Get Name of Current Module in Go

Question

I am attempting to create named loggers automatically for HTTP handlers that I'm writing, where I am passed a function (pointer).

I'm using the code mentioned in this question to get the name of a function:

package utils

import (
    "reflect"
    "runtime"
)

func GetFunctionName(fn interface{}) string {
    value := reflect.ValueOf(fn)
    ptr := value.Pointer()
    ffp := runtime.FuncForPC(ptr)

    return ffp.Name()
}

I'm using this in my main function to try it out like so:

package main

import (
    "github.com/naftulikay/golang-webapp/experiments/functionname/long"
    "github.com/naftulikay/golang-webapp/experiments/functionname/long/nested/path"
    "github.com/naftulikay/golang-webapp/experiments/functionname/utils"
    "log"
)

type Empty struct{}

func main() {
    a := long.HandlerA
    b := path.HandlerB
    c := path.HandlerC

    log.Printf("long.HandlerA: %s", utils.GetFunctionName(a))
    log.Printf("long.nested.path.HandlerB: %s", utils.GetFunctionName(b))
    log.Printf("long.nested.path.HandlerC: %s", utils.GetFunctionName(c))
}

I see output like this:

github.com/naftulikay/golang-webapp/experiments/functionname/long.HandlerA

This is okay but I'd like an output such as long.HandlerA, long.nested.path.HandlerB, etc.

If I could get the Go module name (github.com/naftulikay/golang-webapp/experiments/functionname), I can then use strings.Replace to remove the module name to arrive at long/nested/path.HandlerB, then strings.Replace to replace / with . to finally get to my desired value, which is long.nested.path.HandlerB.

The first question is: can I do better than runtime.FuncForPC(reflect.ValueOf(fn).Pointer()) for getting the qualified path to a function?

If the answer is no, is there a way to get the current Go module name using runtime or reflect so that I can transform the output of runtime.FuncForPC into what I need?

Once again, I'm getting values like:

• github.com/naftulikay/golang-webapp/experiments/functionname/long.HandlerA
• github.com/naftulikay/golang-webapp/experiments/functionname/long/nested/path.HandlerB
• github.com/naftulikay/golang-webapp/experiments/functionname/long/nested/path.HandlerC

And I'd like to get values like:

• long.HandlerA
• long.nested.path.HandlerB
• long.nested.path.HandlerC

EDIT: It appears that Go does not have a runtime representation of modules, and that's okay, if I can do it at compile time that would be fine too. I've seen the codegen documentation and I'm having a hard time figuring out how to write my own custom codegen that can be used from go generate.

Answer 1

The module info is included in the executable binary, and can be acquired using the debug.ReadBuildInfo() function (the only requirement is that the executable must be built using module support, but this is the default in the current version, and likely the only in future versions).

BuildInfo.Path is the current module's path.

Let's say you have the following go.mod file:

module example.com/foo

Example reading the build info:

bi, ok := debug.ReadBuildInfo()
if !ok {
    log.Printf("Failed to read build info")
    return
}

fmt.Println(bi.Main.Path)
// or
fmt.Println(bi.Path)

This will output (try it on the Go Playground):

example.com/foo
example.com/foo

See related: Golang - How to display modules version from inside of code

Answer 2

If your goal is to just have the name of the module available in your program, and if you are okay with setting this value at link time, then you may use the -ldflags build option.

You can get the name of the module with go list -m from within the module directory.

You can place everything in a Makefile or in a shell script:

MOD_NAME=$(go list -m)
go build -ldflags="-X 'main.MODNAME=$MOD_NAME'" -o main ./...

With main.go looking like:

package main

import "fmt"

var MODNAME string

func main() {
    fmt.Println(MODNAME) // example.com
}

---

With the mentioned "golang.org/x/mod/modfile" package, an example might look like:

package main

import (
    "fmt"
    "golang.org/x/mod/modfile"
    _ "embed"
)

//go:embed go.mod
var gomod []byte

func main() {
    f, err := modfile.Parse("go.mod", gomod, nil)
    if err != nil {
        panic(err)
    }
    fmt.Println(f.Module.Mod.Path) // example.com
}

However embedding the entire go.mod file in your use case seems overkill. Of course you could also open the file at runtime, but that means you have to deploy go.mod along with your executable. Setting the module name with -ldflags is more straightforward IMO.