correctly handle offsets bigger than file size

mod/dagmodifier.go

@@ -200,6 +200,16 @@ func (dm *DagModifier) Sync() error {
 	// Number of bytes we're going to write
 	buflen := dm.wrBuf.Len()
 
+	fs, err := fileSize(dm.curNode)
+	if err != nil {
+		return err
+	}
+	if fs < dm.writeStart {
+		if err := dm.expandSparse(int64(dm.writeStart - fs)); err != nil {
+			return err
+		}
+	}
+
 	// overwrite existing dag nodes
 	thisc, err := dm.modifyDag(dm.curNode, dm.writeStart)
 	if err != nil {
@@ -225,8 +235,8 @@ func (dm *DagModifier) Sync() error {
 	}
 
 	dm.writeStart += uint64(buflen)
-
 	dm.wrBuf = nil
+
 	return nil
 }
 

mod/dagmodifier_test.go

@@ -7,12 +7,14 @@ import (
 	"io/ioutil"
 	"testing"
 
+	dag "github.com/ipfs/go-merkledag"
 	h "github.com/ipfs/go-unixfs/importer/helpers"
 	trickle "github.com/ipfs/go-unixfs/importer/trickle"
 	uio "github.com/ipfs/go-unixfs/io"
 	testu "github.com/ipfs/go-unixfs/test"
 
 	u "github.com/ipfs/go-ipfs-util"
+	"github.com/ipfs/go-unixfs"
 )
 
 func testModWrite(t *testing.T, beg, size uint64, orig []byte, dm *DagModifier, opts testu.NodeOpts) []byte {
@@ -410,6 +412,53 @@ func testDagTruncate(t *testing.T, opts testu.NodeOpts) {
 	}
 }
 
+func TestDagSync(t *testing.T) {
+	dserv := testu.GetDAGServ()
+	nd := dag.NodeWithData(unixfs.FilePBData(nil, 0))
+
+	ctx, cancel := context.WithCancel(context.Background())
+	defer cancel()
+
+	dagmod, err := NewDagModifier(ctx, nd, dserv, testu.SizeSplitterGen(512))
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	_, err = dagmod.Write([]byte("test1"))
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	err = dagmod.Sync()
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	err = dagmod.Truncate(0)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	_, err = dagmod.Write([]byte("test2"))
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	err = dagmod.Sync()
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	out, err := ioutil.ReadAll(dagmod)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	if err = testu.ArrComp(out[5:], []byte("test2")); err != nil {
+		t.Fatal(err)
+	}
+}
+
 // TestDagTruncateSameSize tests that a DAG truncated
 // to the same size (i.e., doing nothing) doesn't modify
 // the DAG (its hash).