go.mod placement introduces an ambiguous import error

[bcmills]

The go.mod file introduced for #259 effectively splits the ugorji/go repository into two separate modules: github.com/ugorji/go and github.com/ugorji/go/codec.

A repository without a go.mod file is effectively a single module, so prior to the introduction of that file, the latest tagged version (v1.1.1) had all of the packages in module github.com/ugorji/go, and some existing users already depended on that module (https://github.com/etcd-io/etcd/blob/329be66e8b3f9e2e6af83c123ff89297e49ebd15/go.mod#L45).

Now, if some program combines those existing users with a user of the new module, they receive two distinct copies of each package (via the two distinct modules), and the build fails with an ambiguous import error:

scratch$ go mod init golang.org/issue/scratch
go: creating new go.mod: module golang.org/issue/scratch

scratch$ go get -m github.com/ugorji/go@latest
go: finding github.com/ugorji/go v1.1.1

scratch$ go get -m github.com/ugorji/go/codec@latest
go: finding github.com/ugorji/go/codec latest

scratch$ go list github.com/ugorji/go/...
go: downloading github.com/ugorji/go v1.1.1
go: extracting github.com/ugorji/go v1.1.1
go: downloading github.com/ugorji/go/codec v0.0.0-20181209151446-772ced7fd4c2
go: extracting github.com/ugorji/go/codec v0.0.0-20181209151446-772ced7fd4c2
can't load package: package github.com/ugorji/go/codec: unknown import path "github.com/ugorji/go/codec": ambiguous import: found github.com/ugorji/go/codec in multiple modules:
        github.com/ugorji/go v1.1.1 (/tmp/tmp.m35NK6Fpqr/_gopath/pkg/mod/github.com/ugorji/go@v1.1.1/codec)
        github.com/ugorji/go/codec v0.0.0-20181209151446-772ced7fd4c2 (/tmp/tmp.m35NK6Fpqr/_gopath/pkg/mod/github.com/ugorji/go/codec@v0.0.0-20181209151446-772ced7fd4c2)
can't load package: package github.com/ugorji/go/codec/codecgen: unknown import path "github.com/ugorji/go/codec/codecgen": ambiguous import: found github.com/ugorji/go/codec/codecgen in multiple modules:
        github.com/ugorji/go v1.1.1 (/tmp/tmp.m35NK6Fpqr/_gopath/pkg/mod/github.com/ugorji/go@v1.1.1/codec/codecgen)
        github.com/ugorji/go/codec v0.0.0-20181209151446-772ced7fd4c2 (/tmp/tmp.m35NK6Fpqr/_gopath/pkg/mod/github.com/ugorji/go/codec@v0.0.0-20181209151446-772ced7fd4c2/codecgen)

---

To resolve this problem, I recommend that you add the following line to github.com/ugorji/go/codec/go.mod:

require github.com/ugorji/go v1.1.2-0.20180831062425-e253f1f20942

Until golang/go#27899 is resolved, you may need to re-add the line whenever you run go mod tidy, or add another .go source file to ensure that it is not removed (as described in golang/go#27899 (comment)).

[bcmills]

As a workaround in the interim, anyone running into the ambiguous import error can apply the fix themselves by running

go get github.com/ugorji/go@v1.1.2-0.20180831062425-e253f1f20942

[ugorji]

why v1.1.2-XXX ? I am a big confused, and I don't want to make assumptions.

[ugorji]

As a workaround in the interim, anyone running into the ambiguous import error can apply the fix themselves by running

go get github.com/ugorji/go@v1.1.2-0.20180831062425-e253f1f20942

I don't understand why this works? There's no tag v1.1.2. Is this a git thing, and if so, can you explain?

I truly don't understand the quirks of modules yet. I just want to support a multi-module repo, which is what I had been wanting from the beginning, and this feels like the best time to get it setup. I want folks to treat github.com/ugorji/go/codec as a module in itself (not github.com/ugorji/go)

[ugorji]

This should be fixed now, as at 8fd0f8d . Please let me know if this is fixed for you.

[bcmills]

Thanks for the fix!

why v1.1.2-XXX ?

That's the pseudo-version at which the packages were removed from the root module, so that requirement ensures that only the github.com/ugorji/go/codec module provides those packages.