`Sync()`/`modifyDag()`: correctly handle offsets bigger than file size

[ipfs]

`Sync()`/`modifyDag()`: correctly handle offsets bigger than file size

[kjzz]

Hey @schomatis ,i am doing for this bug now.But i want to know that what i should do when
if cur+bs <= offset?

[schomatis]

I'm not really sure but I think we should replicate the logic from Seek,

go-unixfs/mod/dagmodifier.go

Lines 453 to 457 in 929f9a5

	if int64(newoffset) > fisize {
	if err := dm.expandSparse(int64(newoffset) - fisize); err != nil {
	return 0, err
	}
	}

where, if the offset is bigger than the file length, we expand the file DAG; copying that logic into the Sync function,

go-unixfs/mod/dagmodifier.go

Lines 203 to 225 in 929f9a5

	// overwrite existing dag nodes
	thisc, err := dm.modifyDag(dm.curNode, dm.writeStart)
	if err != nil {
	return err
	}
	dm.curNode, err = dm.dagserv.Get(dm.ctx, thisc)
	if err != nil {
	return err
	}
	// need to write past end of current dag
	if dm.wrBuf.Len() > 0 {
	dm.curNode, err = dm.appendData(dm.curNode, dm.splitter(dm.wrBuf))
	if err != nil {
	return err
	}
	err = dm.dagserv.Add(dm.ctx, dm.curNode)
	if err != nil {
	return err
	}
	}

Before appending the data, and after modifyDag has been called, we should check if writeStart (the offset where we should start writing) is bigger the the file size, and in that case (as before) also expand the file before appending.

Right now if you have to write to the offset 4 in a file of length 0 modifyDag will do nothing (which is correct, there is no existing space to overwrite) and Sync will just append the write buffer to the end of the file, that in this case would be the offset 0 (since that's the end of the file) which is incorrect, we want to fill the file DAG with zeros between the end of the DAG (offset 0) and the place to start writing (offset 4) and only then append the write data.

You'll need to check this patch against the go-ipfs tests, that means using gx-go link to make go-ipfs use your modified go-unixfs and not the Gx version.

[schomatis]

Fixed by #33.