Go: unable to build snap when using go mod and main package is not in the root

[pavel-d]

When using go modules, go build -o <path> will pick up main package only if it's in the root of repository.

Steps to reproduce:

0. Given a go module, where main package is located in ./cli subdirectory
1. Clone https://github.com/pavel-d/snaptest
2. cd snaptest
3. Run snapcraft build --use-lxd --debug

Result:

snapcraft --use-lxd  --debug
The LXD provider is offered as a technology preview for early adopters.
The command line interface, container names or lifecycle handling may change in upcoming releases.
Launching a container.
Waiting for container to be ready
......................
status: done
snapd 2.43.3 from Canonical✓ refreshed
Skipping pull snaptest (already ran)
Building snaptest 
go build -o /root/parts/snaptest/install/bin
go build: no main packages to build
Failed to run 'go build -o /root/parts/snaptest/install/bin' for 'snaptest': Exited with code 1.
Verify that the part is using the correct parameters and try again.
snapcraft-snaptest # snapcraft --version
snapcraft, version 3.10.1

The ./... pattern matches all the packages within the current module and go build will automatically build main packages in subdirectories.

• Have you followed the guidelines for contributing?
• Have you signed the CLA?
• Have you successfully run ./runtests.sh static?
• Have you successfully run ./runtests.sh tests/unit?

---

[cjp256]

Confirmed by https://github.com/golang/go/wiki/Modules#how-to-define-a-module

LGTM, pending tests pass.

[cjp256]

A couple of unit tests need to be updated:
https://travis-ci.org/snapcore/snapcraft/jobs/659245540#L7921

[sergiusens]

This is great, an integration test would be really good though, it can look exactly like the current go mod one but with a main package in a subdir. I can add it to this PR if you cannot do so.

[pavel-d]

@sergiusens, thanks, I've added an integration test.

Unfortunately, I haven't figured out how to run it locally, I hope I did it correctly :)

[cjp256]

Nice!

@pavel-d: If you have lxd installed, you can test it with:

# From inside snapcraft directory, build snapcraft snap to use for testing.
snapcraft snap

# Run spread test locally using LXD.
./runtests.sh spread lxd:tests/spread/plugins/go/run-go-mod-subdir

I am running it now myself...

[cjp256]

Tested, LGTM

[cjp256]

Unfortunately the CLA check is failing because one (or more) of the commits were made with @assembla.com address instead of the @travis-ci.com which is associated with the CLA.

[pavel-d]

@cjp256, oh, thanks. I've fixed my email address in commits.