Segmentation violation with docker build --compress --stream

[thaJeztah]

moved from moby/moby#36579 - just tried, and this still reproduces on Docker 18.03.1

Running echo "FROM alpine" | docker build --compress --stream . -f - from an empty (or non-empty) folder produces this error:

panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x0 pc=0x4554000]

goroutine 50 [running]:
github.com/docker/cli/cli/command/image/build.Compress.func1(0xc42000e040, 0x0, 0x0)
	/go/src/github.com/docker/cli/cli/command/image/build/context.go:389 +0x70
created by github.com/docker/cli/cli/command/image/build.Compress
	/go/src/github.com/docker/cli/cli/command/image/build/context.go:384 +0x102

docker version
Client:
 Version:	18.02.0-ce-rc1
 API version:	1.35
 Go version:	go1.9.2
 Git commit:	5e1d90a
 Built:	Thu Jan 25 00:33:50 2018
 OS/Arch:	darwin/amd64
 Experimental:	true
 Orchestrator:	kubernetes

Server:
 Engine:
  Version:	18.02.0-ce-rc1
  API version:	1.36 (minimum version 1.12)
  Go version:	go1.9.3
  Git commit:	5e1d90a
  Built:	Thu Jan 25 00:40:43 2018
  OS/Arch:	linux/amd64
  Experimental:	true

[thaJeztah]

dump mentions this part of the code;

cli/cli/command/image/build/context.go

Lines 381 to 389 in a104852

	func Compress(buildCtx io.ReadCloser) (io.ReadCloser, error) {
	pipeReader, pipeWriter := io.Pipe()
	go func() {
	compressWriter, err := archive.CompressStream(pipeWriter, archive.Gzip)
	if err != nil {
	pipeWriter.CloseWithError(err)
	}
	defer buildCtx.Close()

[thaJeztah]

Added in a04aa8f (#233)

/cc @dnephin

[dnephin]

I guess using compress with stream should be a validation error because it doesn't send a context so there's no need to compress it.

I always found the stream logic was not well isolated, which made the rest of the code brittle. I attempted to improve this in #294, but it was never merged.