sslutil: fall back to commonName when no dNSName in subjectAltName (issue2798)

Any entries in subjectAltName would prevent fallback to using commonName, but
RFC 2818 says:

    If a subjectAltName extension of type dNSName is present, that MUST
    be used as the identity. Otherwise, the (most specific) Common Name
    field in the Subject field of the certificate MUST be used.

We now only consider dNSNames in subjectAltName.

(dNSName is known as 'DNS' in OpenSSL/Python.)

mercurial/sslutil.py

@@ -48,6 +48,7 def _verifycert(cert, hostname):
         for name in certnames:
             if matchdnsname(name):
                 return None
+        if certnames:
         return _('certificate is for %s') % ', '.join(certnames)
 
     # subject is only checked when subjectAltName is empty

tests/test-url.py

@@ -33,9 +33,13 check(_verifycert(san_cert, 'example.net
       None)
 check(_verifycert(san_cert, 'foo.example.net'),
       None)
-# subject is only checked when subjectAltName is empty
+# no fallback to subject commonName when subjectAltName has DNS
 check(_verifycert(san_cert, 'example.com'),
       'certificate is for *.example.net, example.net')
+# fallback to subject commonName when no DNS in subjectAltName
+san_cert = {'subject': ((('commonName', 'example.com'),),),
+            'subjectAltName': (('IP Address', '8.8.8.8'),)}
+check(_verifycert(san_cert, 'example.com'), None)
 
 # Avoid some pitfalls
 check(_verifycert(cert('*.foo'), 'foo'),
@@ -49,6 +53,10 check(_verifycert({'subject': ()},
 check(_verifycert(None, 'example.com'),
       'no certificate received')
 
+# Unicode (IDN) certname isn't supported
+check(_verifycert(cert(u'\u4f8b.jp'), 'example.jp'),
+      'IDN in certificate not supported')
+
 import doctest
 
 def test_url():
@@ -211,7 +219,3 def test_url():
     """
 
 doctest.testmod(optionflags=doctest.NORMALIZE_WHITESPACE)
-
-# Unicode (IDN) certname isn't supported
-check(_verifycert(cert(u'\u4f8b.jp'), 'example.jp'),
-      'IDN in certificate not supported')