fix(cosmovisor): retry while upgrade-info.json is still empty

tools/cosmovisor/CHANGELOG.md

@@ -36,6 +36,10 @@ Ref: https://keepachangelog.com/en/1.0.0/
 
 ## [Unreleased]
 
+### Bug Fixes
+
+* [#26819](https://github.com/cosmos/cosmos-sdk/pull/26819) Fix the inverted retry condition when `upgrade-info.json` is still empty: the watcher now keeps waiting (up to 10 × 2 ms) until the write lands instead of giving up after the first empty stat.
+
 ### Improvements
 
 * [#23720](https://github.com/cosmos/cosmos-sdk/pull/23720) Get block height from db after node execution fails

tools/cosmovisor/scanner.go

@@ -143,7 +143,8 @@ func (fw *fileWatcher) CheckUpdate(currentUpgrade upgradetypes.Plan) bool {
 					panic(fmt.Errorf("failed to stat upgrade info file: %w", err))
 				}
 			}
-			if stat.Size() == 0 {
+			// stop waiting as soon as the write has landed
+			if stat.Size() != 0 {
 				break
 			}
 		}

tools/cosmovisor/scanner_test.go

@@ -1,8 +1,10 @@
 package cosmovisor
 
 import (
+	"os"
 	"path/filepath"
 	"testing"
+	"time"
 
 	"github.com/stretchr/testify/require"
 
@@ -90,3 +92,26 @@ func TestParseUpgradeInfoFile(t *testing.T) {
 		})
 	}
 }
+
+func TestCheckUpdateWaitsForEmptyFileWrite(t *testing.T) {
+	filename := filepath.Join(t.TempDir(), "upgrade-info.json")
+	require.NoError(t, os.WriteFile(filename, nil, 0o600))
+
+	fw := &fileWatcher{
+		filename:    filename,
+		currentInfo: upgradetypes.Plan{Name: "upgrade1", Height: 1},
+		initialized: true,
+	}
+
+	// Simulate the daemon finishing its write a few milliseconds after the
+	// watcher observed the empty file; CheckUpdate should keep waiting instead
+	// of giving up after the first still-empty stat.
+	go func() {
+		time.Sleep(5 * time.Millisecond)
+		_ = os.WriteFile(filename, []byte(`{"name":"upgrade2","height":10}`), 0o600)
+	}()
+
+	require.True(t, fw.CheckUpdate(upgradetypes.Plan{Name: "upgrade1"}))
+	require.Equal(t, "upgrade2", fw.currentInfo.Name)
+	require.Equal(t, int64(10), fw.currentInfo.Height)
+}