lib/chunkedbuffer: add Buffer.Len() method, which returns the byte length of the data stored in the buffer

lib/chunkedbuffer/chunked_buffer.go

@@ -17,6 +17,7 @@ const chunkSize = 4 * 1024
 type Buffer struct {
 	chunks []*[chunkSize]byte
 
+	// offset is the offset in the last chunk to write data to.
 	offset int
 }
 
@@ -38,6 +39,14 @@ func (cb *Buffer) SizeBytes() int {
 	return len(cb.chunks) * chunkSize
 }
 
+// Len returns the length of the data stored at cb.
+func (cb *Buffer) Len() int {
+	if len(cb.chunks) == 0 {
+		return 0
+	}
+	return (len(cb.chunks)-1)*chunkSize + cb.offset
+}
+
 // MustWrite writes p to cb.
 func (cb *Buffer) MustWrite(p []byte) {
 	for len(p) > 0 {
@@ -134,6 +143,7 @@ func (cb *Buffer) NewReader() filestream.ReadCloser {
 type reader struct {
 	cb *Buffer
 
+	// offset is the offset at cb to read the next data at Read call.
 	offset int
 }
 

lib/chunkedbuffer/chunked_buffer_test.go

@@ -23,6 +23,11 @@ func TestBuffer(t *testing.T) {
 			totalSize += len(b)
 		}
 
+		cbLen := cb.Len()
+		if cbLen != totalSize {
+			t.Fatalf("nexpected Buffer.Len value; got %d; want %d", cbLen, totalSize)
+		}
+
 		size := cb.SizeBytes()
 		if size < totalSize {
 			t.Fatalf("too small SizeBytes; got %d; want at least %d", size, totalSize)