cmd/go: include AllTags in go list

[jimmyfrasche]

https://golang.org/cl/12703044 added AllTags to *go/build.Package so that "[t]ools scanning packages can use this to decide how many variants there are and what they are."

Since it's often necessary to invoke go list rather than use go/build directly, I think the same reasoning applies for adding it to go list.

[jimmyfrasche]

cc: @rsc @ianlancetaylor @bcmills

[mvdan]

Also cc @alandonovan for go/packages.

[bcmills]

How does AllTags actually help with deciding the number and set of variants?

In theory, the number of variants is exponential with the number of tags, but in practice the number of combinations found in build constraints source files is much smaller than the powerset of tags.

[jimmyfrasche]

If you have a directory containing go files and you want to find some set of build tags such that loading the directory with each member of that set in turn leads to every go file in the directory being included at least once, you need to know what tags can affect selection.

If you go through every GOOS ⨯ GOARCH ⨯ release tag ⨯ cgo/!cgo ⨯ … ⨯ etc. you're looking at (way!) too many combinations and most of them will resolve to the same list of included files and you're ignoring user-defined tags and things like a GOOS added after you've compiled the tool doing the search.

I'm not entirely sure yet that AllTags is sufficient to perform such a search (I need to do some sketching/prototyping) but I do believe that it is necessary.

[bcmills]

We shouldn't have the go command build an index for a problem that it doesn't need to solve, so we're probably scanning file contents either way.

Probably the right solution for tools is to obtain the complete list of source files from go list, and then have some library that can take that list of files, mash them through the parser (in PackageClauseOnly mode?), and extract the unique sets of build constraints. I doubt that the build-constraint syntax will change enough over time for version skew to be much of a problem.

[jimmyfrasche]

That would work, too, I imagine.

I am not super familiar with that part of the codebase, but, based on my find/grep/go-to-definition spelunking, it looks like cmd/go/internal/load already computes AllTags, because go/build does. It just doesn't get copied into the struct that go list uses for output.

[bcmills]

Even if it already computes that data, I would argue that we shouldn't expose it unless we think tools will use it appropriately. The cmd/go documentation is already pretty verbose, and I don't want to spend our users' attention on things that have better alternatives anyway.