package main

import (
	"fmt"

	"golang.org/x/sys/unix"
)

// Running as PID 1 there is nothing underneath us: sysfs and devtmpfs are what
// every check, counter and device node below is reached through, and no one
// else is going to mount them. Written as checks like everything else, so this
// is a silent no-op under a running system.

type mountSpec struct {
	dir    string
	fstype string
	magic  int64
	flags  uintptr
}

var wantMounts = []mountSpec{
	{"/proc", "proc", unix.PROC_SUPER_MAGIC, unix.MS_NOSUID | unix.MS_NODEV | unix.MS_NOEXEC},
	{"/sys", "sysfs", unix.SYSFS_MAGIC, unix.MS_NOSUID | unix.MS_NODEV | unix.MS_NOEXEC},
	// devtmpfs reports itself as tmpfs, so an existing tmpfs on /dev is taken
	// as good enough; the kernel populates the nodes either way.
	{"/dev", "devtmpfs", unix.TMPFS_MAGIC, unix.MS_NOSUID},
}

func checkMount(m mountSpec) checkResult {
	res := checkResult{item: "mount " + m.dir}
	var st unix.Statfs_t
	err := unix.Statfs(m.dir, &st)
	if err == unix.ENOENT {
		if err := unix.Mkdir(m.dir, 0o755); err != nil {
			res.err = fmt.Errorf("creating %s: %w", m.dir, err)
			res.fatal = true
			return res
		}
		err = unix.Statfs(m.dir, &st)
	}
	if err != nil {
		res.err = err
		res.fatal = true
		return res
	}
	if int64(st.Type) == m.magic {
		res.state = m.fstype + " already mounted"
		return res
	}
	if err := unix.Mount(m.fstype, m.dir, m.fstype, m.flags, ""); err != nil {
		res.err = err
		res.fatal = true
		return res
	}
	res.fixed = true
	res.state = "mounted " + m.fstype
	return res
}

// Each mount is what the next check stands on, so the first failure ends it.
func mountFilesystems() []checkResult {
	var out []checkResult
	for _, m := range wantMounts {
		res := checkMount(m)
		out = append(out, res)
		if res.fatal {
			return out
		}
	}
	return out
}