Buildkit: Progress is redirected to output on Travis-CI

[moikot]

I'm trying to build my Docker images on Travis-CI and redirect output back to Docker using these options:

 --exporter docker --exporter-opt name="${image}:${tag}" |  docker load 

But it is failing with message:

Error processing tar file(exit status 1): archive/tar: invalid tar header

I investigated the issue and found out that the stderr output used for progress is mixed with stdout and everything is streamed into Docker.

[AkihiroSuda]

Please try mkfifo a.tar; buildctl build --output type=docker,name="${image}:${tag}",dest=a.tar ... & docker load < a.tar & wait

[tonistiigi]

@AkihiroSuda What would be the reason the more hacky version is needed? buildctl should not print any progress output to stdout.

[AkihiroSuda]

I thought it makes sense to use stdout for --progress=plain, but no strong opinion

[tonistiigi]

Ah, didn't realize it was from the progress value. Yes, I think we should fix it and always put the progress to stderr.

[moikot]

@AkihiroSuda The trick with fifo worked perfectly, thank you! How I missed that dest option exists for output so that you can specify a file?

[AkihiroSuda]

The CLI was massively updated in https://github.com/moby/buildkit/pull/807

--output type=docker,name="${image}:${tag}",dest=a.tar can be also written in the legacy form: --exporter docker --exporter-opt name="${image}:${tag}" --exporter-opt output=a.tar.