UnboundLocalError: now_token in /sync for appservice bots using MSC3202 (regression in v1.150.0)

[Just-Insane]

Description

Synapse v1.150.0 causes all /sync requests from appservice bridge bots to fail with HTTP 500 when msc4190_enabled: true. Every bridge bot's sync crashes every ~10 seconds, breaking all E2EE message delivery to/from bridges.

Steps to reproduce

Steps to reproduce:

• Enable MSC4190 in homeserver.yaml: experimental_features: { msc4190_enabled: true }
• Start any mautrix bridge (discord, signal, whatsapp, etc.) with E2EE enabled
• Bridge bot calls GET /_matrix/client/v3/sync?org.matrix.msc3202.device_id=<device_id>&user_id=@bot:example.com
• Synapse returns HTTP 500 on every sync request from the bot

What happened vs expected:
Expected: Bridge bots sync successfully and can participate in E2EE rooms.
Actual: Every sync request returns HTTP 500 with UnboundLocalError: cannot access local variable 'now_token'. All bridges stop receiving/decrypting messages.

Workaround: Setting msc4354_enabled: false fully resolves the issue, indicating the regression was introduced by #19487 (MSC4354 sticky events in legacy /sync).

Homeserver

Self-hosted (matrix.cloudhub.social)

Synapse Version

1.150.0

Installation Method

Docker (matrixdotorg/synapse)

Database

PostgreSQL: single server, no recent restore

Workers

Single process

Platform

Docker container, Linux (Debian)

Configuration

experimental_features:
msc4354_enabled: true # sticky events — disabling this is the workaround

Relevant log output

UnboundLocalError: cannot access local variable 'now_token' where it is not associated with a value
  File ".../synapse/http/server.py", line 335, in _async_render_wrapper
  File ".../synapse/http/server.py", line 576, in _async_render
  File ".../synapse/rest/client/sync.py", line 271, in on_GET
  File ".../synapse/handlers/sync.py", line 365, in wait_for_sync_for_user
  File ".../synapse/util/caches/response_cache.py", line 402, in wrap
  File ".../synapse/util/caches/response_cache.py", line 343, in cb
UnboundLocalError: cannot access local variable 'now_token' where it is not associated with a value

Anything else that would be useful to know?

No response

[tulir]

This is not related to the bug, but note that there's no experimental_features key called msc4190_enabled and never has been, that field is a no-op. Also, the device_id query param was moved to MSC4326. MSC3202 is the solution for replacing /sync, so if you're using /sync, you're not using MSC3202. Both MSC4190 and MSC4326 are already in the spec (but MSC3202 is not)

[Just-Insane]

Ah, yup, my bad. I am using the Matrix-Docker-Ansible-Deploy playbook with the following config:

# See https://github.com/matrix-org/matrix-spec-proposals/pull/4140
matrix_synapse_experimental_features_msc4140_enabled: true

# See `matrix_synapse_experimental_features_msc4140_enabled`.
matrix_synapse_max_event_delay_duration: 730h

# See https://github.com/matrix-org/matrix-spec-proposals/pull/4143
matrix_synapse_experimental_features_msc4143_enabled: true

# See https://github.com/matrix-org/matrix-spec-proposals/pull/4222
matrix_synapse_experimental_features_msc4222_enabled: true

# See:
# - https://github.com/matrix-org/matrix-spec-proposals/pull/4306
# - https://github.com/matrix-org/matrix-spec-proposals/pull/4308
matrix_synapse_experimental_features_msc4306_enabled: true

# See https://github.com/matrix-org/matrix-spec-proposals/pull/4354
matrix_synapse_experimental_features_msc4354_enabled: false

## Bridges
matrix_bridges_encryption_enabled: true
matrix_bridges_encryption_default: true
matrix_bridges_msc4190_enabled: true
matrix_bridges_self_sign_enabled: true
matrix_bridges_relay_enabled: true

With matrix_synapse_experimental_features_msc4354_enabled: true, I get the error from this issue.