Fix double-delta unmarshaling to respect actual min header size

Turns out its valid to have an overall chunk which is smaller than the
full doubleDeltaHeaderBytes size -- if it has a single sample, it
doesn't fill the whole header.  Updated unmarshalling check to respect
this.

storage/local/delta_test.go

@@ -87,7 +87,7 @@ func TestUnmarshalingCorruptedDoubleDeltaReturnsAnError(t *testing.T) {
 	cs[0].marshalToBuf(buf)
 
 	// Corrupt the length to be every possible too-small value
-	for i := 0; i < doubleDeltaHeaderBytes; i++ {
+	for i := 0; i < doubleDeltaHeaderMinBytes; i++ {
 
 		binary.LittleEndian.PutUint16(buf[doubleDeltaHeaderBufLenOffset:], uint16(i))
 

storage/local/doubledelta.go

@@ -33,7 +33,8 @@ import (
 // - base time delta:          8 bytes
 // - base value delta:         8 bytes
 const (
-	doubleDeltaHeaderBytes = 37
+	doubleDeltaHeaderBytes    = 37
+	doubleDeltaHeaderMinBytes = 21 // header isn't full for chunk w/ one sample
 
 	doubleDeltaHeaderBufLenOffset         = 0
 	doubleDeltaHeaderTimeBytesOffset      = 2
@@ -250,8 +251,8 @@ func (c *doubleDeltaEncodedChunk) unmarshal(r io.Reader) error {
 	if int(l) > cap(*c) {
 		return fmt.Errorf("chunk length exceeded during unmarshaling: %d", l)
 	}
-	if int(l) < doubleDeltaHeaderBytes {
-		return fmt.Errorf("chunk length less than header size: %d < %d", l, doubleDeltaHeaderBytes)
+	if int(l) < doubleDeltaHeaderMinBytes {
+		return fmt.Errorf("chunk length less than header size: %d < %d", l, doubleDeltaHeaderMinBytes)
 	}
 
 	*c = (*c)[:l]
@@ -266,8 +267,8 @@ func (c *doubleDeltaEncodedChunk) unmarshalFromBuf(buf []byte) error {
 	if int(l) > cap(*c) {
 		return fmt.Errorf("chunk length exceeded during unmarshaling: %d", l)
 	}
-	if int(l) < doubleDeltaHeaderBytes {
-		return fmt.Errorf("chunk length less than header size: %d < %d", l, doubleDeltaHeaderBytes)
+	if int(l) < doubleDeltaHeaderMinBytes {
+		return fmt.Errorf("chunk length less than header size: %d < %d", l, doubleDeltaHeaderMinBytes)
 	}
 	*c = (*c)[:l]
 	return nil