fix sending on close channel with yet another mutex.

connection.go

@@ -7,6 +7,7 @@ import (
 	"errors"
 	"io"
 	"net/http"
+	"sync"
 	"sync/atomic"
 	"time"
 
@@ -21,6 +22,7 @@ type connection struct {
 	writeQueue   chan writeRequest
 	closed       *atomic.Bool
 	closedNotify chan struct{}
+	closeMutex   sync.Mutex
 }
 
 type writeRequest struct {
@@ -136,7 +138,9 @@ func (c *connection) doClose(code ws.StatusCode, reason string) {
 	wasClosed := c.closed.Swap(true)
 	if !wasClosed {
 		c.conn.Close(code, reason)
+		c.closeMutex.Lock()
 		close(c.closedNotify)
 		close(c.writeQueue)
+		c.closeMutex.Unlock()
 	}
 }

pool.go

@@ -471,7 +471,7 @@ func (pool *Pool) subMany(
 			subscribe:
 				sub, err = relay.Subscribe(ctx, filter, opts)
 				if err != nil {
-					debugLogf("[pool] subscription to %s died: %s -- will retry\n", nm, err)
+					debugLogf("[pool] subscription to %s failed: %s -- will retry\n", nm, err)
 					goto reconnect
 				}
 

relay.go

@@ -216,22 +216,33 @@ func (r *Relay) handleMessage(message string) {
 
 // Write queues an arbitrary message to be sent to the relay.
 func (r *Relay) Write(msg []byte) {
+	r.connection.closeMutex.Lock()
+	defer r.connection.closeMutex.Unlock()
 	select {
-	case r.connection.writeQueue <- writeRequest{msg: msg, answer: nil}:
 	case <-r.connection.closedNotify:
+		return
+	default:
+	}
+	select {
 	case <-r.connectionContext.Done():
+	case r.connection.writeQueue <- writeRequest{msg: msg, answer: nil}:
 	}
 }
 
 // WriteWithError is like Write, but returns an error if the write fails (and the connection gets closed).
 func (r *Relay) WriteWithError(msg []byte) error {
 	ch := make(chan error)
+	r.connection.closeMutex.Lock()
+	defer r.connection.closeMutex.Unlock()
 	select {
-	case r.connection.writeQueue <- writeRequest{msg: msg, answer: ch}:
-	case <-r.connectionContext.Done():
-		return fmt.Errorf("failed to write to %s: %w", r.URL, context.Cause(r.connectionContext))
 	case <-r.connection.closedNotify:
 		return fmt.Errorf("failed to write to %s: <closed>", r.URL)
+	default:
+	}
+	select {
+	case <-r.connectionContext.Done():
+		return fmt.Errorf("failed to write to %s: %w", r.URL, context.Cause(r.connectionContext))
+	case r.connection.writeQueue <- writeRequest{msg: msg, answer: ch}:
 	}
 	return <-ch
 }