Downloader: http panic connCount underflow

[AskAlexSharov]

(I thought that after #9962 easier to reproduce next panics, but seems not).

panic: net/http: internal error: connCount underflow

goroutine 1030540 [running]:
net/http.(*Transport).decConnsPerHost(0xc0008dc9a0, {{0x0, 0x0}, {0xc00afd8980, 0x5}, {0xc0274e0360, 0x2f}, 0x0})
	net/http/transport.go:1511 +0x405
net/http.(*Transport).roundTrip(0xc0008dc9a0, 0xc0039eb9e0)
	net/http/transport.go:618 +0x876
net/http.(*Transport).RoundTrip(...)
	net/http/roundtrip.go:17
github.com/ledgerwatch/erigon-lib/downloader.(*requestHandler).RoundTrip(0xc0008dc9a0, 0xc0039eb9e0)
	github.com/ledgerwatch/erigon-lib@v1.0.0/downloader/downloader.go:154 +0x11d
net/http.send(0xc0039eb9e0, {0x18533c0, 0xc0008dc9a0}, {0xc04c4bf501?, 0xc04c4bf788?, 0x0?})
	net/http/client.go:259 +0x5e4
net/http.(*Client).send(0xc000b927b0, 0xc0039eb9e0, {0x523de5?, 0xc0420d0b60?, 0x0?})
	net/http/client.go:180 +0x98
net/http.(*Client).do(0xc000b927b0, 0xc0039eb9e0)
	net/http/client.go:724 +0x8dc
net/http.(*Client).Do(...)
	net/http/client.go:590
github.com/anacrolix/torrent/webseed.(*Client).NewRequest.func1.1.1()
	github.com/anacrolix/torrent@v1.54.1/webseed/client.go:97 +0x27
created by github.com/anacrolix/torrent/webseed.(*Client).NewRequest.func1.1 in goroutine 1030501
	github.com/anacrolix/torrent@v1.54.1/webseed/client.go:96 +0x90
exit status 2

and

panic: net/http: internal error: connCount underflow

goroutine 63612 [running]:
net/http.(*Transport).decConnsPerHost(0xc000f894a0, {{0x0, 0x0}, {0xc02e36ef80, 0x5}, {0xc02b114ed0, 0x2f}, 0x0})
	net/http/transport.go:1511 +0x405
net/http.(*persistConn).closeLocked(0xc032db2900, {0x1852e20, 0x21bae10})
	net/http/transport.go:2764 +0xe5
net/http.(*persistConn).close(0xc000f894a0?, {0x1852e20?, 0x21bae10?})
	net/http/transport.go:2754 +0xa8
net/http.(*Transport).putOrCloseIdleConn(0xc000f894a0?, 0xc032db2900)
	net/http/transport.go:913 +0x2d
net/http.(*Transport).dialConnFor(0xc000f894a0, 0xc015572fd0)
	net/http/transport.go:1491 +0x11d
created by net/http.(*Transport).queueForDial in goroutine 57934
	net/http/transport.go:1461 +0x1cd
exit status 2

maybe it's because of some resp.Body not closed
maybe it's because another panic happened (but we don't see it) - maybe inside new RoundTrip func

[mh0lt]

I think that the underlying issue is in:

https://github.com/anacrolix/torrent/blob/master/webseed/client.go

func readRequestPartResponses(ctx context.Context, parts []requestPart) (_ []byte, err error) {
	var buf bytes.Buffer
	for _, part := range parts {
		part.start()  <--This calls http.Client Do in a seperate go routine
		err = recvPartResult(ctx, &buf, part) <--This waits on a channel in this go routine and calls resp.Body.Close()
		if err != nil {
			err = fmt.Errorf("reading %q at %q: %w", part.req.URL, part.req.Header.Get("Range"), err)
			break
		}
	}
	return buf.Bytes(), err
}

The code is here:

part.start = func() {
			go func() {
				resp, err := ws.HttpClient.Do(req)
				part.result <- requestPartResult{
					resp: resp,
					err:  err,
				}
			}()
}

and here:

func recvPartResult(ctx context.Context, buf io.Writer, part requestPart) error {
	result := <-part.result
	// Make sure there's no further results coming, it should be a one-shot channel.
	close(part.result)
	if result.err != nil {
		return result.err
	}
	defer result.resp.Body.Close()
	var body io.Reader = result.resp.Body

I dont think the inner go routine is required as the calls are already asynchronous so start can be just:

  func() (resp *http.Response, err  error) {
     return ws.HttpClient.Do(req)
  }()

I think it will remove the potential race condition

[AskAlexSharov]

fixed by anacrolix/torrent#933