cmd/go: list -json includes assembly files among CompiledGoFiles

[alandonovan]

$ go version
go version devel +a4e958ff2d Mon Nov 12 13:44:06 2018 -0500 linux/amd64
$ go list -json -compiled reflect
{
        "Dir": "/usr/local/google/home/adonovan/goroot/src/reflect",
        "ImportPath": "reflect",
        "Name": "reflect",
...
        "GoFiles": [
                "deepequal.go",
                "makefunc.go",
                "swapper.go",
                "type.go",
                "value.go"
        ],
        "CompiledGoFiles": [
                "deepequal.go",
                "makefunc.go",
                "swapper.go",
                "type.go",
                "value.go",
                "asm_amd64.s"   // !
        ],
        "SFiles": [
                "asm_amd64.s"
        ],
...

[gopherbot]

Change https://golang.org/cl/149237 mentions this issue: go/packages: remove .s files from go list's CompiledGoFiles

[josharian]

.cc files too. This is pretty clearly a bug in cmd/go:

• cmd/go/internal/work/exec.go, around line 590:

	var srcfiles []string // .go and non-.go
	srcfiles = append(srcfiles, gofiles...)
	srcfiles = append(srcfiles, sfiles...)
	srcfiles = append(srcfiles, cfiles...)
	srcfiles = append(srcfiles, cxxfiles...)
	b.cacheSrcFiles(a, srcfiles)

• cacheSrcFiles eventually does: c.PutBytes(cache.Subkey(a.actionID, "srcfiles"), buf.Bytes()).

• Later, loadCachedSrcFiles does: list, _, err := c.GetBytes(cache.Subkey(a.actionID, "srcfiles")) and then a.Package.CompiledGoFiles = files.

• This makes its way into the json that cmd/go emits, which go/packages trusts absolutely.

We should make cmd/go filter out non-Go files.

---

There's a comment in the go/packages source code, where it manually skips .s files:

		// Workaround for https://golang.org/issue/28749.
		// TODO(adonovan): delete before go1.12 release.

If we don't fix this for 1.12, we'll have to keep the workaround in go/packages for a long time. And expand it to cover .cc, .c, .S, etc.

I'll send a CL to expand the go/packages workaround, but I think it might be good to know how risky it is to fix this yet in cmd/go.

[gopherbot]

Change https://golang.org/cl/163597 mentions this issue: go/packages: expand CompiledGoFiles workaround