exec.go

package main

import (
	"fmt"
	"os"
	osexec "os/exec"
	"os/signal"
	"runtime"
	"strings"
	"syscall"
)

type environ []string

// Unset an environment variable by key
func (e *environ) Unset(key string) {
	for i := range *e {
		if strings.HasPrefix((*e)[i], key+"=") {
			(*e)[i] = (*e)[len(*e)-1]
			*e = (*e)[:len(*e)-1]
			break
		}
	}
}

// Set adds an environment variable, replacing any existing ones of the same key
func (e *environ) Set(key, val string) {
	e.Unset(key)
	*e = append(*e, key+"="+val)
}

func execSyscall(command string, args []string, env []string) error {
	argv0, err := osexec.LookPath(command)
	if err != nil {
		return fmt.Errorf("Couldn't find the executable '%s': %w", command, err)
	}

	argv := make([]string, 0, 1+len(args))
	argv = append(argv, command)
	argv = append(argv, args...)

	return syscall.Exec(argv0, argv, env)
}

func execCmd(command string, args []string, env []string) error {
	cmd := osexec.Command(command, args...)
	cmd.Stdin = os.Stdin
	cmd.Stdout = os.Stdout
	cmd.Stderr = os.Stderr
	cmd.Env = env

	sigChan := make(chan os.Signal, 1)
	signal.Notify(sigChan)

	if err := cmd.Start(); err != nil {
		return err
	}

	go func() {
		for {
			sig := <-sigChan
			cmd.Process.Signal(sig)
		}
	}()

	if err := cmd.Wait(); err != nil {
		cmd.Process.Signal(os.Kill)
		return fmt.Errorf("Failed to wait for command termination: %w", err)
	}

	waitStatus := cmd.ProcessState.Sys().(syscall.WaitStatus)
	os.Exit(waitStatus.ExitStatus())
	return nil
}

func supportsExecSyscall() bool {
	return runtime.GOOS == "linux" || runtime.GOOS == "darwin" || runtime.GOOS == "freebsd"
}

func run(port string, command string, args []string) error {
	env := environ(os.Environ())
	env.Set("PORT", port)
	if supportsExecSyscall() {
		return execSyscall(command, args, env)
	}
	return execCmd(command, args, env)
}