Stdin builds hang if `BUILDX_EXPERIMENTAL=1` is set

[tonistiigi]

Description

BUILDX_EXPERIMENTAL=1 docker buildx build - <<eot                                                                    !1518
from alpine
eot

Hangs after

 => [internal] connecting to local controller

output.

Without the experimental opt-in all seems to work.

Probably related to the controller switch or the debugging features opt-in (cc @ktock @jedevc )

@crazy-max We should probably add BUILDX_EXPERIMENTAL also to the CI test matrix.

Buildx version

v0.13 (likely v0.12 as well)

[crazy-max]

I was looking at this issue and it hangs at

buildx/build/opt.go

Line 384 in 5c29e6e

magic, err := buf.Peek(archiveHeaderSize * 2)

where no data are being produced so it blocks indefinitely. I wonder if this is linked to

buildx/commands/build.go

Lines 421 to 428 in 5c29e6e

	f := ioset.NewSingleForwarder()
	f.SetReader(dockerCli.In())
	pr, pw := io.Pipe()
	f.SetWriter(pw, func() io.WriteCloser {
	pw.Close() // propagate EOF
	logrus.Debug("propagating stdin close")
	return nil
	})

where one end of the pipe is waiting for the other end to consume or produce data.