fix buffer overwrite issue

[jim3ma]

Ⅰ. Describe what this PR did

RawContent func returns a buffer io, when enable autoReset, the buffer io can be reused by AcquireBufferSize, so, if first piece is processed slow than an other, the faster piece will overwrite buffer.

func (p *Piece) RawContent(noWrapper bool) *bytes.Buffer {
	contents := p.Content.Bytes()
	length := len(contents)
	defer func() {
		if p.autoReset {
			p.ResetContent()
		}
	}()

	if noWrapper {
		return bytes.NewBuffer(contents[:])
	}
	if length >= 5 {
		return bytes.NewBuffer(contents[4 : length-1])
	}
	return nil
}

Ⅱ. Does this pull request fix one issue?

fix #1415

Ⅲ. Why don't you add test cases (unit test/integration test)? (你真的觉得不需要加测试吗？)

N/A

Ⅳ. Describe how to verify it

N/A

Ⅴ. Special notes for reviews

[pouchrobot]

@jim3ma Thanks for your contribution. 🍻
Please sign off in each of your commits.

[jim3ma]

@lowzj @starnop PTAL

[zcc35357949]

You can not ResetContent here in writePieceToFile.
Because the same piece may be used in target writer queue after write finish in client writer queue.
https://github.com/dragonflyoss/Dragonfly/blob/master/dfget/core/downloader/p2p_downloader/client_writer.go#L223

[jim3ma]

when enable cross write, we will read piece content twice? but this is a buffer, it has index, when read twice, I think we need reset some index, #1411 is a good workaround.

[zcc35357949]

when enable cross write, we will read piece content twice? but this is a buffer, it has index, when read twice, I think we need reset some index, #1411 is a good workaround.

#1411 still can not solve this problem. Underlying Buf data will be reused. Piece should be reset after all writer process.

[zcc35357949]

I have rewritten the fix code #1411 , could you help me review whether it can work?

[jim3ma]

great!

[lowzj]

IMOP, the pull request #1411 is an appropriate solution. I think that we can work on it to make it better.