[PATCH 1 of 7] sslutil: better document state of security/ssl module

[Gregory Szorc]

# HG changeset patch
# User Gregory Szorc <gregory.szorc at gmail.com>
# Date 1459112826 25200
#      Sun Mar 27 14:07:06 2016 -0700
# Node ID df8f913a95870ed5262e96d8bd68221bf7aef3cd
# Parent  345f4fa4cc8912bb722ad3e35d68858487420bc6
sslutil: better document state of security/ssl module

Pythons older than 2.7.9 are lacking the modern ssl module
and have horrible security. Let's document this explicitly.

diff --git a/mercurial/sslutil.py b/mercurial/sslutil.py
--- a/mercurial/sslutil.py
+++ b/mercurial/sslutil.py
@@ -14,20 +14,30 @@ import ssl
 import sys
 
 from .i18n import _
 from . import (
     error,
     util,
 )
 
+# Python 2.7.9+ overhauled the built-in SSL/TLS features of Python. It added
+# support for TLS 1.1, TLS 1.2, SNI, system CA stores, etc. These features are
+# all exposed via the "ssl" module.
+#
+# Depending on the version of Python being used, SSL/TLS support is either
+# modern/secure or legacy/insecure. Many operations in this module have
+# separate code paths depending on support in Python.
+
 hassni = getattr(ssl, 'HAS_SNI', False)
 
 _canloaddefaultcerts = False
 try:
+    # ssl.SSLContext was added in 2.7.9 and presence indicates modern
+    # SSL/TLS features are available.
     ssl_context = ssl.SSLContext
     _canloaddefaultcerts = util.safehasattr(ssl_context, 'load_default_certs')
 
     def wrapsocket(sock, keyfile, certfile, ui, cert_reqs=ssl.CERT_NONE,
                    ca_certs=None, serverhostname=None):
         # Allow any version of SSL starting with TLSv1 and
         # up. Note that specifying TLSv1 here prohibits use of
         # newer standards (like TLSv1_2), so this is the right way
@@ -53,16 +63,18 @@ try:
         sslsocket = sslcontext.wrap_socket(sock, server_hostname=serverhostname)
         # check if wrap_socket failed silently because socket had been
         # closed
         # - see http://bugs.python.org/issue13721
         if not sslsocket.cipher():
             raise error.Abort(_('ssl connection failed'))
         return sslsocket
 except AttributeError:
+    # We don't have a modern version of the "ssl" module and are running
+    # Python <2.7.9.
     def wrapsocket(sock, keyfile, certfile, ui, cert_reqs=ssl.CERT_NONE,
                    ca_certs=None, serverhostname=None):
         sslsocket = ssl.wrap_socket(sock, keyfile, certfile,
                                     cert_reqs=cert_reqs, ca_certs=ca_certs,
                                     ssl_version=ssl.PROTOCOL_TLSv1)
         # check if wrap_socket failed silently because socket had been
         # closed
         # - see http://bugs.python.org/issue13721