hgweb: skip body creation of HEAD for most requests

The body is thrown away anyway, so this just wastes a lot of CPU time.
In the case of /archive/, this skips manifest processing and the actual
file archiving, resulting in a huge difference.

The most tricky part here is skipping the Content-Length creation as it
would indicate the output size for the corresponding GET request.

mercurial/hgweb/hgweb_mod.py

@@ -230,6 +230,7 b' class requestcontext:'
 
     def sendtemplate(self, name, **kwargs):
         """Helper function to send a response generated from a template."""
+        if self.req.method != b'HEAD':
         kwargs = pycompat.byteskwargs(kwargs)
         self.res.setbodygen(self.tmpl.generate(name, kwargs))
         return self.res.sendresponse()

mercurial/hgweb/request.py

@@ -485,6 +485,7 b' class wsgiresponse:'
             self._bodybytes is None
             and self._bodygen is None
             and not self._bodywillwrite
+            and self._req.method != b'HEAD'
         ):
             raise error.ProgrammingError(b'response body not defined')
 
@@ -594,6 +595,8 b' class wsgiresponse:'
                 yield chunk
         elif self._bodywillwrite:
             self._bodywritefn = write
+        elif self._req.method == b'HEAD':
+            pass
         else:
             error.ProgrammingError(b'do not know how to send body')
 

mercurial/hgweb/server.py

@@ -151,6 +151,9 b' class _httprequesthandler(httpservermod.'
     def do_GET(self):
         self.do_POST()
 
+    def do_HEAD(self):
+        self.do_POST()
+
     def do_hgweb(self):
         self.sent_headers = False
         path, query = _splitURI(self.path)
@@ -246,7 +249,11 b' class _httprequesthandler(httpservermod.'
             self.send_header(*h)
             if h[0].lower() == 'content-length':
                 self.length = int(h[1])
-        if self.length is None and saved_status[0] != common.HTTP_NOT_MODIFIED:
+        if (
+            self.length is None
+            and saved_status[0] != common.HTTP_NOT_MODIFIED
+            and self.command != 'HEAD'
+        ):
             self._chunked = (
                 not self.close_connection and self.request_version == 'HTTP/1.1'
             )

mercurial/hgweb/webcommands.py

@@ -1299,6 +1299,9 b' def archive(web):'
             b'sendresponse() should not emit data if writing later'
         )
 
+    if web.req.method == b'HEAD':
+        return []
+
     bodyfh = web.res.getbodyfile()
 
     archival.archive(