Make postgres_max_connections scale with the Synapse worker count

With workers enabled, every Synapse process (the main one and each
worker) maintains its own database connection pool of up to
matrix_synapse_database_cp_max (default: 10) connections. The previous
fixed limit of 500 left little headroom on setups running many workers,
where exhausting it manifests as degraded performance and, reportedly,
E2EE misbehavior.

Size the limit as a 200-connection baseline (for everything else that
talks to Postgres) plus cp_max connections per Synapse process, never
going below the previous value of 500. The stock worker presets stay at
500; setups that raise worker counts (or cp_max) get a limit that grows
accordingly. The new matrix_synapse_workers_total_count variable
reflects what the matrix_synapse_workers_*_count variables ask for; it
does not reflect a manually populated
matrix_synapse_workers_enabled_list, and such setups should override
postgres_max_connections themselves.

Fixes #4442

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

group_vars/matrix_servers

@@ -4046,7 +4046,13 @@ postgres_db_name: matrix
 
 postgres_systemd_services_to_stop_for_maintenance_list_auto: "{{ devture_systemd_service_manager_services_list_auto | map(attribute='name') | reject('equalto', (postgres_identifier + '.service')) }}"
 
-postgres_max_connections: "{{ 500 if matrix_synapse_workers_enabled else 200 }}"
+# When Synapse workers are enabled, connection demand grows with the number of Synapse processes:
+# each process (the main one and every worker) maintains its own connection pool of up to `matrix_synapse_database_cp_max` connections.
+# The formula below reserves a 200-connection baseline for everything else talking to Postgres and adds `cp_max` per Synapse process.
+# It never goes below 500, the fixed value used previously, so existing worker setups are not downsized.
+# Note: the worker count reflects the `matrix_synapse_workers_*_count` variables.
+# If you populate `matrix_synapse_workers_enabled_list` manually, consider overriding `postgres_max_connections` yourself.
+postgres_max_connections: "{{ [200 + (matrix_synapse_database_cp_max | int) * ((matrix_synapse_workers_total_count | int) + 1), 500] | max if matrix_synapse_workers_enabled else 200 }}"
 
 postgres_container_image_registry_prefix_upstream: "{{ matrix_container_global_registry_prefix_override if matrix_container_global_registry_prefix_override else postgres_container_image_registry_prefix_upstream_default }}"
 

roles/custom/matrix-synapse/defaults/main.yml

@@ -1239,6 +1239,25 @@ matrix_synapse_workers_background_workers_container_arguments: []
 # `run_background_tasks_on` is meant to point to a worker, which is dedicated to processing background tasks.
 matrix_synapse_run_background_tasks_on: "{{ (matrix_synapse_workers_enabled_list | selectattr('type', 'equalto', 'background') | list)[0].name if (matrix_synapse_workers_enabled and matrix_synapse_workers_enabled_list | selectattr('type', 'equalto', 'background') | list | length > 0) else '' }}"
 
+# matrix_synapse_workers_total_count reflects the total number of Synapse worker processes that the `matrix_synapse_workers_*_count` variables ask for.
+# This is useful for sizing resources that depend on the number of Synapse processes (e.g. the maximum number of Postgres connections).
+# Note: if `matrix_synapse_workers_enabled_list` is populated manually (instead of via the count variables), this count does not reflect it.
+matrix_synapse_workers_total_count: |
+  {{
+    (matrix_synapse_workers_room_workers_count | int)
+    + (matrix_synapse_workers_sync_workers_count | int)
+    + (matrix_synapse_workers_client_reader_workers_count | int)
+    + (matrix_synapse_workers_federation_reader_workers_count | int)
+    + (matrix_synapse_workers_generic_workers_count | int)
+    + (matrix_synapse_workers_pusher_workers_count | int)
+    + (matrix_synapse_workers_federation_sender_workers_count | int)
+    + (matrix_synapse_workers_media_repository_workers_count | int)
+    + (matrix_synapse_workers_appservice_workers_count | int)
+    + (matrix_synapse_workers_user_dir_workers_count | int)
+    + (matrix_synapse_workers_background_workers_count | int)
+    + (matrix_synapse_workers_stream_writers | length)
+  }}
+
 # Default list of workers to spawn.
 #
 # Unless you populate this manually, this list is dynamically generated