTP clients (notably httplib) won't read the HTTP response
         # until the HTTP request has been sent in full. If servers (us) send a
         # response before the HTTP request has been fully sent, the connection
@@ -539,13 +570,6 b' class wsgirequest(object):'
 
             if isinstance(status, ErrorResponse):
                 self.headers.extend(status.headers)
-                if status.code == HTTP_NOT_MODIFIED:
-                    # RFC 2616 Section 10.3.5: 304 Not Modified has cases where
-                    # it MUST NOT include any headers other than these and no
-                    # body
-                    self.headers = [(k, v) for (k, v) in self.headers if
-                                    k in ('Date', 'ETag', 'Expires',
-                                          'Cache-Control', 'Vary')]
                 status = statusmessage(status.code, pycompat.bytestr(status))
             elif status == 200:
                 status = '200 Script output follows'