Patch unsafe producer map access

While doing a little work on #1235, my LLM flagged that the way that
we're accessing `producersByQueueName` today is already a little unsafe
as it may be read in multiple places concurrently. There's a mutex to
protect it somewhat in the `QueueBundle`, but it may still race between
a change there a "notify producer" send.

Here, add one additional `RWMutex` that makes sure to synchronize read
access on the map.

Author: brandur

CHANGELOG.md

@@ -7,6 +7,10 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
+### Fixed
+
+- Fix unsafe concurrent producer map access in client. [PR #1236](https://github.com/riverqueue/river/pull/1236).
+
 ## [0.35.1] - 2026-04-26
 
 ### Fixed

client.go

@@ -636,6 +636,7 @@ type Client[TTx any] struct {
 	periodicJobs           *PeriodicJobBundle
 	pilot                  riverpilot.Pilot
 	producersByQueueName   map[string]*producer
+	producersMu            sync.RWMutex
 	queueMaintainer        *maintenance.QueueMaintainer
 	queueMaintainerLeader  *maintenance.QueueMaintainerLeader
 	queues                 *QueueBundle
@@ -1959,7 +1960,14 @@ func (c *Client[TTx]) insertManyParams(params []InsertManyParams) ([]*rivertype.
 // transaction, the producer wouldn't yet be able to access the new jobs that
 // triggered the notification because they're not committed yet.
 func (c *Client[TTx]) notifyProducerWithoutListenerJobFetch(_ context.Context, res []*rivertype.JobInsertResult) {
-	if c.driver.SupportsListener() || len(c.producersByQueueName) < 1 {
+	if c.driver.SupportsListener() {
+		return
+	}
+
+	c.producersMu.RLock()
+	defer c.producersMu.RUnlock()
+
+	if len(c.producersByQueueName) < 1 {
 		return
 	}
 
@@ -2169,6 +2177,9 @@ func (c *Client[TTx]) validateJobArgs(args JobArgs) error {
 }
 
 func (c *Client[TTx]) addProducer(queueName string, queueConfig QueueConfig) (*producer, error) {
+	c.producersMu.Lock()
+	defer c.producersMu.Unlock()
+
 	if _, alreadyExists := c.producersByQueueName[queueName]; alreadyExists {
 		return nil, &QueueAlreadyAddedError{Name: queueName}
 	}
@@ -2722,7 +2733,14 @@ func (c *Client[TTx]) QueueUpdateTx(ctx context.Context, tx TTx, name string, pa
 // transaction, the producer wouldn't yet be able to access the state that
 // triggered the notification because it's not committed yet.
 func (c *Client[TTx]) notifyProducerWithoutListenerQueueControlEvent(queue string, controlEvent *controlEventPayload) {
-	if c.driver.SupportsListener() || len(c.producersByQueueName) < 1 {
+	if c.driver.SupportsListener() {
+		return
+	}
+
+	c.producersMu.RLock()
+	defer c.producersMu.RUnlock()
+
+	if len(c.producersByQueueName) < 1 {
 		return
 	}