Always send `/rendezvous` routes to the same Synapse worker process (main) to fix QR code login

Related to https://github.com/matrix-org/matrix-spec-proposals/pull/4108

roles/custom/matrix-synapse-reverse-proxy-companion/defaults/main.yml

@@ -230,6 +230,9 @@ matrix_synapse_reverse_proxy_companion_synapse_media_repository_locations: []
 matrix_synapse_reverse_proxy_companion_synapse_user_dir_locations: []
 matrix_synapse_reverse_proxy_companion_client_server_main_override_locations_regex: ^/_matrix/client/(api/v1|r0|v3|unstable)/(account/3pid/|directory/list/room/|pushrules/|rooms/[^/]+/(forget|upgrade|report)|login/sso/redirect/|register)
 matrix_synapse_reverse_proxy_companion_client_server_sso_override_locations_regex: ^(/_matrix/client/(api/v1|r0|v3|unstable)/login/sso/redirect|/_synapse/client/(pick_username|(new_user_consent|oidc/callback|pick_idp|sso_register)$))
+# Related to MSC4108 (https://github.com/matrix-org/matrix-spec-proposals/pull/4108)
+matrix_synapse_reverse_proxy_companion_client_server_qr_code_login_locations_regex: ^(/_matrix/client/(unstable|v1)/org.matrix.msc4108/rendezvous|/_synapse/client/rendezvous)$
+
 matrix_synapse_reverse_proxy_companion_federation_override_locations_regex: ^/_matrix/federation/v1/openid/userinfo$
 
 # synapse content caching

roles/custom/matrix-synapse-reverse-proxy-companion/templates/nginx/conf.d/matrix-synapse-reverse-proxy-companion.conf.j2

@@ -125,6 +125,23 @@ server {
 			proxy_max_temp_file_size 0;
 		}
 
+		# QR code login (`rendezvous`) locations need to go to the same Synapse process.
+		# It doesn't necessarily need to be the main process, but it needs to be consistent.
+		# For simplicity, we'll send them to the main process though.
+		location ~ {{ matrix_synapse_reverse_proxy_companion_client_server_qr_code_login_locations_regex }} {
+			{# FIXME: This block was copied from the main Synapse fallback below.  It would be better to have it in one place and avoid duplication. #}
+			{# Use the embedded DNS resolver in Docker containers to discover the service #}
+			resolver {{ matrix_synapse_reverse_proxy_companion_http_level_resolver }} valid=5s;
+			set $backend "{{ matrix_synapse_reverse_proxy_companion_client_api_addr }}";
+			proxy_pass http://$backend;
+
+			proxy_set_header Host $host;
+
+			client_body_buffer_size 25M;
+			client_max_body_size {{ matrix_synapse_reverse_proxy_companion_client_api_client_max_body_size_mb }}M;
+			proxy_max_temp_file_size 0;
+		}
+
 		{# Workers redirects BEGIN #}
 
 		{% if generic_workers | length > 0 %}