sslutil: add a config knob to support TLS (default) or SSLv23 (bc) (issue4038)

Prior to this change, we default to SSLv23, which is insecure because
it allows use of SSLv2. Unfortunately, there's no constant for OpenSSL
to let us use SSLv3 or TLS - we have to pick one or the other. We
expose a knob to revert to pre-TLS SSL for the user that has an
ancient server that lacks proper TLS support.

mercurial/sslutil.py

@@ -14,10 +14,13 b' try:'
     # avoid using deprecated/broken FakeSocket in python 2.6
     import ssl
     CERT_REQUIRED = ssl.CERT_REQUIRED
-    def ssl_wrap_socket(sock, keyfile, certfile,
+    PROTOCOL_SSLv23 = ssl.PROTOCOL_SSLv23
+    PROTOCOL_TLSv1 = ssl.PROTOCOL_TLSv1
+    def ssl_wrap_socket(sock, keyfile, certfile, ssl_version=PROTOCOL_TLSv1,
                 cert_reqs=ssl.CERT_NONE, ca_certs=None):
         sslsocket = ssl.wrap_socket(sock, keyfile, certfile,
-                cert_reqs=cert_reqs, ca_certs=ca_certs)
+                                    cert_reqs=cert_reqs, ca_certs=ca_certs,
+                                    ssl_version=ssl_version)
         # check if wrap_socket failed silently because socket had been closed
         # - see http://bugs.python.org/issue13721
         if not sslsocket.cipher():
@@ -26,9 +29,12 b' try:'
 except ImportError:
     CERT_REQUIRED = 2
 
+    PROTOCOL_SSLv23 = 2
+    PROTOCOL_TLSv1 = 3
+
     import socket, httplib
 
-    def ssl_wrap_socket(sock, key_file, cert_file,
+    def ssl_wrap_socket(sock, key_file, cert_file, ssl_version=PROTOCOL_TLSv1,
                         cert_reqs=CERT_REQUIRED, ca_certs=None):
         if not util.safehasattr(socket, 'ssl'):
             raise util.Abort(_('Python SSL support not found'))
@@ -84,15 +90,22 b' def _verifycert(cert, hostname):'
 
 def sslkwargs(ui, host):
     cacerts = ui.config('web', 'cacerts')
+    forcetls = ui.configbool('ui', 'tls', default=True)
+    if forcetls:
+        ssl_version = PROTOCOL_TLSv1
+    else:
+        ssl_version = PROTOCOL_SSLv23
     hostfingerprint = ui.config('hostfingerprints', host)
+    kws = {'ssl_version': ssl_version,
+           }
     if cacerts and not hostfingerprint:
         cacerts = util.expandpath(cacerts)
         if not os.path.exists(cacerts):
             raise util.Abort(_('could not find web.cacerts: %s') % cacerts)
-        return {'ca_certs': cacerts,
+        kws.update({'ca_certs': cacerts,
                 'cert_reqs': CERT_REQUIRED,
-                }
-    return {}
+                    })
+    return kws
 
 class validator(object):
     def __init__(self, ui, host):