cmd/go: make cache keys for libraries in GOROOT insensitive to `-trimpath`

[bcmills]

In CL 380915 we observed that libraries in GOROOT contain the literal string $GOROOT instead of their absolute paths, even when -trimpath is not set.

As a result, we should (theoretically) compile the same bytes for those libraries regardless of whether -trimpath is set. We can therefore omit -trimpath from the cache key, allowing object files already in the cache to be reused in more cases.

We should add a script test that verifies that the library object files really don't depend on the -trimpath flag, make the flags passed to the compiler ignore cfg.BuildTrimpath for those libraries, and then update cmd/go/internal/work to omit it from the cache key.

(Note that we cannot omit it from the cache key for executables, since those are stamped with the actual GOROOT or GOROOT_FINAL during linking.)

[bcmills]

(This issue was noticed while investigating #57328.)

[juliandanciu]

Saw the help wanted tag on this issue. May I try my hand at it? It would be my first. (@mvdan , does the 👍🏼 ❤️ on the initial comment mean you are already working on it?)

[mvdan]

My reactions are only in support. In general, if anyone was working on it, it would be assigned.

[quantonganh]

We should add a script test that verifies that the library object files really don't depend on the -trimpath flag

An expanded build ID has the following format:

actionID(binary)/actionID(main.a)/contentID(main.a)/contentID(binary)

Given 2 builds with and without -trimpath:

$ go tool buildid a.out
jeyy1T5dc2drVv2G8FBq/-gUKbztC4KAaY-khMcPo/LpRHUI2WYOhYr4OcEpMe/pBhAE-qkrmmJf4qPqOn1

$ go tool buildid b.out
OkMoPTcQGbTbAGdw-pcM/-gUKbztC4KAaY-khMcPo/LpRHUI2WYOhYr4OcEpMe/5KJVugRnLU7aFU9NZsNL

Is it possible to just compare actionID(main.a) and actionID(main.b) (-gUKbztC4KAaY-khMcPo in this case) using testscript?

[quantonganh]

I found a way to do it:

go build -o a.out
go tool buildid a.out
cp stdout a-buildid.txt
exec awk -F/ '{print $2}' a-buildid.txt
cp stdout a-actionid.txt

go build -x -o b.out -trimpath
go tool buildid b.out
cp stdout b-buildid.txt
exec awk -F/ '{print $2}' b-buildid.txt
cp stdout b-actionid.txt
cmp a-actionid.txt b-actionid.txt

[gopherbot]

Change https://go.dev/cl/542435 mentions this issue: cmd/go: omit -trimpath from the cache key