server: always close http socket if responding with an error (issue6033)

It's possible for hgweb to respond _very_ early with an error if we're
catching certain types of errors. When we do, we need to tell the client
the socket is toast when there's a POST involved because otherwise there
can be lingering POST data on the socket that will confuse any future
requests on the socket. This manifested as a flaky failure on Linux in an
lfs extension test and a reliable failure on FreeBSD. With this patch
applied, test-lfs-serve-access.t now passes for me on FreeBSD.

Differential Revision: https://phab.mercurial-scm.org/D5498

mercurial/hgweb/server.py

@@ -127,6 +127,13 b' class _httprequesthandler(httpservermod.'
             and not path.startswith(self.server.prefix + b'/')):
             self._start_response(pycompat.strurl(common.statusmessage(404)),
                                  [])
+            if self.command == 'POST':
+                # Paranoia: tell the client we're going to close the
+                # socket so they don't try and reuse a socket that
+                # might have a POST body waiting to confuse us. We do
+                # this by directly munging self.saved_headers because
+                # self._start_response ignores Connection headers.
+                self.saved_headers = [(r'Connection', r'Close')]
             self._write(b"Not Found")
             self._done()
             return