Go: cmd/compile: reject var main in package main

[tfausak]

What version of Go are you using?

1.8

What operating system and processor architecture are you using?

The Go Playground

What did you do?

https://play.golang.org/p/HBKmQFBAuv

package main

var main = func() {}

What did you expect to see?

Either a compile-time error telling me that main must be a function (not a variable), or for it to work. The spec says:

The main package must have package name main and declare a function main that takes no arguments and returns no value.

I didn't expect func() main {} and var main = func() {} to behave differently, but I understand that the latter isn't actually a function declaration.

What did you see instead?

panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0xffffffff addr=0x0 pc=0xd0940]

goroutine 1 [running]:
panic(0x9b520, 0xd0998)
    /usr/local/go/src/runtime/panic.go:531 +0x220
runtime.panicmem()
    /usr/local/go/src/runtime/panic.go:63 +0x80
runtime.sigpanic()
    /usr/local/go/src/runtime/os_nacl.go:60 +0x60
runtime.main()
    /usr/local/go/src/runtime/proc.go:194 +0x2e0
runtime.goexit()
    /usr/local/go/src/runtime/asm_amd64p32.s:1014 +0x1

[ALTree]

This is enough to trigger a runtime panic:

package main

var main int

I guess the question here is if we want the compiler to catch bad main names at compile time, or just let it crash at run time.

[ianlancetaylor]

I think the compiler should catch this. The language spec says that the package main is required to declare a function main.

[gopherbot]

Change https://golang.org/cl/53450 mentions this issue: Guard to panic whenmain = func() { }was declared

[griesemer]

@ALTree, @ianlancetaylor I don't believe that is correct: The spec says that a program is created by linking a single, unimported package main and that that package must contain a function main. It doesn't say that a package main cannot have variables, types, or constants called main. In fact, it is possible to have a library called main. For instance, given two files/packages tmp/a/a.go:

package main

var main int // <<< this is ok!

func F() { println("main.F") }

and tmp/x.go:

package main

import a "./a/a"

func main() {
    a.F()
}

it is possible to compile, link, and execute:

cd tmp
cd a
go tool compile a.go
cd ..
go tool compile x.go
go tool link x.o
./a.out

produces

main.F

as output.

This is not a compiler problem, this is a linker issue. We (or I) have long maintained (from day one in fact) that the main package shouldn't be special from the language's point of view; it's just a package. Similarly, identifiers called main shouldn't be special either.

Only when we link does it matter because we need to get to an entry point.

[ianlancetaylor]

I think that is less true these days than it used to be. For example, the compiler rejects import "main". It also rejects

package main

func main() int { return 0 }

[griesemer]

@ianlancetaylor Fair enough, but I'm not convinced yet that we should erode this further. There's a lot of implicit assumptions here that are not really written down in the spec. I also don't see much technical reason. It's basically an additional special case in the compiler where there doesn't need to be one as far as I can tell.

[mdempsky]

I was thinking the same objection as @griesemer yesterday: according to the Go spec, a package named "main" is not necessary the "main package".

Though seeing as cmd/go doesn't actually support packages named "main" and the compiler already treats packages named "main" as the main package for purposes of validating the "main" function's signature, I'm sort of inclined to just accept adding a compiler error against declaring main.main as a non-function.

[timakin]

I respect @griesemer 's idea, but honestly, as @mdempsky says, I also regard main as a little bit different definition from others.
localpkg.Name or node named main is often used for validation on compiler, so I was not so hesitated to add lines to validate var/const/type declarations whether they include main .

[griesemer]

@mdempsky I'm not per se against that but it would be an error that is not backed by the spec. As it is, the implementation is inconsistent. My suggestion: Let's see if the linker fix is trivial and if so, let's do it there. Otherwise, let's go back to the compiler. The main goal is to provide an good error message to the user rather than a crash. Also, this is really an esoteric issue, so not worth spending a lot of engineering complexity on it.

[timakin]

@griesemer Hi, I tried to fix this issue from the aspect of cmd/link.
Could you please check this commit?
https://go-review.googlesource.com/c/54330

[gopherbot]

Change https://golang.org/cl/54330 mentions this issue: cmd/link: changedvar main = func() { }to alert on build

[go101]

btw, for good or bad, the issue makes it is possible that a Go program can run okay without a main entry function.

package main

import (
    "fmt"
    "time"
)

var main int

func init() {
    for {
        time.Sleep(time.Second)
        fmt.Println("aaa")
    }
}

[rsc]

The compiler should be the one to reject this. As Ian noted it already checks func main's signature in package main. Package main is special.

[gopherbot]

Change https://golang.org/cl/79435 mentions this issue: cmd/compile: error if main.main is not a function