sslutil: introduce a function for determining host-specific settings

This patch marks the beginning of a series that introduces a new,
more configurable, per-host security settings mechanism. Currently,
we have global settings (like web.cacerts and the --insecure argument).
We also have per-host settings via [hostfingerprints].

Global security settings are good for defaults, but they don't
provide the amount of control often wanted. For example, an
organization may want to require a particular CA is used for a
particular hostname.

[hostfingerprints] is nice. But it currently assumes SHA-1.
Furthermore, there is no obvious place to put additional per-host
settings.

Subsequent patches will be introducing new mechanisms for defining
security settings, some on a per-host basis. This commits starts
the transition to that world by introducing the _hostsettings
function. It takes a ui and hostname and returns a dict of security
settings. Currently, it limits itself to returning host fingerprint
info.

We foreshadow the future support of non-SHA1 hashing algorithms
for verifying the host fingerprint by making the "certfingerprints"
key a list of tuples instead of a list of hashes.

We add this dict to the hgstate property on the socket and use it
during socket validation for checking fingerprints. There should be
no change in behavior.

mercurial/sslutil.py

@@ -106,6 +106,23 b' except AttributeError:'
 
             return ssl.wrap_socket(socket, **args)
 
+def _hostsettings(ui, hostname):
+    """Obtain security settings for a hostname.
+
+    Returns a dict of settings relevant to that hostname.
+    """
+    s = {
+        # List of 2-tuple of (hash algorithm, hash).
+        'certfingerprints': [],
+    }
+
+    # Fingerprints from [hostfingerprints] are always SHA-1.
+    for fingerprint in ui.configlist('hostfingerprints', hostname, []):
+        fingerprint = fingerprint.replace(':', '').lower()
+        s['certfingerprints'].append(('sha1', fingerprint))
+
+    return s
+
 def _determinecertoptions(ui, host):
     """Determine certificate options for a connections.
 
@@ -217,6 +234,7 b' def wrapsocket(sock, keyfile, certfile, '
     sslsocket._hgstate = {
         'caloaded': caloaded,
         'hostname': serverhostname,
+        'settings': _hostsettings(ui, serverhostname),
         'ui': ui,
     }
 
@@ -292,6 +310,7 b' def validatesocket(sock, strict=False):'
     """
     host = sock._hgstate['hostname']
     ui = sock._hgstate['ui']
+    settings = sock._hgstate['settings']
 
     try:
         peercert = sock.getpeercert(True)
@@ -305,15 +324,13 b' def validatesocket(sock, strict=False):'
 
     # If a certificate fingerprint is pinned, use it and only it to
     # validate the remote cert.
-    hostfingerprints = ui.configlist('hostfingerprints', host)
     peerfingerprint = util.sha1(peercert).hexdigest()
     nicefingerprint = ":".join([peerfingerprint[x:x + 2]
         for x in xrange(0, len(peerfingerprint), 2)])
-    if hostfingerprints:
+    if settings['certfingerprints']:
         fingerprintmatch = False
-        for hostfingerprint in hostfingerprints:
-            if peerfingerprint.lower() == \
-                    hostfingerprint.replace(':', '').lower():
+        for hash, fingerprint in settings['certfingerprints']:
+            if peerfingerprint.lower() == fingerprint:
                 fingerprintmatch = True
                 break
         if not fingerprintmatch: