backend/retry: hide final log for stat() method

stat is only used to check the config file's existence. We don't want
log output in this case.

internal/backend/retry/backend_retry.go

@@ -221,12 +221,19 @@ func (be *Backend) Load(ctx context.Context, h backend.Handle, length int, offse
 
 // Stat returns information about the File identified by h.
 func (be *Backend) Stat(ctx context.Context, h backend.Handle) (fi backend.FileInfo, err error) {
-	err = be.retry(ctx, fmt.Sprintf("Stat(%v)", h),
+	// see the call to `cancel()` below for why this context exists
+	statCtx, cancel := context.WithCancel(ctx)
+	defer cancel()
+
+	err = be.retry(statCtx, fmt.Sprintf("Stat(%v)", h),
 		func() error {
 			var innerError error
 			fi, innerError = be.Backend.Stat(ctx, h)
 
 			if be.Backend.IsNotExist(innerError) {
+				// stat is only used to check the existence of the config file.
+				// cancel the context to suppress the final error message if the file is not found.
+				cancel()
 				// do not retry if file is not found, as stat is usually used  to check whether a file exists
 				return backoff.Permanent(innerError)
 			}

internal/backend/retry/backend_retry_test.go

@@ -400,7 +400,11 @@ func TestBackendStatNotExists(t *testing.T) {
 	}
 
 	TestFastRetries(t)
-	retryBackend := New(be, 10, nil, nil)
+	retryBackend := New(be, 10, func(s string, err error, d time.Duration) {
+		t.Fatalf("unexpected error output %v", s)
+	}, func(s string, i int) {
+		t.Fatalf("unexpected log output %v", s)
+	})
 
 	_, err := retryBackend.Stat(context.TODO(), backend.Handle{})
 	test.Assert(t, be.IsNotExistFn(err), "unexpected error %v", err)