Retry well known on fail.

If we have recently seen a valid well-known for a domain we want to
retry on (non-final) errors a few times, to handle temporary blips in
networking/etc.

synapse/http/federation/matrix_federation_agent.py

@@ -51,17 +51,17 @@ class MatrixFederationAgent(object):
             SRVResolver impl to use for looking up SRV records. None to use a default
             implementation.
 
-        _well_known_cache (TTLCache|None):
-            TTLCache impl for storing cached well-known lookups. None to use a default
-            implementation.
+        _well_known_resolver (WellKnownResolver|None):
+            WellKnownResolver to use to perform well-known lookups. None to use a
+            default implementation.
     """
 
     def __init__(
         self,
         reactor,
         tls_client_options_factory,
         _srv_resolver=None,
-        _well_known_cache=None,
+        _well_known_resolver=None,
     ):
         self._reactor = reactor
         self._clock = Clock(reactor)
@@ -76,15 +76,17 @@ def __init__(
         self._pool.maxPersistentPerHost = 5
         self._pool.cachedConnectionTimeout = 2 * 60
 
-        self._well_known_resolver = WellKnownResolver(
-            self._reactor,
-            agent=Agent(
+        if _well_known_resolver is None:
+            _well_known_resolver = WellKnownResolver(
                 self._reactor,
-                pool=self._pool,
-                contextFactory=tls_client_options_factory,
-            ),
-            well_known_cache=_well_known_cache,
-        )
+                agent=Agent(
+                    self._reactor,
+                    pool=self._pool,
+                    contextFactory=tls_client_options_factory,
+                ),
+            )
+
+        self._well_known_resolver = _well_known_resolver
 
     @defer.inlineCallbacks
     def request(self, method, uri, headers=None, bodyProducer=None):

synapse/http/federation/well_known_resolver.py

@@ -38,6 +38,13 @@
 # period to cache failure to fetch .well-known for
 WELL_KNOWN_INVALID_CACHE_PERIOD = 1 * 3600
 
+# period to cache failure to fetch .well-known if there has recently been a
+# valid well-known for that domain.
+WELL_KNOWN_DOWN_CACHE_PERIOD = 2 * 60
+
+# period to remember there was a valid well-known after valid record expires
+WELL_KNOWN_REMEMBER_DOMAIN_HAD_VALID = 2 * 3600
+
 # cap for .well-known cache period
 WELL_KNOWN_MAX_CACHE_PERIOD = 48 * 3600
 
@@ -49,11 +56,16 @@
 # we'll start trying to refetch 1 minute before it expires.
 WELL_KNOWN_GRACE_PERIOD_FACTOR = 0.2
 
+# Number of times we retry fetching a well-known for a domain we know recently
+# had a valid entry.
+WELL_KNOWN_RETRY_ATTEMPTS = 3
+
 
 logger = logging.getLogger(__name__)
 
 
 _well_known_cache = TTLCache("well-known")
+_had_valid_well_known_cache = TTLCache("had-valid-well-known")
 
 
 @attr.s(slots=True, frozen=True)
@@ -65,14 +77,20 @@ class WellKnownResolver(object):
     """Handles well-known lookups for matrix servers.
     """
 
-    def __init__(self, reactor, agent, well_known_cache=None):
+    def __init__(
+        self, reactor, agent, well_known_cache=None, had_well_known_cache=None
+    ):
         self._reactor = reactor
         self._clock = Clock(reactor)
 
         if well_known_cache is None:
             well_known_cache = _well_known_cache
 
+        if had_well_known_cache is None:
+            had_well_known_cache = _had_valid_well_known_cache
+
         self._well_known_cache = well_known_cache
+        self._had_valid_well_known_cache = had_well_known_cache
         self._well_known_agent = RedirectAgent(agent)
 
     @defer.inlineCallbacks
@@ -100,7 +118,7 @@ def get_well_known(self, server_name):
         # requests for the same server in parallel?
         try:
             with Measure(self._clock, "get_well_known"):
-                result, cache_period = yield self._do_get_well_known(server_name)
+                result, cache_period = yield self._fetch_well_known(server_name)
 
         except _FetchWellKnownFailure as e:
             if prev_result and e.temporary:
@@ -111,18 +129,28 @@ def get_well_known(self, server_name):
 
             result = None
 
-            # add some randomness to the TTL to avoid a stampeding herd every hour
-            # after startup
-            cache_period = WELL_KNOWN_INVALID_CACHE_PERIOD
-            cache_period += random.uniform(0, WELL_KNOWN_DEFAULT_CACHE_PERIOD_JITTER)
+            if self._had_valid_well_known_cache.get(server_name, False):
+                # We have recently seen a valid well-known record for this
+                # server, so we cache the lack of well-known for a shorter time.
+                cache_period = WELL_KNOWN_DOWN_CACHE_PERIOD
+                cache_period += random.uniform(
+                    0, WELL_KNOWN_DEFAULT_CACHE_PERIOD_JITTER
+                )
+            else:
+                # add some randomness to the TTL to avoid a stampeding herd every hour
+                # after startup
+                cache_period = WELL_KNOWN_INVALID_CACHE_PERIOD
+                cache_period += random.uniform(
+                    0, WELL_KNOWN_DEFAULT_CACHE_PERIOD_JITTER
+                )
 
         if cache_period > 0:
             self._well_known_cache.set(server_name, result, cache_period)
 
         return WellKnownLookupResult(delegated_server=result)
 
     @defer.inlineCallbacks
-    def _do_get_well_known(self, server_name):
+    def _fetch_well_known(self, server_name):
         """Actually fetch and parse a .well-known, without checking the cache
 
         Args:
@@ -134,24 +162,17 @@ def _do_get_well_known(self, server_name):
         Returns:
             Deferred[Tuple[bytes,int]]: The lookup result and cache period.
         """
-        uri = b"https://%s/.well-known/matrix/server" % (server_name,)
-        uri_str = uri.decode("ascii")
-        logger.info("Fetching %s", uri_str)
+
+        had_valid_well_known = bool(
+            self._had_valid_well_known_cache.get(server_name, False)
+        )
 
         # We do this in two steps to differentiate between possibly transient
         # errors (e.g. can't connect to host, 503 response) and more permenant
         # errors (such as getting a 404 response).
-        try:
-            response = yield make_deferred_yieldable(
-                self._well_known_agent.request(b"GET", uri)
-            )
-            body = yield make_deferred_yieldable(readBody(response))
-
-            if 500 <= response.code < 600:
-                raise Exception("Non-200 response %s" % (response.code,))
-        except Exception as e:
-            logger.info("Error fetching %s: %s", uri_str, e)
-            raise _FetchWellKnownFailure(temporary=True)
+        response, body = yield self._make_well_known_request(
+            server_name, retry=had_valid_well_known
+        )
 
         try:
             if response.code != 200:
@@ -161,8 +182,11 @@ def _do_get_well_known(self, server_name):
             logger.info("Response from .well-known: %s", parsed_body)
 
             result = parsed_body["m.server"].encode("ascii")
+        except defer.CancelledError:
+            # Bail if we've been cancelled
+            raise
         except Exception as e:
-            logger.info("Error fetching %s: %s", uri_str, e)
+            logger.info("Error parsing well-known for %s: %s", server_name, e)
             raise _FetchWellKnownFailure(temporary=False)
 
         cache_period = _cache_period_from_headers(
@@ -177,8 +201,62 @@ def _do_get_well_known(self, server_name):
             cache_period = min(cache_period, WELL_KNOWN_MAX_CACHE_PERIOD)
             cache_period = max(cache_period, WELL_KNOWN_MIN_CACHE_PERIOD)
 
+        # We got a success, mark as such in the cache
+        self._had_valid_well_known_cache.set(
+            server_name,
+            bool(result),
+            cache_period + WELL_KNOWN_REMEMBER_DOMAIN_HAD_VALID,
+        )
+
         return (result, cache_period)
 
+    @defer.inlineCallbacks
+    def _make_well_known_request(self, server_name, retry):
+        """Make the well known request.
+
+        This will retry the request if requested and it fails (with unable
+        to connect or receives a 5xx error).
+
+        Args:
+            server_name (bytes)
+            retry (bool): Whether to retry the request if it fails.
+
+        Returns:
+            Deferred[tuple[IResponse, bytes]] Returns the response object and
+            body. Response may be a non-200 response.
+        """
+        uri = b"https://%s/.well-known/matrix/server" % (server_name,)
+        uri_str = uri.decode("ascii")
+
+        i = 0
+        while True:
+            i += 1
+
+            logger.info("Fetching %s", uri_str)
+            try:
+                response = yield make_deferred_yieldable(
+                    self._well_known_agent.request(b"GET", uri)
+                )
+                body = yield make_deferred_yieldable(readBody(response))
+
+                if 500 <= response.code < 600:
+                    raise Exception("Non-200 response %s" % (response.code,))
+
+                return response, body
+            except defer.CancelledError:
+                # Bail if we've been cancelled
+                raise
+            except Exception as e:
+                logger.info("Retry: %s", retry)
+                if not retry or i >= WELL_KNOWN_RETRY_ATTEMPTS:
+                    logger.info("Error fetching %s: %s", uri_str, e)
+                    raise _FetchWellKnownFailure(temporary=True)
+
+                logger.info("Error fetching %s: %s. Retrying", uri_str, e)
+
+            # Sleep briefly in the hopes that they come back up
+            yield self._clock.sleep(0.5)
+
 
 def _cache_period_from_headers(headers, time_now=time.time):
     cache_controls = _parse_cache_control(headers)