wireproto: add config knob for http header length limit

Well-behaved Mercurial clients will respect the httpheader capability by not
sending http headers longer than the given limit in bytes. The limit is
currently hard-coded at 1024 bytes, a safe value for any web server.

Since parsing headers is a notable factor in web server performance, tuning
header size can nontrivially improve performance for request-heavy operations
(eg. obsolete marker negotiation). Exposing the maximum header length limit
as a configuration setting is a simple way to enable such tuning.

mercurial/help/config.txt

@@ -1291,6 +1291,10 b' Controls generic server settings.'
     checking that all new file revisions specified in manifests are
     present. Default is False.
 
+``maxhttpheaderlen``
+    Instruct HTTP clients not to send request headers longer than this
+    many bytes. Default is 1024.
+
 ``smtp``
 --------
 

mercurial/wireproto.py

@@ -624,7 +624,8 b' def _capabilities(repo, proto):'
         capsblob = bundle2.encodecaps(bundle2.getrepocaps(repo))
         caps.append('bundle2=' + urllib.quote(capsblob))
     caps.append('unbundle=%s' % ','.join(changegroupmod.bundlepriority))
-    caps.append('httpheader=1024')
+    caps.append(
+        'httpheader=%d' % repo.ui.configint('server', 'maxhttpheaderlen', 1024))
     return caps
 
 # If you are writing an extension and consider wrapping this function. Wrap