fix: deliver the STOMP ERROR frame before closing the websocket

[dkrizan]

Fixes the flaky WebsocketAuthenticationTest "unauthenticated" tests — and the production behaviour behind them.

Symptom

On billing CI (tolgee/billing#321), server-app:runWebsocketTests failed 4 runs out of 4, each time on exactly one test out of 89 and a different one each run (unauthenticated with expired PAT, unauthenticated on a user topic, an OAuth token presented only on the handshake…, unauthenticated with expired PAK). All four go through waitForUnauthenticated(), and the diagnostic in WebsocketTestHelper logged the same thing each time:

Expected websocket authentication status UNAUTHENTICATED never observed; transitions=[CONNECTION_LOST]

That's the case the helper describes: the server's ERROR frame was lost in the flush-before-close window.

Root cause

Taken from the Spring 7.0.9 source and confirmed with a local repro:

1. An unauthenticated SUBSCRIBE is rejected by throwing (WebSocketConfig). With no error handler configured, StompSubProtocolHandler.sendErrorMessage sends an ERROR frame and closes with PROTOCOL_ERROR in the finally straight after.
2. The session is a ConcurrentWebSocketSessionDecorator. If another thread holds its flush lock — the outbound thread that has just written CONNECTED and is still doing SockJS heartbeat bookkeeping — tryFlushMessageBuffer() fails and the ERROR is only buffered. sendMessage returns normally.
3. close() sets closeInProgress. When the lock holder gets back to the buffer, shouldNotSend() is true and it discards the ERROR.

The client sees a bare close. In the failing CI trace, the server handled SUBSCRIBE 3 ms after CONNECT; in a passing re-attempt of the same test the gap was 20 ms. The race only opens when SUBSCRIBE lands inside the CONNECTED write's tail, which is why it shows up under CPU contention.

Why this isn't only a test problem: since #3900 the webapp deactivates only when the ERROR frame carries Unauthenticated, and treats a bare disconnect as a server failure and reconnects. When this race hits, it reopens the reconnect loop #3900 closed.

Fix

• ErrorFrameFlushingSessionDecorator: subclass of Spring's decorator. On a PROTOCOL_ERROR close it waits until no sendMessage is in progress, then closes. A finished send leaves nothing buffered, because the lock holder re-checks the buffer before it leaves. The exception is a failed write, and then nobody is left to send the rest, so waiting on the buffer itself would only stall. Once the close starts, new sends are dropped, as Spring does after closeInProgress: nothing follows the ERROR, and further rejected frames don't queue more ERRORs. The wait wakes on a Condition when the last send finishes. It ends early once the active send has overrun sendTimeLimit. Other close statuses keep Spring's behaviour, and in the normal case there's nothing to wait for.
• WebSocketBrokerConfiguration (proxyBeanMethods = false, like its parent): decorateSession is Spring's documented hook, but the handler is built inline in a @Bean method. This class extends DelegatingWebSocketMessageBrokerConfiguration, which is all @EnableWebSocketMessageBroker imports, and overrides only subProtocolWebSocketHandler. The annotation is removed from WebSocketConfig.

Rejected alternatives:

• Relaxing the test — would hide the production reconnect loop, and the helper's own note asks not to.
• preservePublishOrder — the ERROR is written directly on the inbound thread, so outbound ordering doesn't cover it, and it would change slow-client handling for every session.
• A custom StompSubProtocolErrorHandler — sendToClient still closes straight after the send.

Tests

• WebsocketErrorFrameDeliveryTest (new) reproduces the race deterministically. A test-only session wrapper, after CONNECTED is written, holds the flush lock until the rejected SUBSCRIBE has arrived and the close is requested. Without the fix that close reaches the wrapper after closeInProgress is set, so the ERROR is always dropped: 3/3 runs failed with transitions=[CONNECTION_LOST], the CI signature. With the fix the close is held back instead, the hold ends on its timeout, the ERROR is written and the close follows: 3/3 passed.
• ErrorFrameFlushingSessionDecoratorTest (new) covers the decorator on its own:
  • the buffered ERROR is written before the close, and a frame sent after the close starts is dropped;
  • a frame stranded by a failed write doesn't hold the close (1001 ms before the change);
  • the wait ends once the active send has overrun sendTimeLimit.
• io.tolgee.websocket.*: 67 tests, 0 failed. BatchJobsGeneralWithRedisTest / BatchJobsGeneralWithoutRedisTest: 27 tests, 0 failed.
• ktlint clean on :api and :server-app.

The repro widens the window on purpose. It proves the mechanism and that the fix covers it. The fix was also run inside billing CI, where the flake occurred (tolgee/billing#321, full re-run against the first version of this commit, cd2a17a25, before the review follow-ups added the wait bound): runWebsocketTests passed with 0 failures and no retries.

Follow-up

Reported upstream as spring-projects/spring-framework#37328, with a Spring-only reproduction that fails on 7.0.9 and 7.1.0-M1. Once it's fixed, delete ErrorFrameFlushingSessionDecorator and WebSocketBrokerConfiguration, and put @EnableWebSocketMessageBroker back. Keep WebsocketErrorFrameDeliveryTest: it should keep passing on the fixed Spring.

Summary by CodeRabbit

• Bug Fixes

  • Improved delivery of WebSocket protocol error messages when another message is still being sent.
  • Reduced the chance of buffered error messages being discarded when a connection closes due to a protocol error, while limiting how long the close waits for pending sends.

• Tests

  • Added coverage for unauthenticated error delivery during concurrent message sending and for closing after a send exceeds its time limit.

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• backend/api/src/main/kotlin/io/tolgee/websocket/ErrorFrameFlushingSessionDecorator.kt

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📝 Walkthrough

Walkthrough

The change adds a session decorator that tracks active sends and waits before a protocol-error close, within the remaining send-time limit. The STOMP broker applies the decorator to WebSocket sessions. Tests cover send ordering, wait limits, and unauthenticated ERROR frame delivery during CONNECTED frame flushing.

Changes

WebSocket error delivery

Layer / File(s)	Summary
Flush protocol-error frames backend/api/src/main/kotlin/io/tolgee/websocket/ErrorFrameFlushingSessionDecorator.kt, backend/app/src/test/kotlin/io/tolgee/websocket/ErrorFrameFlushingSessionDecoratorTest.kt	The decorator tracks active sends and skips new sends after closing starts. For PROTOCOL_ERROR, it waits for active sends within the remaining send-time limit. Unit tests check frame ordering, send failures, and the wait limit.
Wire and verify session decorator backend/api/src/main/kotlin/io/tolgee/websocket/WebSocketBrokerConfiguration.kt, backend/api/src/main/kotlin/io/tolgee/websocket/WebSocketConfig.kt, backend/app/src/test/kotlin/io/tolgee/websocket/WebsocketErrorFrameDeliveryTest.kt	The broker configuration wraps sessions with the decorator and sets the sub-protocol handler phase. WebSocketConfig no longer enables the broker directly. The integration test checks unauthenticated ERROR frame delivery while CONNECTED frame flushing is held.

Priority: ⬇️ Low

Estimated code review effort: 3 (Moderate) | ~25 minutes

Change: Bug fix

Merge Risk: ⚪ Minimal · up to 9f905

The protocol-error close no longer restarts the full send-time wait. No actionable merge risk is established.

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 5.56% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 18 functions across 4 files.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly and concisely describes the main change: ensuring the STOMP ERROR frame is delivered before the WebSocket closes.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

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Inline comments:
In
`@backend/api/src/main/kotlin/io/tolgee/websocket/ErrorFrameFlushingSessionDecorator.kt`:
- Line 38: Update the PROTOCOL_ERROR close-wait logic in
ErrorFrameFlushingSessionDecorator to account for the active delegate send’s
elapsed time via timeSinceSendStarted. Compute a non-negative remainingMillis
budget, derive the deadline with System.nanoTime, and use the same monotonic
clock in the wait-loop condition so closing cannot restart the full
sendTimeLimit.

In
`@backend/app/src/test/kotlin/io/tolgee/websocket/WebsocketErrorFrameDeliveryTest.kt`:
- Line 62: Update WebsocketErrorFrameDeliveryTest to replace the fixed
FLUSH_LOCK_HOLD_MS sleep with a test-only inbound hook that detects the
invalid-JWT SUBSCRIBE before rejection, keeps the post-CONNECTED hold active
until that hook fires, then releases it. Ensure CONNECTED is still sent before
SUBSCRIBE and avoid using SessionSubscribeEvent, which does not observe rejected
subscriptions.

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📒 Files selected for processing (4)

• backend/api/src/main/kotlin/io/tolgee/websocket/ErrorFrameFlushingSessionDecorator.kt
• backend/api/src/main/kotlin/io/tolgee/websocket/WebSocketBrokerConfiguration.kt
• backend/api/src/main/kotlin/io/tolgee/websocket/WebSocketConfig.kt
• backend/app/src/test/kotlin/io/tolgee/websocket/WebsocketErrorFrameDeliveryTest.kt

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• backend/api/src/main/kotlin/io/tolgee/websocket/WebSocketConfig.kt

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