x/tools/gopls: improve support for authoring cgo packages

[heschi]

Forked from #31561. This issue is not for users of cgo packages; that issue is #35720.

Beta instructions: #35721 (comment) contains instructions on how to build a version of Go and gopls that should work well for authoring cgo packages.

Files that import "C" are not really valid Go. They depend heavily on extra information that is produced by the go tool when it builds the package. They can be parsed, but not type checked, though there is some rudimentary support for working around the issues by setting the go/types.Config.FakeImportC flag.

Many people have suggested that gopls use the package's GoFiles rather than its CompiledGoFiles. The GoFiles are the original (invalid) files. Using those may give a slightly better experience for people editing the package, but it will completely break anything that depends on the type information, since the package will no longer type check. We would have to evaluate the effects very carefully before making that change in the current architecture.

We could consider type checking cgo packages twice, once with the real code and once with the user's code. That would be a significant architectural change.

Alternatively, we could run the cgo tool as the .go files change to produce the real code. This is a much more robust approach, but the cost of running cgo processing may be too high, especially on large packages.

No matter what, this is a large project that will need a lot of testing and thought. It's unlikely we'll make huge improvements here very soon.

[OneOfOne]

A suggestion (not sure how viable it is) would be to use clangd if it's available and proxy the C code to it.

[heschi]

That's probably overkill, but does point to an approach I didn't mention: tightly integrate cgo processing into gopls so that we can mitigate the performance problems of running the actual cgo tool. As you imply, the expensive part of cgo is learning about the C code. If we could separate that part from the code generation part, we could cache the C information and run just code generation when .go files changed. That might be the ideal solution, but would require changes to the cgo tool's implementation.

[mdempsky]

Possibly related to #16623?

(It's been a while since I've worked on any go/types-based tools though. I seem to recall something changed where it became easier to process cgo-using packages, if still not perfect, and working on #16623 dropped in priority for me... maybe column position information?)

[heschi]

Definitely related. If the net result of that change was that GoFiles was [x.go, y.go] and CompiledGoFiles was [x.go,y.go,$GOCACHE/.../_cgo_gotypes.go] per #16623 (comment) then I think we would be in much better shape.

[mdempsky]

@heschik To confirm, by "much better shape" do you mean you think there would still be further work to do to address this issue? (Seems like this would address #35720 too, actually.)

Looking at CLs 33677 and 33678 again, I think they're probably in reasonable shape to polish up for the next release cycle. I think the main issues are:

1. Is go/types.Config.UsesCgo the right API for enabling that setting within go/types?

2. CL 33678 needs to set go/types.Config.UsesCgo instead of FakeImportC. (CL 33677 PS1 originally just extended the semantics of FakeImportC; PS4 instead added UsesCgo, but it looks like I never pushed a matching revision of CL 33678.)

3. It probably also needs to be ported to go/packages too? How does gopls actually invoke go/types?

[heschi]

Further work: Yes -- we'd still have to regenerate the generated file sometimes. But that's a much more manageable concern.

#35720: I think we want a fix for that before 1.15 is released. Otherwise I'd say yeah.

If you have a set of patches that work against master, even if the API is rough, I'm happy to work to get go/packages working on top of it if that's necessary. But as long as go list -json returns the right stuff in CompiledGoFiles there shouldn't be anything much to do for either go/packages or gopls -- both just follow go list's lead.

[mdempsky]

@heschik FYI, I just rebased CL 33677 and fixed merge conflicts. I verified that tests still pass, but haven't done anything more extensive than that.

I don't see CompiledGoFiles when I run go list -json net though. Is cmd/go supposed to be running cmd/cgo?

[mdempsky]

Nevermind. Read the docs and realized it needs to be invoked as go list -json -compiled net.

[mdempsky]

I think cmd/go will need a new flag to indicate that you want the unprocessed versions of CgoFiles rather than the processed versions. Old tools might not be able to handle the processed versions, and we can't just break those. I'm not sure what the best way to control this is.

However, for the short term, here's what I think you can do within go/packages:

1. CompiledGoFiles consists of (in this order): GoFiles, cgo(CgoFiles), SFiles, CFiles, CXXFiles.

2. The cgo(CgoFiles) list consists of 1 entry per CgoFiles, plus 2 more fixed entres. _cgo_types.go is the first element (and is what provides the C pseudo-package declarations); the last one is _cgo_dynimport.go (which just provides compiler directives that influence linker behavior, and isn't relevant to go/types).

3. You want to pass to go/types: GoFiles, CgoFiles, and the first file from cgo(CgoFiles). I.e., rather than taking CompiledGoFiles and filtering out the non-go files, take GoFiles; and if CgoFiles is non-empty, take CgoFiles and CompiledGoFiles[len(GoFiles)] too.

4. You also need to set go/types.Config.UsesCgo = true.

[mdempsky]

https://go-review.googlesource.com/c/tools/+/208264 is an ugly hack to do the above. It passes go test, but I haven't tried using it with gopls.