cmd/go: go install: don’t fail when go.mod can’t be updated on a read-only system

[stapelberg]

$ go version
go version go1.13beta1 linux/amd64

I talked about this issue with @bcmills over GopherCon 2019 and he recommended I file this issue for tracking.

I am using cmd/go in a build environment where the entire system is read-only, but I have a writable copy of the source tree I’m building (e.g. github.com/junegunn/fzf). The system provides a read-only module cache (at /ro/gopath/pkg/mod) which contains all the dependencies of the source tree.

I’m using replace directives to enforce that my environment’s module versions are used, which can be different from the versions listed in go.mod/go.sum. See #29410 (comment) for details.

When running go install, it fails to lock the module cache because it’s read-only:

go: failed to lock file at /ro/gopath/pkg/mod/cache/lock

So far so good—it’s reasonable to not continuing when the side lock can’t be acquired. But, when specifying -mod=readonly, I’m getting:

go: updates to go.mod needed, disabled by -mod=readonly

I don’t know that go install should error out here. Certainly it could just continue with whatever changes needed just being retained in memory? As a user, I’m interested in the binary installation, not the side effect updates.

Thanks for considering,

[gopherbot]

Change https://golang.org/cl/204521 mentions this issue: cmd/go: default to mod=readonly when the go.mod file is read-only

[gopherbot]

Change https://golang.org/cl/204878 mentions this issue: cmd/go: make commands other than 'tidy' prune go.mod less agressively

[bcmills]

it fails to lock the module cache because it’s read-only:

That should be fixed by https://golang.org/cl/205637, which is now in.

@stapelberg, provided that the go.sum file is complete (or complete enough) and the go.mod file is consistent, this should now be fixed at head (and in Go 1.14 when that is released). Please give it a spin and let us know if there are any gaps remaining.