Attempt to reload dll in a golang program

README.md

I found that when we attempt to use the DLL generated by GO we get the following error:

fatal error: bad sweepgen in refill

runtime stack:
runtime.throw({0x807f8e?, 0xc0000c3df8?})
        C:/Program Files/Go/src/runtime/panic.go:992 +0x76
runtime.(*mcache).refill(0x271caaf0a28, 0x2)
        C:/Program Files/Go/src/runtime/mcache.go:156 +0x1ec
runtime.(*mcache).nextFree(0x271caaf0a28, 0x2)
        C:/Program Files/Go/src/runtime/malloc.go:886 +0x85
runtime.mallocgc(0x8, 0x7f69a0, 0x1)
        C:/Program Files/Go/src/runtime/malloc.go:1085 +0x40c
runtime.growslice(0x7f69a0, {0x0?, 0x76ad79?, 0x7be52d?}, 0xec7e1ff708?)
        C:/Program Files/Go/src/runtime/slice.go:278 +0x28d
runtime.doaddtimer(0x1c000032a00, 0xc0000e0000)
        C:/Program Files/Go/src/runtime/time.go:295 +0x95
runtime.modtimer(0xc0000e0000, 0x5af67e66487c, 0x0, 0x80d220, {0x7f6160?, 0xc00004a000}, 0x0)
        C:/Program Files/Go/src/runtime/time.go:493 +0x27c
runtime.resettimer(0x86a618?, 0x86a040?)
        C:/Program Files/Go/src/runtime/time.go:540 +0x2d
runtime.resetForSleep(0xc00004a000?, 0x400000002?)
        C:/Program Files/Go/src/runtime/time.go:203 +0x25
runtime.park_m(0xc00004a000?)
        C:/Program Files/Go/src/runtime/proc.go:3325 +0x8b
runtime.mcall()
        C:/Program Files/Go/src/runtime/asm_amd64.s:425 +0x4a

goroutine 1 [sleep]:
time.Sleep(0x3b9aca00)

When generating the DLL with g++ the module replacement works as expected. And it's possible to replace the function while the main program is still running.

Relevant reported issues:

• golang/go#22192
• golang/go#19282

Alternatives (to review in the future):

• https://github.com/edwingeng/hotswap

build_libs.ps1

if ($IsWindows) {

    Push-Location ./lib

    # C:\tools\msys64\usr\bin\env.exe `
    #     MSYSTEM=MINGW64 `
    #     CHERE_INVOKING=1 `
    #     /usr/bin/bash -lc "go build -buildmode=c-shared -gcflags='all=-N -l' -o ../dist/lib.dll ."

    C:\tools\msys64\usr\bin\env.exe `
        MSYSTEM=MINGW64 `
        CHERE_INVOKING=1 `
        /usr/bin/bash -lc "g++ -shared -o ../dist/lib.dll mathlib.c"


    Pop-Location

    if ($args[0] -eq 'all') {

        Push-Location ./uselib

        C:\tools\msys64\usr\bin\env.exe `
            MSYSTEM=MINGW64 `
            CHERE_INVOKING=1 `
            /usr/bin/bash -lc "go build -gcflags='all=-N -l' -o ../dist/uselib.exe ."

        Pop-Location
    }
}

lib-lib.go

package main

import "C"

//export Add
func Add(a, b int) int {
	return a + b + 80
}

func main() {}

// go build -buildmode=c-shared -gcflags='all=-N -l' -o ../dist/lib.dll .

lib-mathlib.c

#include <windows.h>
#include <stdlib.h>

extern "C" int Add(int a, int b)
{
  return a + b;
}



BOOL WINAPI DllMain(HINSTANCE hinstDLL, DWORD fdwReason, LPVOID lpReserved)
{
    return TRUE;
}

// g++ -shared -o ../dist/lib.dll mathlib.c

uselib-main.go

package main

import (
	"io"
	"os"
	"syscall"
	"time"
)

func main() {

	var lastModified time.Time
	var add *syscall.Proc
	var mathLib *syscall.DLL

	for {
		dllName := "lib.dll"

		file, err := os.Stat(dllName)

		if err != nil {
			println("Failed read the DLL file")
			return
		}

		modTime := file.ModTime()

		if modTime != lastModified {
			println(modTime.GoString(), lastModified.GoString())
			lastModified = modTime

			if mathLib != nil {
				mathLib.Release()
			}

			tmpDllName := lastModified.Format("0405") + "_" + dllName
			err := Copy(dllName, tmpDllName)

			if err != nil {
				println("Failed to copy the dll file")
			}

			mathLib = syscall.MustLoadDLL(tmpDllName)
			add = mathLib.MustFindProc("Add")
		}

		res, _, _ := add.Call(4, 7)

		println(res)
		time.Sleep(1 * time.Second)
	}
}

// Copy the src file to dst. Any existing file will be overwritten and will not
// copy file attributes.
func Copy(src, dst string) error {
	in, err := os.Open(src)
	if err != nil {
		return err
	}
	defer in.Close()

	out, err := os.Create(dst)
	if err != nil {
		return err
	}
	defer out.Close()

	_, err = io.Copy(out, in)
	if err != nil {
		return err
	}
	return out.Close()
}