[dbnode] Ensure index data consistency

src/dbnode/persist/fs/merger.go

@@ -89,7 +89,7 @@ func (m *merger) Merge(
 	flushPreparer persist.FlushPreparer,
 	nsCtx namespace.Context,
 	onFlush persist.OnFlushSeries,
-) (err error) {
+) (persist.DataCloser, error) {
 	var (
 		reader         = m.reader
 		blockAllocSize = m.blockAllocSize
@@ -114,10 +114,12 @@ func (m *merger) Merge(
 			},
 			FileSetType: persist.FileSetFlushType,
 		}
+		closer persist.DataCloser
+		err    error
 	)
 
 	if err := reader.Open(openOpts); err != nil {
-		return err
+		return closer, err
 	}
 	defer func() {
 		// Only set the error here if not set by the end of the function, since
@@ -129,7 +131,7 @@ func (m *merger) Merge(
 
 	nsMd, err := namespace.NewMetadata(nsID, nsOpts)
 	if err != nil {
-		return err
+		return closer, err
 	}
 	prepareOpts := persist.DataPrepareOptions{
 		NamespaceMetadata: nsMd,
@@ -141,7 +143,7 @@ func (m *merger) Merge(
 	}
 	prepared, err := flushPreparer.PrepareData(prepareOpts)
 	if err != nil {
-		return err
+		return closer, err
 	}
 
 	var (
@@ -191,15 +193,15 @@ func (m *merger) Merge(
 
 	// The merge is performed in two stages. The first stage is to loop through
 	// series on disk and merge it with what's in the merge target. Looping
-	// through disk in the first stage is done intentionally to read disk
+	// through disk in the first stage is prepared intentionally to read disk
 	// sequentially to optimize for spinning disk access. The second stage is to
 	// persist the rest of the series in the merge target that were not
 	// persisted in the first stage.
 
 	// First stage: loop through series on disk.
 	for id, tagsIter, data, checksum, err := reader.Read(); err != io.EOF; id, tagsIter, data, checksum, err = reader.Read() {
 		if err != nil {
-			return err
+			return closer, err
 		}
 		idsToFinalize = append(idsToFinalize, id)
 
@@ -211,7 +213,7 @@ func (m *merger) Merge(
 		ctx.Reset()
 		mergeWithData, hasInMemoryData, err := mergeWith.Read(ctx, id, blockStart, nsCtx)
 		if err != nil {
-			return err
+			return closer, err
 		}
 		if hasInMemoryData {
 			segmentReaders = appendBlockReadersToSegmentReaders(segmentReaders, mergeWithData)
@@ -222,7 +224,7 @@ func (m *merger) Merge(
 		tags, err := convert.TagsFromTagsIter(id, tagsIter, identPool)
 		tagsIter.Close()
 		if err != nil {
-			return err
+			return closer, err
 		}
 		tagsToFinalize = append(tagsToFinalize, tags)
 
@@ -232,15 +234,15 @@ func (m *merger) Merge(
 		if len(segmentReaders) == 1 && hasInMemoryData == false {
 			segment, err := segmentReaders[0].Segment()
 			if err != nil {
-				return err
+				return closer, err
 			}
 
 			if err := persistSegmentWithChecksum(id, tags, segment, checksum, prepared.Persist); err != nil {
-				return err
+				return closer, err
 			}
 		} else {
 			if err := persistSegmentReaders(id, tags, segmentReaders, iterResources, prepared.Persist); err != nil {
-				return err
+				return closer, err
 			}
 		}
 		// Closing the context will finalize the data returned from
@@ -278,12 +280,11 @@ func (m *merger) Merge(
 			return err
 		}, nsCtx)
 	if err != nil {
-		return err
+		return closer, err
 	}
 
-	// Close the flush preparer, which writes the rest of the files in the
-	// fileset.
-	return prepared.Close()
+	// NB(bodu): Return a deferred closer so that we can guarantee that cold index writes are persisted first.
+	return prepared.DeferClose()
 }
 
 func appendBlockReadersToSegmentReaders(segReaders []xio.SegmentReader, brs []xio.BlockReader) []xio.SegmentReader {

src/dbnode/persist/fs/merger_test.go

@@ -437,6 +437,7 @@ func testMergeWith(
 	reader := mockReaderFromData(ctrl, diskData)
 
 	var persisted []persistedData
+	var deferClosed bool
 	preparer := persist.NewMockFlushPreparer(ctrl)
 	preparer.EXPECT().PrepareData(gomock.Any()).Return(
 		persist.PreparedDataPersist{
@@ -450,7 +451,13 @@ func testMergeWith(
 				})
 				return nil
 			},
-			Close: func() error { return nil },
+			DeferClose: func() (persist.DataCloser, error) {
+				return func() error {
+					require.False(t, deferClosed)
+					deferClosed = true
+					return nil
+				}, nil
+			},
 		}, nil)
 	nsCtx := namespace.Context{}
 
@@ -463,8 +470,11 @@ func testMergeWith(
 		BlockStart: startTime,
 	}
 	mergeWith := mockMergeWithFromData(t, ctrl, diskData, mergeTargetData)
-	err := merger.Merge(fsID, mergeWith, 1, preparer, nsCtx, &persist.NoOpColdFlushNamespace{})
+	close, err := merger.Merge(fsID, mergeWith, 1, preparer, nsCtx, &persist.NoOpColdFlushNamespace{})
 	require.NoError(t, err)
+	require.False(t, deferClosed)
+	require.NoError(t, close())
+	require.True(t, deferClosed)
 
 	assertPersistedAsExpected(t, persisted, expectedData)
 }

src/dbnode/persist/fs/persist_manager.go

@@ -200,7 +200,7 @@ func NewPersistManager(opts Options) (persist.Manager, error) {
 
 func (pm *persistManager) reset() {
 	pm.status = persistManagerIdle
-	pm.start = timeZero 
+	pm.start = timeZero
 	pm.count = 0
 	pm.bytesWritten = 0
 	pm.worked = 0
@@ -490,6 +490,7 @@ func (pm *persistManager) PrepareData(opts persist.DataPrepareOptions) (persist.
 
 	prepared.Persist = pm.persist
 	prepared.Close = pm.closeData
+	prepared.DeferClose = pm.deferCloseData
 
 	return prepared, nil
 }
@@ -544,6 +545,10 @@ func (pm *persistManager) closeData() error {
 	return pm.dataPM.writer.Close()
 }
 
+func (pm *persistManager) deferCloseData() (persist.DataCloser, error) {
+	return pm.dataPM.writer.DeferClose()
+}
+
 // DoneFlush is called by the databaseFlushManager to finish the data persist process.
 func (pm *persistManager) DoneFlush() error {
 	pm.Lock()

src/dbnode/persist/fs/types.go

@@ -92,6 +92,9 @@ type DataFileSetWriter interface {
 	// WriteAll will write the id and all byte slices and returns an error on a write error.
 	// Callers must not call this method with a given ID more than once.
 	WriteAll(id ident.ID, tags ident.Tags, data []checked.Bytes, checksum uint32) error
+
+	// DeferClose returns a DataCloser that defers writing of a checkpoint file.
+	DeferClose() (persist.DataCloser, error)
 }
 
 // SnapshotMetadataFileWriter writes out snapshot metadata files.
@@ -557,7 +560,7 @@ type Merger interface {
 		flushPreparer persist.FlushPreparer,
 		nsCtx namespace.Context,
 		onFlush persist.OnFlushSeries,
-	) error
+	) (persist.DataCloser, error)
 }
 
 // NewMergerFn is the function to call to get a new Merger.