Review feedback

Signed-off-by: Chris Marchbanks <csmarchbanks@gmail.com>

head_test.go

@@ -1024,6 +1024,7 @@ func TestWalRepair(t *testing.T) {
 	}{
 		"invalid_record": {
 			func(rec []byte) []byte {
+				// Do not modify the base record because it is Logged multiple times.
 				res := make([]byte, len(rec))
 				copy(res, rec)
 				res[0] = byte(RecordInvalid)

wal/live_reader.go

@@ -174,7 +174,7 @@ func (r *LiveReader) buildRecord() (bool, error) {
 			r.snappyBuf = r.snappyBuf[:0]
 		}
 
-		compressed := r.hdr[0]&8 == 8
+		compressed := r.hdr[0]&snappyMask != 0
 		if compressed {
 			r.snappyBuf = append(r.snappyBuf, temp...)
 		} else {
@@ -188,6 +188,9 @@ func (r *LiveReader) buildRecord() (bool, error) {
 		if rt == recLast || rt == recFull {
 			r.index = 0
 			if compressed && len(r.snappyBuf) > 0 {
+				// The snappy library uses `len` to calculate if we need a new buffer.
+				// In order to allocate as few buffers as possible make the length
+				// equal to the capacity.
 				r.rec = r.rec[:cap(r.rec)]
 				r.rec, err = snappy.Decode(r.rec, r.snappyBuf)
 				if err != nil {

wal/reader.go

@@ -72,7 +72,7 @@ func (r *Reader) next() (err error) {
 		}
 		r.total++
 		r.curRecTyp = recTypeFromHeader(hdr[0])
-		compressed := hdr[0]&8 == 8
+		compressed := hdr[0]&snappyMask != 0
 
 		// Gobble up zero bytes.
 		if r.curRecTyp == recPageTerm {
@@ -139,6 +139,9 @@ func (r *Reader) next() (err error) {
 		}
 		if r.curRecTyp == recLast || r.curRecTyp == recFull {
 			if compressed && len(r.snappyBuf) > 0 {
+				// The snappy library uses `len` to calculate if we need a new buffer.
+				// In order to allocate as few buffers as possible make the length
+				// equal to the capacity.
 				r.rec = r.rec[:cap(r.rec)]
 				r.rec, err = snappy.Decode(r.rec, r.snappyBuf)
 				return err

wal/wal.go

@@ -477,6 +477,14 @@ func (w *WAL) flushPage(clear bool) error {
 	return nil
 }
 
+// First Byte of header format:
+// [ 4 bits unallocated] [1 bit snappy compression flag] [ 3 bit record type ]
+
+const (
+	recTypeMask = 7
+	snappyMask  = 8
+)
+
 type recType uint8
 
 const (
@@ -488,7 +496,7 @@ const (
 )
 
 func recTypeFromHeader(header byte) recType {
-	return recType(header & 7)
+	return recType(header & recTypeMask)
 }
 
 func (t recType) String() string {
@@ -553,7 +561,9 @@ func (w *WAL) log(rec []byte, final bool) error {
 
 	compressed := false
 	if w.compress && len(rec) > 0 {
-		// Allow Snappy to use the full capacity of the buffer.
+		// The snappy library uses `len` to calculate if we need a new buffer.
+		// In order to allocate as few buffers as possible make the length
+		// equal to the capacity.
 		w.snappyBuf = w.snappyBuf[:cap(w.snappyBuf)]
 		w.snappyBuf = snappy.Encode(w.snappyBuf, rec)
 		if len(w.snappyBuf) < len(rec) {