//go:build linux

package system

import (
	"os"
	"path/filepath"
	"strings"
)

// MountIsShutdown reports whether the TOP mount at path is a dead filesystem — ext4's `shutdown` super option, set when
// the device under it went away (a pulled drive). It reads /proc/self/mountinfo only: NO disk I/O, so it never hangs on
// the dead device (gates.md: health checks issue no block I/O). A timed `stat` is NOT a substitute: it answered „ok" on
// a dead drive from the cache (measured 2026-10-14, `15` §10 K4). Pinned by TestMountIsShutdown_*.
func MountIsShutdown(path string) bool {
	data, err := os.ReadFile(mountinfoPath)
	if err != nil {
		return false
	}
	return mountinfoShutdown(string(data), path)
}

var mountinfoPath = "/proc/self/mountinfo"

// mountinfoShutdown: the LAST mountinfo line whose mount point is path (the top of a stack) carries `shutdown` in its
// super options (the third field after " - ").
func mountinfoShutdown(mountinfo, path string) bool {
	path = filepath.Clean(path)
	dead := false
	found := false
	for _, line := range strings.Split(mountinfo, "\n") {
		f := strings.Fields(line)
		if len(f) < 5 || unescapeMountinfo(f[4]) != path {
			continue
		}
		found = true
		dead = false
		if i := strings.Index(line, " - "); i >= 0 {
			tail := strings.Fields(line[i+3:])
			if len(tail) >= 3 {
				for _, o := range strings.Split(tail[2], ",") {
					if o == "shutdown" {
						dead = true
					}
				}
			}
		}
	}
	return found && dead
}

// unescapeMountinfo undoes mountinfo's octal escapes for space, tab, newline and backslash.
func unescapeMountinfo(s string) string {
	r := strings.NewReplacer(`\040`, " ", `\011`, "\t", `\012`, "\n", `\134`, `\`)
	return r.Replace(s)
}