Making a Go program build with Go modules can be not a small change
In theory, at some point in the future Go will stop supporting
the traditional GOPATH mode. When this happens,
if you want to still build old Go programs that you have sitting
around in checked out version control repositories, you will need
to modularize them. Once upon a time, I thought that this would be
as simple as going to the root of your copy of the repo, then running
go mod init ...' and '
go mod tidy'. Unfortunately, life is not
this simple and there can be at least two complications.
The first complication is moved and renamed repositories for modules,
if the moved module has a
go.mod that declares its new name. For
example what is now github.com/hexops/vecty was once github.com/gopherjs/vecty. In a non-modular Go build, you
can still import it under the old path and it will work. However,
the moment you attempt to modularize the program, '
go mod tidy'
will complain and stop:
github.com/gopherjs/vecty: email@example.com: parsing go.mod: module declares its path as: github.com/hexops/vecty but was required as: github.com/gopherjs/vecty
In theory you may be able to get this to work with a go.mod
In practice my attempts to do this resulted in '
go mod tidy' errors
go: firstname.lastname@example.org used for two different module paths (github.com/gopherjs/vecty and github.com/hexops/vecty)
(You also need to get the version number or other version identifier of the moved repository.)
The general fix is to edit every import of packages from the module
to use the new location. Then you can run '
go mod tidy' without it
The second complication is modules that have moved to versions above
v1, possibly very far past v1; for example, github.com/google/go-github is up to v37, and modularized
at v18 (it doesn't even have a tagged v1). A GOPATH build of the
program you're trying to modularize will use whatever version of
the repository you have checked out, which may well be the current
one, and the code will import it as a version without a version
suffix (as '
github.com/google/go-github'). When you run '
tidy', Go will attempt to find the most recent tag (or version of
the repository) that doesn't have a
go.mod file, and specify that
version in your
go.mod with a '
+incompatible' tag. Depending on
how far Go had to rewind, this may be a version of the package that
is far older than the program expects.
go.mod existed for a v1 version, I suspect that '
tidy' will pick that in this case. But I haven't tried to test
it, partly for lack of a suitable module to test against. With
github.com/google/go-github, I get 'v17.0.0+incompatible',
the last tagged version before it was modularized.)
Again the fix is to edit the program's source code to change every
import of the package to use the proper versioned package. Instead
of importing, say, '
github.com/google/go-github/github', you would
(There may be other tools to do this package import renaming, but this is the one I could find.)
The unfortunate part of all of this is that it requires you to make changes to files that will be under version control in the repo. If the upstream updates things in the future, this will probably make your life more complicated.
(In some cases, '
go mod tidy' may insist that you clean up imports
in code that's in sub-packages in the repository that aren't actually
imported and used in the program itself.)