Optionally Run Cross Build?

[bitbier]

This is more a question and I'll tag it as such:

Is there a way to optionally check some kind of environment variable that if it exists, will not run cross-build-start? I ask because I've been trying to figure out a way of keeping the same Dockerfile for both building in CI (x86) and building on balena (ARM). Right now I have to have two separate Dockerfiles to build the same image minus the wrapping the installation code inside the

RUN [ "cross-build-start" ]
...
RUN [ "cross-build-end" ]

Thanks!

[bitbier]

Oh, there are no tags so can't tag this issue as a question.

[petrosagg]

@bitbier I banged my head for a long time trying to accomplish architecture parameterized dockerfiles but unfortunately Docker won't replace variables.

However you made me think about it again and I think there is a solution! It's a bit more verbose but I think if we have a dockerfile like this:

FROM balenalib/crossbuild

FROM raspbian
COPY --from=0 /xbuild /bin

RUN [ "xbuild", "start" ]
RUN ...                                                                                                                                                                                             
RUN [ "xbuild", "end" ]

And balenalib/crossbuild is set up as a multiarchitecture repo then Docker will automatically pull the right image and so on ARM xbuild could be a no-op but in x86 do what it currently does.

@nghiant2710 we should build this, it should be fairly easy to make

[bitbier]

Nice that would be awesome

[nghiant2710]

@petrosagg sorry, I missed this one. I will set up the crossbuild repository but how about we update balena-xbuild, https://github.com/balena-io-library/armv7hf-debian-qemu/blob/master/resin-xbuild.go#L44-L48 we just need to change here a bit, so if qemu doesn't exist (both arm and aarch64) then it's not ARM image and we do nothing here?

[jgentes]

We've got this working with:

# cross-build for x86
RUN if [ %%BALENA_ARCH%% = "i386" ] ; then [ "cross-build-start" ] ; fi

[bitbier]

I haven't tested this out yet, but doesn't the balena variables only exist for Dockerfile.template or will it apply those values on any Dockerfile being used for building and not just the Dockerfile.template.

Also these variables are only applied when building in Balena, so if you have Circle CI or some other CI process, you may need to do something like:

RUN if [ "%%BALENA_ARCH%%" = "\%\%BALENA_ARCH\%\%" ] || [ "%%BALENA_ARCH%%" = "i386"] ; then [ "cross-build-start" ] ; fi

^ untested.

May work to see if I can get this done sometime this week, but it would be nice to just have the multi-arch repo for the crossbuild as suggested above.

[petrosagg]

@jgentes @bitbier I don't think any approach based on RUN statements can work. Have you tried running the Dockerfile on both x86 and ARM machines?

The reason is that the if statement itself will be ran by /bin/bash which is a binary built either for ARM or x86 so when you try to run it on the wrong platform it won't even arrive at the point of running cross-build-start.

[bitbier]

I haven't. Right now we just have two separate Dockerfiles, one for CI and one for balena build/deployments. I was going to try it later this week, but I also think that you are right. I don't think it will work because it'll run in bash or sh like you stated above.

[jgentes]

@petrosagg I have tested it, and it works, however I'm only building on Balena - one app for x86 and the other for ARM. My base image is debian-node:10 so perhaps that's why it works on both.

[petrosagg]

Right, that's why. Our balena builders do some magic when they have to cross compile. Namely, we bind mount a qemu binary in the container and we have binfmt set up for emulation

[jgentes]

Another note.. building locally, my box shows up as amd64 so my line ultimately reads:

RUN if [ %%BALENA_ARCH%% = "i386" ] || [ %%BALENA_ARCH%% = "amd64" ] ; then [ "cross-build-start" ] ; fi