bug#30287: Go compiler tries writing to the store

[Leo Famulari <leo@famulari.name>]

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On Mon, Jan 29, 2018 at 10:50:10AM +0100, Peter Mikkelsen wrote:
> Hi guix,

Hi, thanks for this report! I saw your brief conversation on #guix with
civodul. Unfortunately I've only been paying attention to packaging Go
software with Guix, so I didn't notice this issue.

> For example, after setting the environment variable GOPATH to
> /home/peter/go, and creating a small hello world program in there, I
> would normally be able to run 'go install' in that folder, and the
> hello world binary would end up in my gopath. But on guixSD this is
> not the case, since it seems like 'go install' is trying to compile
> every single library provided by to go package itself, and that means
> writing to the store, which is not possible.

This is definitely a problem that we need to fix. However, I'm not sure
where to start...

I'll try debugging by checking if this works on Nix, and seeing what
they do differently.

> So my question is: Is there a way to prevent 'go install' compiling
> things in the store, maybe by pre-compiling during installation of the
> go package? Or is there another preferred way to develop go
> applications on guix, since what i do now is 'go build', but that gets
> really slow with larger projects, at it recompiles everything
> everytime if i understand it right.

The compiler should never try to write to the store; something is wrong
with how we build it.

Changing the subject slightly, yes, `go build` always recompiles some
things:

In general, for Go libraries (they call them "packages"), `go build`
always discards the compiled objects. You have to use `go install` to
retain them and achieve a caching speedup.

For Go executable programs (called "commands"), `go build` does keep the
compiled executable.

To quote the docs [0]:

"Build compiles the packages named by the import paths, along with their
dependencies, but it does not install the results. [...] When compiling
multiple packages or a single non-main package, build compiles the
packages but discards the resulting object, serving only as a check that
the packages can be built."

https://golang.org/cmd/go/

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