Only keep 1 WAL entry (#22)

* Only keep 1 WAL entry; #21 * cleanup and remove more unused code
Preetam · Jan 7, 2017 · 4ee9e5a · 4ee9e5a
1 parent 95719a7
commit 4ee9e5a
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Showing 2 changed files with 30 additions and 98 deletions.
diff --git a/lm2.go b/lm2.go
@@ -32,9 +32,8 @@ type Collection struct {
 }
 
 type fileHeader struct {
-	Head              int64
-	LastCommit        int64
-	LastValidLogEntry int64
+	Head       int64
+	LastCommit int64
 }
 
 func (h fileHeader) bytes() []byte {
@@ -611,13 +610,11 @@ func (c *Collection) Update(wb *WriteBatch) (int64, error) {
 	// out to the WAL.
 	c.LastCommit = currentOffset
 	walEntry.Push(newWALRecord(0, c.fileHeader.bytes()))
-	logCommit, err := c.wal.Append(walEntry)
+	_, err = c.wal.Append(walEntry)
 	if err != nil {
 		return 0, err
 	}
 
-	c.LastValidLogEntry = logCommit
-
 	// Update + fsync data file header.
 
 	for _, walRec := range walEntry.records {
@@ -668,7 +665,7 @@ func NewCollection(file string, cacheSize int) (*Collection, error) {
 
 	// write file header
 	c.fileHeader.Head = 0
-	c.fileHeader.LastCommit = int64(8 * 3)
+	c.fileHeader.LastCommit = int64(8 * 2)
 	c.f.Seek(0, 0)
 	err = binary.Write(c.f, binary.LittleEndian, c.fileHeader)
 	if err != nil {
@@ -710,7 +707,7 @@ func OpenCollection(file string, cacheSize int) (*Collection, error) {
 	}
 	c.cache.c = c
 
-	// read file header
+	// Read file header.
 	c.f.Seek(0, 0)
 	err = binary.Read(c.f, binary.LittleEndian, &c.fileHeader)
 	if err != nil {
@@ -722,26 +719,9 @@ func OpenCollection(file string, cacheSize int) (*Collection, error) {
 	lastEntry, err := c.wal.ReadLastEntry()
 	if err != nil {
 		// Maybe latest WAL write didn't succeed.
-		// Read the last known good one.
-		err = c.wal.SetOffset(c.LastValidLogEntry)
-		if err != nil {
-			// Nothing else to do. Bail out.
-			c.Close()
-			return nil, err
-		}
-		lastEntry, err = c.wal.ReadEntry()
-		if err != nil {
-			if c.wal.fileSize > 0 {
-				// Nothing else to do. Bail out.
-				c.Close()
-				return nil, err
-			}
-		} else {
-			c.wal.Truncate()
-		}
-	}
-
-	if lastEntry != nil {
+		// Truncate.
+		c.wal.Truncate()
+	} else {
 		// Apply last WAL entry again.
 		for _, walRec := range lastEntry.records {
 			n, err := c.f.WriteAt(walRec.Data, walRec.Offset)
@@ -754,6 +734,14 @@ func OpenCollection(file string, cacheSize int) (*Collection, error) {
 				return nil, errors.New("lm2: incomplete data write")
 			}
 		}
+
+		// Reread file header because it could have been updated
+		c.f.Seek(0, 0)
+		err = binary.Read(c.f, binary.LittleEndian, &c.fileHeader)
+		if err != nil {
+			c.Close()
+			return nil, fmt.Errorf("lm2: error reading file header: %v", err)
+		}
 	}
 
 	c.f.Truncate(c.LastCommit)

diff --git a/wal.go b/wal.go
@@ -13,9 +13,7 @@ const (
 )
 
 type wal struct {
-	f              *os.File
-	fileSize       int64
-	lastGoodOffset int64
+	f *os.File
 }
 
 type walEntryHeader struct {
@@ -25,8 +23,7 @@ type walEntryHeader struct {
 }
 
 type walEntryFooter struct {
-	Magic       uint32
-	StartOffset int64
+	Magic uint32
 }
 
 type walEntry struct {
@@ -52,8 +49,7 @@ func newWALEntry() *walEntry {
 			NumRecords: 0,
 		},
 		walEntryFooter: walEntryFooter{
-			Magic:       walFooterMagic,
-			StartOffset: 0,
+			Magic: walFooterMagic,
 		},
 	}
 }
@@ -86,15 +82,8 @@ func openWAL(filename string) (*wal, error) {
 		return nil, err
 	}
 
-	stat, err := f.Stat()
-	if err != nil {
-		f.Close()
-		return nil, err
-	}
-
 	return &wal{
-		f:        f,
-		fileSize: stat.Size(),
+		f: f,
 	}, nil
 }
 
@@ -114,13 +103,6 @@ func newWAL(filename string) (*wal, error) {
 }
 
 func (w *wal) Append(entry *walEntry) (int64, error) {
-	startOffset, err := w.f.Seek(0, 2)
-	if err != nil {
-		w.Truncate()
-		return 0, errors.New("lm2: couldn't get offset")
-	}
-	entry.StartOffset = startOffset
-
 	buf := bytes.NewBuffer(nil)
 	for _, rec := range entry.records {
 		buf.Write(rec.Bytes())
@@ -129,71 +111,40 @@ func (w *wal) Append(entry *walEntry) (int64, error) {
 
 	binary.Write(buf, binary.LittleEndian, entry.walEntryFooter)
 
-	err = binary.Write(w.f, binary.LittleEndian, entry.walEntryHeader)
-	if err != nil {
-		w.Truncate()
-		return 0, err
-	}
+	headerBuf := bytes.NewBuffer(nil)
+	binary.Write(headerBuf, binary.LittleEndian, entry.walEntryHeader)
+	entryBytes := append(headerBuf.Bytes(), buf.Bytes()...)
 
-	n, err := w.f.Write(buf.Bytes())
+	n, err := w.f.WriteAt(entryBytes, 0)
 	if err != nil {
 		w.Truncate()
 		return 0, err
 	}
 
-	if n != buf.Len() {
+	if n != len(entryBytes) {
 		w.Truncate()
 		return 0, errors.New("lm2: incomplete WAL write")
 	}
 
-	currentOffset, err := w.f.Seek(0, 2)
-	if err != nil {
-		w.Truncate()
-		return 0, errors.New("lm2: couldn't get offset")
-	}
-
 	err = w.f.Sync()
 	if err != nil {
 		w.Truncate()
 		return 0, err
 	}
 
-	w.lastGoodOffset = currentOffset
-	return startOffset, nil
+	return 0, nil
 }
 
 func (w *wal) ReadLastEntry() (*walEntry, error) {
-	// Read footer
-	const footerSize = 12
-	_, err := w.f.Seek(-footerSize, 2)
-	if err != nil {
-		return nil, errors.New("lm2: error seeking to WAL footer")
-	}
-	footer := walEntryFooter{}
-	err = binary.Read(w.f, binary.LittleEndian, &footer)
-	if err != nil {
-		return nil, errors.New("lm2: error reading WAL footer")
-	}
-	if footer.Magic != walFooterMagic {
-		return nil, errors.New("lm2: invalid WAL footer magic")
-	}
-
-	// Read entry.
-
-	_, err = w.f.Seek(footer.StartOffset, 0)
+	_, err := w.f.Seek(0, 0)
 	if err != nil {
 		return nil, errors.New("lm2: error seeking to WAL entry start")
 	}
 
-	return w.ReadEntry()
-}
-
-func (w *wal) SetOffset(offset int64) error {
-	_, err := w.f.Seek(offset, 0)
-	return err
+	return w.readEntry()
 }
 
-func (w *wal) ReadEntry() (*walEntry, error) {
+func (w *wal) readEntry() (*walEntry, error) {
 	entry := newWALEntry()
 
 	err := binary.Read(w.f, binary.LittleEndian, &entry.walEntryHeader)
@@ -243,18 +194,11 @@ func (w *wal) ReadEntry() (*walEntry, error) {
 		return nil, errors.New("lm2: invalid WAL footer magic")
 	}
 
-	currentOffset, err := w.f.Seek(0, 2)
-	if err != nil {
-		return nil, errors.New("lm2: couldn't get offset")
-	}
-
-	w.lastGoodOffset = currentOffset
-
 	return entry, nil
 }
 
 func (w *wal) Truncate() error {
-	return w.f.Truncate(w.lastGoodOffset)
+	return w.f.Truncate(0)
 }
 
 func (w *wal) Close() {