Fix data race on Connection logger field

connection.go

@@ -77,7 +77,10 @@ type Connection struct {
 	inboundCtx    context.Context
 	inboundCancel context.CancelCauseFunc
 
-	logger *slog.Logger
+	// logger is atomic because SetLogger may be called after NewConnection has
+	// already spawned the receive/processNotifications goroutines that read it
+	// via loggerOrDefault.
+	logger atomic.Pointer[slog.Logger]
 
 	notifyMu sync.Mutex
 	// notifyCond coordinates response-scoped waits for sequential notification processing.
@@ -122,11 +125,11 @@ func NewConnection(handler MethodHandler, peerInput io.Writer, peerOutput io.Rea
 
 // SetLogger installs a logger used for internal connection diagnostics.
 // If unset, logs are written via the default logger.
-func (c *Connection) SetLogger(l *slog.Logger) { c.logger = l }
+func (c *Connection) SetLogger(l *slog.Logger) { c.logger.Store(l) }
 
 func (c *Connection) loggerOrDefault() *slog.Logger {
-	if c.logger != nil {
-		return c.logger
+	if l := c.logger.Load(); l != nil {
+		return l
 	}
 	return slog.Default()
 }

connection_logger_test.go

@@ -0,0 +1,57 @@
+package acp
+
+import (
+	"context"
+	"encoding/json"
+	"io"
+	"log/slog"
+	"sync"
+	"testing"
+)
+
+// TestConnectionSetLogger_ConcurrentWithReceive guards the logger field against
+// a data race: NewConnection spawns receive/processNotifications before the
+// caller has any chance to install a logger, so a later SetLogger writes the
+// field while those goroutines read it through loggerOrDefault.
+//
+// Unparseable input makes receive log, which is the read side of the race.
+// Run under -race; without synchronization on the field this fails.
+func TestConnectionSetLogger_ConcurrentWithReceive(t *testing.T) {
+	inR, inW := io.Pipe()
+	outR, outW := io.Pipe()
+	t.Cleanup(func() {
+		_ = inW.Close()
+		_ = outW.Close()
+		_ = inR.Close()
+		_ = outR.Close()
+	})
+
+	// Drain peer output so the connection never blocks on a write.
+	go func() { _, _ = io.Copy(io.Discard, outR) }()
+
+	c := NewConnection(func(ctx context.Context, method string, params json.RawMessage) (any, *RequestError) {
+		return nil, nil
+	}, outW, inR)
+
+	discard := slog.New(slog.NewTextHandler(io.Discard, nil))
+	// Install before any input arrives so the parse errors below stay off stderr.
+	c.SetLogger(discard)
+
+	const iterations = 100
+
+	var wg sync.WaitGroup
+	wg.Add(1)
+	go func() {
+		defer wg.Done()
+		for i := 0; i < iterations; i++ {
+			if _, err := inW.Write([]byte("not json\n")); err != nil {
+				return
+			}
+		}
+	}()
+
+	for i := 0; i < iterations; i++ {
+		c.SetLogger(discard)
+	}
+	wg.Wait()
+}