Add support for DTLS timeouts (#1180)

Add support for DTLS timeouts

When performing a DTLS handshake, the DTLS state machine may need to be
updated based on the passage of time, for instance in response to packet
loss.

OpenSSL supports this by means of the `DTLSv1_get_timeout` and
`DTLSv1_handle_timeout` methods, both of which are included in
cryptography's bindings. This change adds Python wrappers for these
methods in the `Connection` class.

1 files changed, 42 insertions, 4 deletions

diff --git a/tests/test_ssl.py b/tests/test_ssl.py
index bfc5ca8..e6c0cdc 100644
--- a/tests/test_ssl.py
+++ b/tests/test_ssl.py
@@ -9,6 +9,7 @@ import datetime
 import gc
 import select
 import sys
+import time
 import uuid
 from errno import (
     EAFNOSUPPORT,
@@ -4369,10 +4370,11 @@ class TestDTLS:
     # new versions of OpenSSL, this is unnecessary, but harmless, because the
     # DTLS state machine treats it like a network hiccup that duplicated a
     # packet, which DTLS is robust against.
-    def test_it_works_at_all(self):
-        # arbitrary number larger than any conceivable handshake volley
-        LARGE_BUFFER = 65536
 
+    # Arbitrary number larger than any conceivable handshake volley.
+    LARGE_BUFFER = 65536
+
+    def test_it_works_at_all(self):
         s_ctx = Context(DTLS_METHOD)
 
         def generate_cookie(ssl):
@@ -4403,7 +4405,7 @@ class TestDTLS:
 
         def pump_membio(label, source, sink):
             try:
-                chunk = source.bio_read(LARGE_BUFFER)
+                chunk = source.bio_read(self.LARGE_BUFFER)
             except WantReadError:
                 return False
             # I'm not sure this check is needed, but I'm not sure it's *not*
@@ -4483,3 +4485,39 @@ class TestDTLS:
             assert 0 < c.get_cleartext_mtu() < 500
         except NotImplementedError:  # OpenSSL 1.1.0 and earlier
             pass
+
+    def test_timeout(self, monkeypatch):
+        c_ctx = Context(DTLS_METHOD)
+        c = Connection(c_ctx)
+
+        # No timeout before the handshake starts.
+        assert c.DTLSv1_get_timeout() is None
+        assert c.DTLSv1_handle_timeout() is False
+
+        # Start handshake and check there is data to send.
+        c.set_connect_state()
+        try:
+            c.do_handshake()
+        except SSL.WantReadError:
+            pass
+        assert c.bio_read(self.LARGE_BUFFER)
+
+        # There should now be an active timeout.
+        seconds = c.DTLSv1_get_timeout()
+        assert seconds is not None
+
+        # Handle the timeout and check there is data to send.
+        time.sleep(seconds)
+        assert c.DTLSv1_handle_timeout() is True
+        assert c.bio_read(self.LARGE_BUFFER)
+
+        # After the maximum number of allowed timeouts is reached,
+        # DTLSv1_handle_timeout will return -1.
+        #
+        # Testing this directly is prohibitively time consuming as the timeout
+        # duration is doubled on each retry, so the best we can do is to mock
+        # this condition.
+        monkeypatch.setattr(_lib, "DTLSv1_handle_timeout", lambda x: -1)
+
+        with pytest.raises(Error):
+            c.DTLSv1_handle_timeout()