tsdb: fix flaky TestWBLCorruption on Windows

TestWBLCorruption reads WBL segment sizes via os.ReadDir(...).Info().Size().
On Windows that size comes from the directory metadata cache, which is not
updated immediately for a segment that WL.NextSegment closes asynchronously
(Sync only fsyncs the current segment, not the previous one). This made the
"655 is not greater than 655" assertion fail intermittently.

Read the sizes through an open file handle instead (os.Open + File.Stat),
which uses GetFileInformationByHandle and returns the authoritative EOF
regardless of the directory cache or the async close.

Signed-off-by: György Krajcsovits <gyorgy.krajcsovits@grafana.com>
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Signed-off-by: György Krajcsovits <gyorgy.krajcsovits@grafana.com>

tsdb/db_test.go

@@ -8010,14 +8010,25 @@ func TestWBLCorruption(t *testing.T) {
 	// We corrupt WBL after the sample at 255. So everything added later
 	// should be deleted after replay.
 
+	// segmentSize returns the on-disk size of the named WBL segment. It opens
+	// the file and stats the handle instead of relying on os.ReadDir's cached
+	// directory metadata, which on Windows can be stale for a segment that was
+	// just closed asynchronously by WL.NextSegment.
+	segmentSize := func(name string) int64 {
+		f, err := os.Open(path.Join(db.head.wbl.Dir(), name))
+		require.NoError(t, err)
+		defer f.Close()
+		fi, err := f.Stat()
+		require.NoError(t, err)
+		return fi.Size()
+	}
+
 	// Checking where we corrupt it.
 	require.NoError(t, db.head.wbl.Sync()) // Syncing to make sure wbl is flushed in windows.
 	files, err := os.ReadDir(db.head.wbl.Dir())
 	require.NoError(t, err)
 	require.Len(t, files, 2)
-	f1, err := files[1].Info()
-	require.NoError(t, err)
-	corruptIndex := f1.Size()
+	corruptIndex := segmentSize(files[1].Name())
 	corruptFilePath := path.Join(db.head.wbl.Dir(), files[1].Name())
 
 	// Corrupt the WBL by adding a malformed record.
@@ -8038,15 +8049,9 @@ func TestWBLCorruption(t *testing.T) {
 	files, err = os.ReadDir(db.head.wbl.Dir())
 	require.NoError(t, err)
 	require.Len(t, files, 3)
-	f1, err = files[1].Info()
-	require.NoError(t, err)
-	require.Greater(t, f1.Size(), corruptIndex)
-	f0, err := files[0].Info()
-	require.NoError(t, err)
-	require.Greater(t, f0.Size(), int64(100))
-	f2, err := files[2].Info()
-	require.NoError(t, err)
-	require.Greater(t, f2.Size(), int64(100))
+	require.Greater(t, segmentSize(files[1].Name()), corruptIndex)
+	require.Greater(t, segmentSize(files[0].Name()), int64(100))
+	require.Greater(t, segmentSize(files[2].Name()), int64(100))
 
 	verifySamples := func(expSamples []chunks.Sample) {
 		sort.Slice(expSamples, func(i, j int) bool {
@@ -8082,12 +8087,8 @@ func TestWBLCorruption(t *testing.T) {
 	files, err = os.ReadDir(db.head.wbl.Dir())
 	require.NoError(t, err)
 	require.Len(t, files, 3)
-	f0, err = files[0].Info()
-	require.NoError(t, err)
-	require.Greater(t, f0.Size(), int64(100))
-	f2, err = files[2].Info()
-	require.NoError(t, err)
-	require.Equal(t, int64(0), f2.Size())
+	require.Greater(t, segmentSize(files[0].Name()), int64(100))
+	require.Equal(t, int64(0), segmentSize(files[2].Name()))
 	require.Equal(t, corruptFilePath, path.Join(db.head.wbl.Dir(), files[1].Name()))
 
 	// Verifying that everything after the corruption point is set to 0.