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// Package reexec facilitates the busybox style reexec of a binary.
//
// Handlers can be registered with a name and the argv 0 of the exec of
// the binary will be used to find and execute custom init paths.
//
// It is used to work around forking limitations when using Go.
package reexec
import (
"context"
"fmt"
"os"
"os/exec"
"path/filepath"
"runtime"
)
var registeredInitializers = make(map[string]func())
// Register adds an initialization func under the specified name. It panics
// if the given name is already registered.
func Register(name string, initializer func()) {
if filepath.Base(name) != name {
panic(fmt.Sprintf("reexec func does not expect a path component: %q", name))
}
if _, exists := registeredInitializers[name]; exists {
panic(fmt.Sprintf("reexec func already registered under name %q", name))
}
registeredInitializers[name] = initializer
}
// Init is called as the first part of the exec process and returns true if an
// initialization function was called.
func Init() bool {
if initializer, ok := registeredInitializers[filepath.Base(os.Args[0])]; ok {
initializer()
return true
}
return false
}
// Command returns an [*exec.Cmd] with its Path set to the path of the current
// binary using the result of [Self].
//
// On Linux, the Pdeathsig of [*exec.Cmd.SysProcAttr] is set to SIGTERM.
// This signal is sent to the process when the OS thread that created
// the process dies.
//
// It is the caller's responsibility to ensure that the creating thread is
// not terminated prematurely. See https://go.dev/issue/27505 for more details.
func Command(args ...string) *exec.Cmd {
return command(args...)
}
// CommandContext is like [Command] but includes a context. It uses
// [exec.CommandContext] under the hood.
//
// The provided context is used to interrupt the process
// (by calling cmd.Cancel or [os.Process.Kill])
// if the context becomes done before the command completes on its own.
//
// CommandContext sets the command's Cancel function to invoke the Kill method
// on its Process, and leaves its WaitDelay unset. The caller may change the
// cancellation behavior by modifying those fields before starting the command.
func CommandContext(ctx context.Context, args ...string) *exec.Cmd {
return commandContext(ctx, args...)
}
// Self returns the path to the current process's binary.
//
// On Linux, it returns "/proc/self/exe", which provides the in-memory version
// of the current binary. This makes it safe to delete or replace the on-disk
// binary (os.Args[0]).
//
// On Other platforms, it attempts to look up the absolute path for os.Args[0],
// or otherwise returns os.Args[0] as-is. For example if current binary is
// "my-binary" at "/usr/bin/" (or "my-binary.exe" at "C:\" on Windows),
// then it returns "/usr/bin/my-binary" and "C:\my-binary.exe" respectively.
func Self() string {
if runtime.GOOS == "linux" {
return "/proc/self/exe"
}
return naiveSelf()
}
func naiveSelf() string {
name := os.Args[0]
if filepath.Base(name) == name {
if lp, err := exec.LookPath(name); err == nil {
return lp
}
}
// handle conversion of relative paths to absolute
if absName, err := filepath.Abs(name); err == nil {
return absName
}
// if we couldn't get absolute name, return original
// (NOTE: Go only errors on Abs() if os.Getwd fails)
return name
}
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