proposal: runtime: add go:linkname on runtime.firstmoduledata

[timandy]

Goals

Add //go:linkname firstmoduledata on runtime.firstmoduledata with go1.23 release.

We want to access it by go:linkname, and keep compatibility with the old go versions.

Proposal Details

runtime.firstmoduledata provides many useful informations for low level develop.

For example: Get the file path of main function or test function.

// get path of the entry file, may be a /xx/xx_test.go or /xx/main.go
func getEntryFilePath() string {
	mainFile := getMainFile()
	if !isTestMode(mainFile) {
		return mainFile
	}
	firstModule := unsafe.Pointer(&firstmoduledata)
	fileTab := *(*[]byte)(reflection.Add(firstModule, filetabField.Offset))
	lastFiles := getLastFiles(fileTab)
	for _, file := range lastFiles {
		if isTestFile(file) {
			return file
		}
	}
	panic("can not found entry file path")
}

[tianon]

I think what Ian meant in #67401 (comment) was making proposals for actual exported interfaces for the data you need (such as your getEntryFilePath example), not just a proposal for more linkname (which is very very likely to be simply rejected, per the discussion in that other thread).

[ianlancetaylor]

Yes. This is not the kind of proposal I meant. I meant a maintainable way to access whatever kind of information you are looking for from runtime.firstmoduledata. For example, a proposal for a function to return the file path of the main function. (If you write such a proposal, please be sure to explain why the information is useful.)

I'm going to close this proposal. There is simply no way that we will adopt it. Sorry.