sslutil: document and slightly refactor validation logic

This main purpose of this patch is to make it clearer that fingerprint
pinning takes precedence over CA verification. This will make
subsequent refactoring to the validation code easier to read.

mercurial/sslutil.py

@@ -264,8 +264,6 b' class validator(object):'
 
     def __call__(self, sock, strict=False):
         host = self.host
-        cacerts = self.ui.config('web', 'cacerts')
-        hostfingerprints = self.ui.configlist('hostfingerprints', host)
 
         if not sock.cipher(): # work around http://bugs.python.org/issue13721
             raise error.Abort(_('%s ssl connection error') % host)
@@ -278,6 +276,10 b' class validator(object):'
         if not peercert:
             raise error.Abort(_('%s certificate error: '
                                'no certificate received') % host)
+
+        # If a certificate fingerprint is pinned, use it and only it to
+        # validate the remote cert.
+        hostfingerprints = self.ui.configlist('hostfingerprints', host)
         peerfingerprint = util.sha1(peercert).hexdigest()
         nicefingerprint = ":".join([peerfingerprint[x:x + 2]
             for x in xrange(0, len(peerfingerprint), 2)])
@@ -294,7 +296,11 b' class validator(object):'
                                  hint=_('check hostfingerprint configuration'))
             self.ui.debug('%s certificate matched fingerprint %s\n' %
                           (host, nicefingerprint))
-        elif cacerts != '!':
+            return
+
+        # No pinned fingerprint. Establish trust by looking at the CAs.
+        cacerts = self.ui.config('web', 'cacerts')
+        if cacerts != '!':
             msg = _verifycert(peercert2, host)
             if msg:
                 raise error.Abort(_('%s certificate error: %s') % (host, msg),