Refactor request sending to have better excpetions (#4358)

* Correctly retry and back off if we get a HTTPerror response

* Refactor request sending to have better excpetions

MatrixFederationHttpClient blindly reraised exceptions to the caller
without differentiating "expected" failures (e.g. connection timeouts
etc) versus more severe problems (e.g. programming errors).

This commit adds a RequestSendFailed exception that is raised when
"expected" failures happen, allowing the TransactionQueue to log them as
warnings while allowing us to log other exceptions as actual exceptions.

1 files changed, 4 insertions, 3 deletions

diff --git a/synapse/rest/media/v1/media_repository.py b/synapse/rest/media/v1/media_repository.py
index e117836e9a..bdffa97805 100644
--- a/synapse/rest/media/v1/media_repository.py
+++ b/synapse/rest/media/v1/media_repository.py
@@ -30,6 +30,7 @@ from synapse.api.errors import (
     FederationDeniedError,
     HttpResponseException,
     NotFoundError,
+    RequestSendFailed,
     SynapseError,
 )
 from synapse.metrics.background_process_metrics import run_as_background_process
@@ -372,10 +373,10 @@ class MediaRepository(object):
                         "allow_remote": "false",
                     }
                 )
-            except twisted.internet.error.DNSLookupError as e:
-                logger.warn("HTTP error fetching remote media %s/%s: %r",
+            except RequestSendFailed as e:
+                logger.warn("Request failed fetching remote media %s/%s: %r",
                             server_name, media_id, e)
-                raise NotFoundError()
+                raise SynapseError(502, "Failed to fetch remote media")
 
             except HttpResponseException as e:
                 logger.warn("HTTP error fetching remote media %s/%s: %s",