POST /_matrix/client/r0/publicised_groups takes *ages*, blocking clients

[richvdh]

when matrix.org is being slow, requests to POST /_matrix/client/r0/publicised_groups block for minutes at a time. Since riot likes to send such requests very frequently (element-hq/element-web#14998), it's easy for this to wedge the whole client.

2020-08-18 17:36:16,449 - synapse.http.matrixfederationclient - 414 - INFO - POST-3555358- {POST-O-9291391} [t2l.io] Sending request: POST matrix://t2l.io/_matrix/federation/v1/get_groups_publicised; timeout 60.000000s
2020-08-18 17:36:16,657 - synapse.http.matrixfederationclient - 163 - INFO - POST-3555358- {POST-O-9291391} [t2l.io] Completed: 200 OK
2020-08-18 17:36:16,657 - synapse.http.matrixfederationclient - 414 - INFO - POST-3555358- {POST-O-9291403} [matrix.org] Sending request: POST matrix://matrix.org/_matrix/federation/v1/get_groups_publicised; timeout 60.000000s
2020-08-18 17:37:16,661 - synapse.http.matrixfederationclient - 442 - INFO - POST-3555358- Failed to send request: [<twisted.python.failure.Failure twisted.internet.defer.CancelledError: >]
2020-08-18 17:37:16,661 - synapse.http.matrixfederationclient - 507 - WARNING - POST-3555358- {POST-O-9291403} [matrix.org] Request failed: POST matrix://matrix.org/_matrix/federation/v1/get_groups_publicised: ResponseNeverReceived:[CancelledError()]
2020-08-18 17:37:17,480 - synapse.http.matrixfederationclient - 414 - INFO - POST-3555358- {POST-O-9291403} [matrix.org] Sending request: POST matrix://matrix.org/_matrix/federation/v1/get_groups_publicised; timeout 60.000000s
2020-08-18 17:37:17,481 - synapse.http.federation.matrix_federation_agent - 250 - INFO - POST-3555358- Connecting to matrix-federation.matrix.org:443
2020-08-18 17:38:17,635 - synapse.http.matrixfederationclient - 442 - INFO - POST-3555358- Failed to send request: [<twisted.python.failure.Failure twisted.internet.defer.CancelledError: >]
2020-08-18 17:38:17,635 - synapse.http.matrixfederationclient - 507 - WARNING - POST-3555358- {POST-O-9291403} [matrix.org] Request failed: POST matrix://matrix.org/_matrix/federation/v1/get_groups_publicised: ResponseNeverReceived:[CancelledError()]
2020-08-18 17:38:26,151 - synapse.http.matrixfederationclient - 414 - INFO - POST-3555358- {POST-O-9291403} [matrix.org] Sending request: POST matrix://matrix.org/_matrix/federation/v1/get_groups_publicised; timeout 60.000000s
2020-08-18 17:38:26,152 - synapse.http.federation.matrix_federation_agent - 250 - INFO - POST-3555358- Connecting to matrix-federation.matrix.org:443
2020-08-18 17:39:26,155 - synapse.http.matrixfederationclient - 442 - INFO - POST-3555358- Failed to send request: [<twisted.python.failure.Failure twisted.internet.defer.CancelledError: >]
2020-08-18 17:39:26,155 - synapse.http.matrixfederationclient - 507 - WARNING - POST-3555358- {POST-O-9291403} [matrix.org] Request failed: POST matrix://matrix.org/_matrix/federation/v1/get_groups_publicised: ResponseNeverReceived:[CancelledError()]
2020-08-18 17:39:28,273 - synapse.http.matrixfederationclient - 414 - INFO - POST-3555358- {POST-O-9291403} [matrix.org] Sending request: POST matrix://matrix.org/_matrix/federation/v1/get_groups_publicised; timeout 60.000000s
2020-08-18 17:40:28,276 - synapse.http.matrixfederationclient - 442 - INFO - POST-3555358- Failed to send request: [<twisted.python.failure.Failure twisted.internet.defer.CancelledError: >]
2020-08-18 17:40:28,276 - synapse.http.matrixfederationclient - 507 - WARNING - POST-3555358- {POST-O-9291403} [matrix.org] Request failed: POST matrix://matrix.org/_matrix/federation/v1/get_groups_publicised: ResponseNeverReceived:[CancelledError()]
2020-08-18 17:40:28,277 - synapse.http.matrixfederationclient - 414 - INFO - POST-3555358- {POST-O-9297748} [half-shot.uk] Sending request: POST matrix://half-shot.uk/_matrix/federation/v1/get_groups_publicised; timeout 60.000000s
2020-08-18 17:40:28,278 - synapse.http.federation.matrix_federation_agent - 250 - INFO - POST-3555358- Connecting to chaotic.half-shot.uk:8448
2020-08-18 17:40:28,347 - synapse.http.matrixfederationclient - 163 - INFO - POST-3555358- {POST-O-9297748} [half-shot.uk] Completed: 200 OK
2020-08-18 17:40:28,348 - synapse.access.http.8008 - 310 - INFO - POST-3555358- 81.2.100.169 - 8008 - {@richvdh:sw1v.org} Processed request: 251.899sec/0.001sec (0.009sec, 0.004sec) (0.000sec/0.000sec/0) 771B 200 "POST /_matrix/client/r0/publicised_groups HTTP/1.1" "Mozilla/5.0 (X11; Linux x86_64) AppleWebKit/537.36 (KHTML, like Gecko) Element-Nightly/2020080401 Chrome/83.0.4103.122 Electron/9.1.2 Safari/537.36" [0 dbevts]

[clokep]

Communities/groups are being removed, see #11584.