Faster Remote Room Joins: Synapse will try to resync from itself.

[reivilibre]

Tested as at v1.78.0rc1.

2023-02-21 15:19:15,209 - synapse.handlers.federation - 1981 - INFO - sync_partial_state_room-24 - Syncing state for room !fuzEIyMtiFYSFEjRCI:libera.chat via matrix.org instead
2023-02-21 15:19:15,209 - synapse.handlers.federation_event - 627 - INFO - sync_partial_state_room-24 - Updating state for $0IdYaOlZVqiCRRb1EF4jXCsxIsE4eW1KPm_etjY8FM4
2023-02-21 15:19:15,750 - synapse.handlers.federation_event - 1068 - INFO - sync_partial_state_room-24-$0IdYaOlZVqiCRRb1EF4jXCsxIsE4eW1KPm_etjY8FM4- - Event $0IdYaOlZVqiCRRb1EF4jXCsxIsE4eW1KPm_etjY8FM4 is missing prev_events ['$-aFRBQqhtO1GtE_mNaZLVm6cCcoQriQANKshvF8ZQaE']: calculating state for a backwards extremity
2023-02-21 15:19:15,751 - synapse.handlers.federation_event - 1098 - INFO - sync_partial_state_room-24-$0IdYaOlZVqiCRRb1EF4jXCsxIsE4eW1KPm_etjY8FM4- - Requesting state after missing prev_event $-aFRBQqhtO1GtE_mNaZLVm6cCcoQriQANKshvF8ZQaE
2023-02-21 15:19:15,752 - synapse.http.federation.well_known_resolver - 253 - INFO - sync_partial_state_room-24-$0IdYaOlZVqiCRRb1EF4jXCsxIsE4eW1KPm_etjY8FM4--$-aFRBQqhtO1GtE_mNaZLVm6cCcoQriQANKshvF8ZQaE--- - Fetching https://matrix.org/.well-known/matrix/server
2023-02-21 15:19:15,937 - synapse.http.federation.well_known_resolver - 190 - INFO - sync_partial_state_room-24-$0IdYaOlZVqiCRRb1EF4jXCsxIsE4eW1KPm_etjY8FM4--$-aFRBQqhtO1GtE_mNaZLVm6cCcoQriQANKshvF8ZQaE--- - Response from .well-known: {'m.server': 'matrix-federation.matrix.org:443'}
2023-02-21 15:19:16,059 - synapse.http.matrixfederationclient - 629 - INFO - sync_partial_state_room-24-$0IdYaOlZVqiCRRb1EF4jXCsxIsE4eW1KPm_etjY8FM4--$-aFRBQqhtO1GtE_mNaZLVm6cCcoQriQANKshvF8ZQaE--- - {GET-O-52239} [matrix.org] Got response headers: 403 Forbidden
2023-02-21 15:19:16,060 - synapse.http.matrixfederationclient - 709 - WARNING - sync_partial_state_room-24-$0IdYaOlZVqiCRRb1EF4jXCsxIsE4eW1KPm_etjY8FM4--$-aFRBQqhtO1GtE_mNaZLVm6cCcoQriQANKshvF8ZQaE--- - {GET-O-52239} [matrix.org] Request failed: GET matrix://matrix.org/_matrix/federation/v1/state_ids/%21fuzEIyMtiFYSFEjRCI%3Alibera.chat?event_id=%24-aFRBQqhtO1GtE_mNaZLVm6cCcoQriQANKshvF8ZQaE: HttpResponseException('403: Forbidden')
2023-02-21 15:19:16,060 - synapse.handlers.federation_event - 1122 - WARNING - sync_partial_state_room-24-$0IdYaOlZVqiCRRb1EF4jXCsxIsE4eW1KPm_etjY8FM4- - Error attempting to resolve state at missing prev_events: 403: Forbidden

Probably a very easy fix: just ignore our own server name in the list of servers to resync from.

Noticed whilst debugging #15115.

[DMRobertson]

I guess this is exacerbated by #12801.