Add a lower bound for TTL on well known results.

It costs both us and the remote server for us to fetch the well known
for every single request we send, so we add a minimum cache period. This
is set to 5m so that we still honour the basic premise of "refetch
frequently".

2 files changed, 6 insertions, 2 deletions

diff --git a/synapse/http/federation/matrix_federation_agent.py b/synapse/http/federation/matrix_federation_agent.py
index a0d5139839..79e488c942 100644
--- a/synapse/http/federation/matrix_federation_agent.py
+++ b/synapse/http/federation/matrix_federation_agent.py
@@ -47,6 +47,9 @@ WELL_KNOWN_INVALID_CACHE_PERIOD = 1 * 3600
 # cap for .well-known cache period
 WELL_KNOWN_MAX_CACHE_PERIOD = 48 * 3600
 
+# lower bound for .well-known cache period
+WELL_KNOWN_MIN_CACHE_PERIOD = 5 * 60
+
 logger = logging.getLogger(__name__)
 well_known_cache = TTLCache("well-known")
 
@@ -356,6 +359,7 @@ class MatrixFederationAgent(object):
             cache_period += random.uniform(0, WELL_KNOWN_DEFAULT_CACHE_PERIOD_JITTER)
         else:
             cache_period = min(cache_period, WELL_KNOWN_MAX_CACHE_PERIOD)
+            cache_period = max(cache_period, WELL_KNOWN_MIN_CACHE_PERIOD)
 
         return (result, cache_period)
 
diff --git a/tests/http/federation/test_matrix_federation_agent.py b/tests/http/federation/test_matrix_federation_agent.py
index 4255add097..5e709c0c17 100644
--- a/tests/http/federation/test_matrix_federation_agent.py
+++ b/tests/http/federation/test_matrix_federation_agent.py
@@ -953,7 +953,7 @@ class MatrixFederationAgentTests(TestCase):
         well_known_server = self._handle_well_known_connection(
             client_factory,
             expected_sni=b"testserv",
-            response_headers={b"Cache-Control": b"max-age=10"},
+            response_headers={b"Cache-Control": b"max-age=1000"},
             content=b'{ "m.server": "target-server" }',
         )
 
@@ -969,7 +969,7 @@ class MatrixFederationAgentTests(TestCase):
         self.assertEqual(r, b"target-server")
 
         # expire the cache
-        self.reactor.pump((10.0,))
+        self.reactor.pump((1000.0,))
 
         # now it should connect again
         fetch_d = self.do_get_well_known(b"testserv")