URL preview blacklisting fixes (#5155)

Prevents a SynapseError being raised inside of a IResolutionReceiver and instead opts to just return 0 results. This thus means that we have to lump a failed lookup and a blacklisted lookup together with the same error message, but the substitute should be generic enough to cover both cases.

synapse/rest/media/v1/preview_url_resource.py

@@ -31,6 +31,7 @@ from six.moves import urllib_parse as urlparse
 from canonicaljson import json
 
 from twisted.internet import defer
+from twisted.internet.error import DNSLookupError
 from twisted.web.resource import Resource
 from twisted.web.server import NOT_DONE_YET
 
@@ -328,9 +329,18 @@ class PreviewUrlResource(Resource):
                 # handler will return a SynapseError to the client instead of
                 # blank data or a 500.
                 raise
+            except DNSLookupError:
+                # DNS lookup returned no results
+                # Note: This will also be the case if one of the resolved IP
+                # addresses is blacklisted
+                raise SynapseError(
+                    502, "DNS resolution failure during URL preview generation",
+                    Codes.UNKNOWN
+                )
             except Exception as e:
                 # FIXME: pass through 404s and other error messages nicely
                 logger.warn("Error downloading %s: %r", url, e)
+
                 raise SynapseError(
                     500, "Failed to download content: %s" % (
                         traceback.format_exception_only(sys.exc_info()[0], e),