V3: Fix to use the version of goa specified in go.mod file, when 'goa gen'/'goa example'

[ikawaha]

Fix issue #2181

[raphael]

Thank you for the great issue report and the PR! A couple of questions:

1. Would it make sense to search for "go.mod" recursively going up the directories to support the case where a single Go module hosts multiple Goa projects, e.g.:

root
├── go.mod
├── project1
│   └── design
└── project2
    └── design

Here the goa command would be run within project1 and project2 and it would be nice if it could pickup the go.mod file under root like the go command does.

2. Instead of adding a dependency on a third party package ("github.com/sirkon/goproxy/gomod") would it be better to just copy the modfile package from the Go source code: https://github.com/golang/go/tree/master/src/cmd/go/internal/modfile. It's unfortunate that it is an internal package but it seems better to copy and give proper attribution than to add a dependency on another package.

Thank you again, this is a great contribution!

[ikawaha]

Thank you for your comment !

1. The location of go.mod to be used can be found by go env.
   (cf. https://dave.cheney.net/2013/06/14/you-dont-need-to-set-goroot-really)

$ echo $GOMOD

$ go env GOMOD
/Users/ikawaha/go/src/github.com/ikawaha/calc/go.mod

So, I fixed to use go env MOD instead of "go.mod".

2. That's right.
   However, go/internal/modfile (It can not be used alone. I think there needs modfetch, modload etc) seems a bit complicated. I think that it is best to be porting go/internal, but how about using this package until then?

[raphael]

Looks good, thanks

[raphael]

Looks like this function can never return an error which is good so the error return value can be removed.

[raphael]

Thinking a bit more about parsing go.mod: we really don't need a full parser. All we need is to find the line that starts with goa.design/goa/v3 and capture what's after. Seems a little heuristic here might be better than bringing in all these dependencies. Thank you!

[ikawaha]

I replaced the third party package with a heuristic parser.
Does it make sense?

[ikawaha]

It may be better to output the version of goa when go get. 🤔

[raphael]

The parser looks great! Thank you for making the change. I'm happy to merge the PR as is.

[ikawaha]

That's all.

[ikawaha]

Oops

[raphael]

I wonder if we need to print the version on the console? this may break scripts that wrap goa and capture its output.

[raphael]

Thank you!

[ikawaha]

What about displaying the version of goa modules by 'go get', when 'goa version' command run? If it looks good, I create another pull request.

[raphael]

Not sure about goa version but it would probably be OK to show it when running with the --debug flag.