Fix WAL errors and add tests for it

wal.go

@@ -358,11 +358,14 @@ Loop:
 	if err := csf.Close(); err != nil {
 		return errors.Wrap(err, "close tmp file")
 	}
-
+	if err := renameFile(csf.Name(), candidates[0].Name()); err != nil {
-	if err := renameFile(csf.Name(), candidates[len(candidates)-1].Name()); err != nil {
 		return err
 	}
+
 	for _, f := range candidates[1:] {
+		if err := f.Close(); err != nil {
+			return errors.Wrap(err, "close obsolete WAL segment file")
+		}
 		if err := os.RemoveAll(f.Name()); err != nil {
 			return errors.Wrap(err, "delete WAL segment file")
 		}
@@ -371,6 +374,17 @@ Loop:
 		return err
 	}
 
+	// The file object of csf still holds the name before rename. Recreate it so
+	// subsequent truncations do not look at a non-existant file name.
+	csf.File, err = w.openSegmentFile(candidates[0].Name())
+	if err != nil {
+		return err
+	}
+	// We don't need it to be open.
+	if err := csf.Close(); err != nil {
+		return err
+	}
+
 	w.mtx.Lock()
 	w.files = append([]*segmentFile{csf}, w.files[len(candidates):]...)
 	w.mtx.Unlock()

wal_test.go

@@ -122,8 +122,8 @@ func TestSegmentWAL_cut(t *testing.T) {
 
 func TestSegmentWAL_Truncate(t *testing.T) {
 	const (
-		numMetrics = 250
+		numMetrics = 50
-		batch      = 50
+		batch      = 10
 	)
 	series, err := readPrometheusLabels("testdata/20k.series", numMetrics)
 	require.NoError(t, err)
@@ -160,10 +160,24 @@ func TestSegmentWAL_Truncate(t *testing.T) {
 		keep = append(keep, uint64(i))
 	}
 
+	err = w.Truncate(1000, newListPostings(keep))
+	require.NoError(t, err)
+
+	var expected []RefSeries
+
+	for i := 1; i <= numMetrics; i++ {
+		if i%2 == 1 || uint64(i) >= boundarySeries {
+			expected = append(expected, RefSeries{Ref: uint64(i), Labels: series[i-1]})
+		}
+	}
+
+	// Call Truncate once again to see whether we can read the written file without
+	// creating a new WAL.
 	err = w.Truncate(1000, newListPostings(keep))
 	require.NoError(t, err)
 	require.NoError(t, w.Close())
 
+	// The same again with a new WAL.
 	w, err = OpenSegmentWAL(dir, nil, 0)
 	require.NoError(t, err)
 
@@ -175,15 +189,7 @@ func TestSegmentWAL_Truncate(t *testing.T) {
 		return nil
 	}, nil, nil)
 
-	var expected []RefSeries
+	require.Equal(t, expected, readSeries)
-
-	for i := 1; i <= numMetrics; i++ {
-		if i%2 == 1 || uint64(i) >= boundarySeries {
-			expected = append(expected, RefSeries{Ref: uint64(i), Labels: series[i-1]})
-		}
-	}
-
-	require.Equal(t, len(expected), len(readSeries))
 }
 
 // Symmetrical test of reading and writing to the WAL via its main interface.