how to deal with absent go.mod file in tool package when GO111MODULE=on

[Dan Kortschak]

As part of our testing we need to install a tool that currently does
not have go.mod/go.sum files. Since we test on Go versions both with
and without module support and want to test using modules on the
versions that have module support we need to use an approach that works
in both of those situations. We use git clone and checkout to get the
version we want.

However, this fails when GO111MODULE=on because go install wants to
find a go.mod and does not. Note that the tool has no external
dependencies.

```
$ mkdir -p /home/travis/gopath/src/github.com/goccmack
$ git clone https://github.com/goccmack/gocc.git /home/travis/gopath/src/github.com/goccmack/gocc
Cloning into '/home/travis/gopath/src/github.com/goccmack/gocc'...
$ cd /home/travis/gopath/src/github.com/goccmack/gocc
$ git checkout 0e2cfc030005b281b2e5a2de04fa7fe1d5063722
Note: checking out '0e2cfc030005b281b2e5a2de04fa7fe1d5063722'.

You are in 'detached HEAD' state. You can look around, make experimental
changes and commit them, and you can discard any commits you make in this
state without impacting any branches by performing another checkout.

If you want to create a new branch to retain commits you create, you may
do so (now or later) by using -b with the checkout command again. Example:

  git checkout -b <new-branch-name>

HEAD is now at 0e2cfc0... Merge pull request #78 from shivansh/fix/Makefile
$ go install
go: cannot find main module, but found .git/config in /home/travis/gopath/src/github.com/goccmack/gocc
to create a module there, run:
go mod init
```

My approach to dealing with this is to set GO111MODULE=auto for the go
install operation, but this is very kludgy and not guaranteed to work
if GO111MODULE=auto/off is not respected in the future by one Go
version (we support 3 minor versions of Go, so this could potentially
happen).

What is the recommended approach for this situation, assuming the tool
authors do not want to add a go.mod file?

[roger peppe]

It's not a standard tool, but you could use github.com/myitcv/gobin.

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[Dan Kortschak]

Thanks, though I don't think that will help us for our specific
problem. gobin does not compile for <go1.11 (prior to os.UserCacheDir)
so one Go version that we support will fail there without additional
logic to guard against this. At the moment what we have is a git clone
and a go install that work provided we shunt into GO111MODULE=off/auto
or there is go.mod in the repo (which doesn't exist upstream).

Also an additional detail, the change that introduced this is
65c206[1], so I can see why it's happening. Perhaps the only way around
it for us is to conditionally `go mod init || true` in testing to make
sure there is a go.mod present (short of adding go.mod upstream which I
have a PR for).

Dan


[1]https://github.com/golang/go/commit/65c2069a9f30cb6fa2c512d17dc0ad65
4d621da9