address comments from CR

License: MIT
Signed-off-by: Jeromy <jeromyj@gmail.com>

mod/dagmodifier.go

@@ -368,19 +368,31 @@ func (dm *DagModifier) Seek(offset int64, whence int) (int64, error) {
 		return 0, err
 	}
 
+	fisize, err := dm.Size()
+	if err != nil {
+		return 0, err
+	}
+
+	var newoffset uint64
 	switch whence {
 	case os.SEEK_CUR:
-		dm.curWrOff += uint64(offset)
-		dm.writeStart = dm.curWrOff
+		newoffset = dm.curWrOff + uint64(offset)
 	case os.SEEK_SET:
-		dm.curWrOff = uint64(offset)
-		dm.writeStart = uint64(offset)
+		newoffset = uint64(offset)
 	case os.SEEK_END:
 		return 0, ErrSeekEndNotImpl
 	default:
 		return 0, ErrUnrecognizedWhence
 	}
 
+	if offset > fisize {
+		if err := dm.expandSparse(offset - fisize); err != nil {
+			return 0, err
+		}
+	}
+	dm.curWrOff = newoffset
+	dm.writeStart = newoffset
+
 	if dm.read != nil {
 		_, err = dm.read.Seek(offset, whence)
 		if err != nil {

mod/dagmodifier_test.go

@@ -487,6 +487,53 @@ func TestSparseWrite(t *testing.T) {
 	}
 }
 
+func TestSeekPastEndWrite(t *testing.T) {
+	dserv := getMockDagServ(t)
+	_, n := getNode(t, dserv, 0)
+	ctx, cancel := context.WithCancel(context.Background())
+	defer cancel()
+
+	dagmod, err := NewDagModifier(ctx, n, dserv, sizeSplitterGen(512))
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	buf := make([]byte, 5000)
+	u.NewTimeSeededRand().Read(buf[2500:])
+
+	nseek, err := dagmod.Seek(2500, os.SEEK_SET)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	if nseek != 2500 {
+		t.Fatal("failed to seek")
+	}
+
+	wrote, err := dagmod.Write(buf[2500:])
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	if wrote != 2500 {
+		t.Fatal("incorrect write amount")
+	}
+
+	_, err = dagmod.Seek(0, os.SEEK_SET)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	out, err := ioutil.ReadAll(dagmod)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	if err = arrComp(out, buf); err != nil {
+		t.Fatal(err)
+	}
+}
+
 func BenchmarkDagmodWrite(b *testing.B) {
 	b.StopTimer()
 	dserv := getMockDagServ(b)