x/build: make precise Git commit hash → Go version mapping readily available #42376
Description
There are some places in x/build that try to determine a Go version of a Go commit in order to make some decisions (what builders should be run, etc.). So far we've been relying on heuristics to approximate the answer. There's some information in the name of the branch itself. For example, "release-branch.go1.15" can be parsed so we know that any commit on it is Go 1.15. If we know the latest Go release is Go 1.15.x, then we can guess that
a commit on "master" or "dev.link" branch is likely a development version of Go 1.(15+1).
By now, there is a package internal/goversion in the Go tree that tracks the Go version precisely. It's possible to create a service that processes new Git commits as they're pushed and do the work needed to determine the precise Go version of that Git commit (for example, by reading the git tree), then serve answers quickly.
This is an issue for tracking progress made towards having a service that's able to answer such "commit hash -> precise Go version" queries. (Including things like agreeing we want to do this, describing a design, etc.)
CC @golang/release, @mvdan.