bpo-35925: Skip SSL server tests that fail due to weak external certs.

Modern Linux distros such as Debian Buster have default OpenSSL system configurations that reject connections to servers with weak certificates by default. This causes our test suite run with external networking resources enabled to skip these tests when they encounter such a failure. Fixing the network servers being used is a separate issue.
python · gpshead · May 6, 2019 · May 6, 2019 · May 6, 2019 · May 6, 2019
commit fe016a21a1739233518f59e12ee1a671de032cc5
diff --git a/Lib/test/test_httplib.py b/Lib/test/test_httplib.py
@@ -4,6 +4,7 @@
 import itertools
 import os
 import array
+import re
 import socket
 import threading
 
@@ -1619,14 +1620,30 @@ def test_networked_good_cert(self):
         # We feed the server's cert as a validating cert
         import ssl
         support.requires('network')
-        with support.transient_internet('self-signed.pythontest.net'):
+        selfsigned_pythontestdotnet = 'self-signed.pythontest.net'
+        with support.transient_internet(selfsigned_pythontestdotnet):
             context = ssl.SSLContext(ssl.PROTOCOL_TLS_CLIENT)
             self.assertEqual(context.verify_mode, ssl.CERT_REQUIRED)
             self.assertEqual(context.check_hostname, True)
             context.load_verify_locations(CERT_selfsigned_pythontestdotnet)
-            h = client.HTTPSConnection('self-signed.pythontest.net', 443, context=context)
-            h.request('GET', '/')
-            resp = h.getresponse()
+            try:
+                h = client.HTTPSConnection(selfsigned_pythontestdotnet, 443,
+                                           context=context)
+                h.request('GET', '/')
+                resp = h.getresponse()
+            except ssl.SSLError as ssl_err:
+                ssl_err_str = str(ssl_err)
+                # In the error message of [SSL: CERTIFICATE_VERIFY_FAILED] on
+                # modern Linux distros (Debian Buster, etc) default OpenSSL
+                # configurations it'll fail saying "key too weak" until we
+                # address https://bugs.python.org/issue36816 to use a proper
+                # key size on self-signed.pythontest.net.
+                if re.search(r'(?i)key.too.weak', ssl_err_str):
+                    raise unittest.SkipTest(
+                        f'Got {ssl_err_str} trying to connect '
+                        f'to {selfsigned_pythontestdotnet}. '
+                        'See https://bugs.python.org/issue36816.')
+                raise
             server_string = resp.getheader('server')
             resp.close()
             h.close()

diff --git a/Lib/test/test_nntplib.py b/Lib/test/test_nntplib.py
@@ -6,6 +6,7 @@
 import functools
 import contextlib
 import os.path
+import re
 import threading
 
 from test import support
@@ -21,6 +22,13 @@
 TIMEOUT = 30
 certfile = os.path.join(os.path.dirname(__file__), 'keycert3.pem')
 
+if ssl is not None:
+    SSLError = ssl.SSLError
+else:
+    class SSLError(Exception):
+        """Non-existent exception class when we lack SSL support."""
+        reason = "This will never be raised."
+
 # TODO:
 # - test the `file` arg to more commands
 # - test error conditions
@@ -261,14 +269,21 @@ def is_connected():
                 return False
             return True
 
-        with self.NNTP_CLASS(self.NNTP_HOST, timeout=TIMEOUT, usenetrc=False) as server:
-            self.assertTrue(is_connected())
-            self.assertTrue(server.help())
-        self.assertFalse(is_connected())
-
-        with self.NNTP_CLASS(self.NNTP_HOST, timeout=TIMEOUT, usenetrc=False) as server:
-            server.quit()
-        self.assertFalse(is_connected())
+        try:
+            with self.NNTP_CLASS(self.NNTP_HOST, timeout=TIMEOUT, usenetrc=False) as server:
+                self.assertTrue(is_connected())
+                self.assertTrue(server.help())
+            self.assertFalse(is_connected())
+
+            with self.NNTP_CLASS(self.NNTP_HOST, timeout=TIMEOUT, usenetrc=False) as server:
+                server.quit()
+            self.assertFalse(is_connected())
+        except SSLError as ssl_err:
+            # matches "[SSL: DH_KEY_TOO_SMALL] dh key too small"
+            if re.search(r'(?i)KEY.TOO.SMALL', ssl_err.reason):
+                raise unittest.SkipTest(f"Got {ssl_err} connecting "
+                                        f"to {self.NNTP_HOST!r}")
+            raise
 
 
 NetworkedNNTPTestsMixin.wrap_methods()
@@ -294,6 +309,12 @@ def setUpClass(cls):
             try:
                 cls.server = cls.NNTP_CLASS(cls.NNTP_HOST, timeout=TIMEOUT,
                                             usenetrc=False)
+            except SSLError as ssl_err:
+                # matches "[SSL: DH_KEY_TOO_SMALL] dh key too small"
+                if re.search(r'(?i)KEY.TOO.SMALL', ssl_err.reason):
+                    raise unittest.SkipTest(f"{cls} got {ssl_err} connecting "
+                                            f"to {cls.NNTP_HOST!r}")
+                raise
             except EOF_ERRORS:
                 raise unittest.SkipTest(f"{cls} got EOF error on connecting "
                                         f"to {cls.NNTP_HOST!r}")