git/odb/pack: teach *OffsetReaderAt to handle positioned reads

git/odb/pack/io.go

@@ -0,0 +1,27 @@
+package pack
+
+import "io"
+
+// OffsetReaderAt transforms an io.ReaderAt into an io.Reader by beginning and
+// advancing all reads at the given offset.
+type OffsetReaderAt struct {
+	// r is the data source for this instance of *OffsetReaderAt.
+	r io.ReaderAt
+
+	// o if the number of bytes read from the underlying data source, "r".
+	// It is incremented upon reads.
+	o int64
+}
+
+// Read implements io.Reader.Read by reading into the given []byte, "p" from the
+// last known offset provided to the OffsetReaderAt.
+//
+// It returns any error encountered from the underlying data stream, and
+// advances the reader forward by "n", the number of bytes read from the
+// underlying data stream.
+func (r *OffsetReaderAt) Read(p []byte) (n int, err error) {
+	n, err = r.r.ReadAt(p, r.o)
+	r.o += int64(n)
+
+	return n, err
+}

git/odb/pack/io_test.go

@@ -0,0 +1,53 @@
+package pack
+
+import (
+	"bytes"
+	"testing"
+
+	"github.com/git-lfs/git-lfs/errors"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestOffsetReaderAtReadsAtOffset(t *testing.T) {
+	bo := &OffsetReaderAt{
+		r: bytes.NewReader([]byte{0x0, 0x1, 0x2, 0x3}),
+		o: 1,
+	}
+
+	var x1 [1]byte
+	n1, e1 := bo.Read(x1[:])
+
+	assert.NoError(t, e1)
+	assert.Equal(t, 1, n1)
+
+	assert.EqualValues(t, 0x1, x1[0])
+
+	var x2 [1]byte
+	n2, e2 := bo.Read(x2[:])
+
+	assert.NoError(t, e2)
+	assert.Equal(t, 1, n2)
+	assert.EqualValues(t, 0x2, x2[0])
+}
+
+func TestOffsetReaderPropogatesErrors(t *testing.T) {
+	expected := errors.New("git/odb/pack: testing")
+	bo := &OffsetReaderAt{
+		r: &ErrReaderAt{Err: expected},
+		o: 1,
+	}
+
+	n, err := bo.Read(make([]byte, 1))
+
+	assert.Equal(t, expected, err)
+	assert.Equal(t, 0, n)
+}
+
+type ErrReaderAt struct {
+	Err error
+}
+
+func (e *ErrReaderAt) ReadAt(p []byte, at int64) (n int, err error) {
+	return 0, e.Err
+}