fix(events): serialize websocket writes per connection

[cesar-carlos]

Summary

• Fixes panic concurrent write to websocket connection under concurrent event dispatch (HistorySync / Receipts).
• Each *websocket.Conn is wrapped in wsConn with a per-connection write mutex; map access still uses RLock.
• Adds race-detector regression tests for instance and broadcast clients.

Closes #99

Test plan

• go test -race ./pkg/events/websocket/...
• Manual: enable WebSocket on an instance, trigger HistorySync / high receipt volume, confirm process stays up

Made with Cursor

Summary by Sourcery

Serialize websocket writes per connection to prevent concurrent write panics and add regression tests exercising concurrent instance and broadcast event delivery.

Bug Fixes:

• Prevent concurrent writes to a single websocket connection by introducing a per-connection write mutex wrapper.

Tests:

• Add race-detector style concurrency tests for instance-specific and broadcast websocket producers.

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Reviewer's Guide

Introduces a per-connection wrapper around gorilla/websocket connections to serialize writes and adds race-detector tests that exercise concurrent instance-specific and broadcast websocket event delivery.

Sequence diagram for serialized websocket writes via wsConn

sequenceDiagram
    participant Producer as websocketProducer
    participant Map as clients_and_broadcast
    participant WsConn as wsConn
    participant WS as websocket_Conn

    Producer->>Map: Produce(queueName, payload, instanceID)
    Map-->>Producer: wsConn for instanceID
    Producer->>WsConn: writeJSON(message)
    activate WsConn
    WsConn->>WsConn: writeMu.Lock()
    WsConn->>WS: WriteJSON(message)
    WS-->>WsConn: error or nil
    WsConn->>WsConn: writeMu.Unlock()
    deactivate WsConn
    WsConn-->>Producer: error or nil

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File-Level Changes

Change	Details	Files
Serialize all websocket writes per connection using a mutex-protected wrapper type to prevent concurrent writer panics while keeping map access concurrency as before.	Introduce wsConn struct holding *websocket.Conn plus a sync.Mutex and provide a writeJSON helper that locks around WriteJSON calls. Change websocketProducer client maps and broadcast slices to store *wsConn instead of *websocket.Conn and update constructor accordingly. Wrap new connections in wsConn when adding instance-specific and broadcast clients, and update removal logic to compare underlying *websocket.Conn. Update Produce to call wsConn.writeJSON for both instance-specific and broadcast sends while still guarding client lookups with clientsMux RLock.	pkg/events/websocket/websocket_producer.go
Add concurrency-focused tests that validate websocketProducer handles many concurrent Produce calls for both instance-specific and broadcast connections without races or errors.	Add a helper logger factory using temporary directories to avoid external logging side effects in tests. Add a dialTestWS helper that spins up an httptest server, upgrades to websocket, returns the server-side *websocket.Conn, and drains client reads. Add TestProduceConcurrentWrites with subtests that create many goroutines issuing Produce calls against a single instance client and a single broadcast client, asserting no errors occur under concurrency.	pkg/events/websocket/websocket_producer_test.go

Assessment against linked issues

Issue	Objective	Addressed	Explanation
#99	Prevent panic 'concurrent write to websocket connection' by serializing writes on each Gorilla WebSocket connection used by websocketProducer.Produce	✅
#99	Ensure both per-instance and broadcast WebSocket clients use the serialized-write mechanism when sending events under concurrent load	✅
#99	Add automated tests (including with -race) that exercise concurrent Produce calls for instance and broadcast clients to guard against regressions	✅

Possibly linked issues

• Panic: concurrent write to websocket connection when sending WebSocket events (0.7.2) #99: PR wraps each websocket.Conn with a per-connection write mutex, fixing the concurrent write panic reported.

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