package main

import (
	"context"
	"os"
	"os/exec"
	"runtime"
	"sync/atomic"
)

// startedFrom is the path this process's executable had when it started: what a verb's command line
// is named in a process listing, and what is run where the image cannot be named otherwise.
var startedFrom, _ = os.Executable()

// ownImage is how a process names the executable it is running, as long as it runs, wherever the file
// has gone since. On Linux the kernel keeps it: /proc/self/exe is the running image itself, not a path,
// so it is valid after the file is renamed into a directory this process may not enter, or deleted —
// which is what the node-engine's witness does to a build it replaces (novox/hq issue 289). Read in the
// child, it names the child's image, which until the exec is this process's.
//
// os.Executable reads the same link and returns the path it points at *now*: correct at start and
// wrong the moment the file moves, which is the fault. A variable so a test can name another.
var ownImage = func() string {
	if runtime.GOOS == "linux" {
		if _, err := os.Stat("/proc/self/exe"); err == nil {
			return "/proc/self/exe"
		}
	}
	return startedFrom
}

// selfCommand is this binary run with a command line: the image this process runs, named in a process
// listing as the path it started from.
func selfCommand(ctx context.Context, argv []string) *exec.Cmd {
	cmd := exec.CommandContext(ctx, ownImage(), argv...)
	if startedFrom != "" {
		cmd.Args[0] = startedFrom
	}
	return cmd
}

// handingOver is set when the serving controller begins to stop — a signal from its supervisor, a lease
// lost. From then a verb that would run a command is refused as a handover rather than started and
// killed with this process: the caller asks again, and the controller after this one answers
// (novox/hq issue 289).
var handingOver atomic.Bool