test(sharedtest): make FakeLDClient.Close idempotent

internal/sharedtest/testclient/fake_client.go

@@ -36,6 +36,7 @@ type FakeLDClient struct {
 	dataSourceStatus *interfaces.DataSourceStatus
 	initialized      bool
 	lock             sync.Mutex
+	closeOnce        sync.Once
 }
 
 type CapturedLDClient struct {
@@ -68,9 +69,13 @@ func (c *FakeLDClient) GetDataStoreStatus() sdks.DataStoreStatusInfo {
 	return sdks.DataStoreStatusInfo{Available: true}
 }
 
+// Close is idempotent, matching the real SDK client: Relay may tear a client down from more than
+// one code path, and the second call must not panic.
 func (c *FakeLDClient) Close() error {
 	if c.CloseCh != nil {
-		close(c.CloseCh)
+		c.closeOnce.Do(func() {
+			close(c.CloseCh)
+		})
 	}
 	return nil
 }

internal/sharedtest/testclient/fake_client_test.go

@@ -0,0 +1,42 @@
+package testclient
+
+import (
+	"sync"
+	"testing"
+
+	"github.com/launchdarkly/ld-relay/v8/config"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+func TestFakeLDClientCloseIsIdempotent(t *testing.T) {
+	c := &FakeLDClient{Key: config.SDKKey("key"), CloseCh: make(chan struct{})}
+
+	require.NoError(t, c.Close())
+	require.NoError(t, c.Close())
+
+	select {
+	case <-c.CloseCh:
+	default:
+		assert.Fail(t, "CloseCh was not closed")
+	}
+}
+
+func TestFakeLDClientCloseIsSafeForConcurrentCallers(t *testing.T) {
+	c := &FakeLDClient{Key: config.SDKKey("key"), CloseCh: make(chan struct{})}
+
+	var wg sync.WaitGroup
+	for range 20 {
+		wg.Go(func() {
+			assert.NoError(t, c.Close())
+		})
+	}
+	wg.Wait()
+
+	select {
+	case <-c.CloseCh:
+	default:
+		assert.Fail(t, "CloseCh was not closed")
+	}
+}