-mod=readonly caused go install p@version to fail

[ma...@eliasnaur.com]

Hi,

I have GOFLAGS=-mod=readonly in my environment to avoid unnoticed changes to go.mod and go.sum. However, I'm getting a weird error from the new in 1.16 "go install p@version" form to install a Go program:

$ go version

go version devel +2de7866470 Fri Dec 18 18:25:14 2020 +0000 darwin/amd64

$ cd $(mktemp -d)

$ GOFLAGS=-mod=readonly go install gioui.org/cmd/gogio@40c082e94961adaf55886b45710a54d0247ff1bf
go install gioui.org/cmd/gogio@40c082e94961adaf55886b45710a54d0247ff1bf: gioui.org/c...@v0.0.0-20201218091821-40c082e94961: missing go.sum entry

Clearing GOFLAGS does work:

$ GOFLAGS= go install gioui.org/cmd/gogio@40c082e94961adaf55886b45710a54d0247ff1bf
go: downloading gioui.org v0.0.0-20201218091821-40c082e94961
go: downloading gioui.org/cmd v0.0.0-20201218091821-40c082e94961

I can't explain the difference, nor the error. Can you? In case it makes a difference, I have a checkout of gioui.org/cmd/gogio at the above revision where `go mod tidy` is a no-op.

Elias

[ma...@eliasnaur.com]

Filed as https://github.com/golang/go/issues/43278