package main

// session.go is the same file in the bundles whose tools act in the operator's graphical session
// (xclip, dmenu): how a process the node's tool runtime launched reaches that session.
//
// The runtime is a system service running as the operator account (novox/hq ADR 0175 §4), in the
// machine's own mount namespace, and is given no session words: no DISPLAY, no XAUTHORITY. An X
// server accepts a client that names its display and presents the cookie in the authority file, and
// both are the account's: the display's socket is in /tmp/.X11-unix, and the cookie file is
// readable by the account. So the session is found, not configured:
//
//  1. the process's own DISPLAY, when the runtime happens to have one;
//  2. else the DISPLAY and XAUTHORITY of the account's own running processes, read from
//     /proc/<pid>/environ (the window manager's, by preference), whose socket exists;
//  3. else the only X socket there is, with the authority file in the account's home.
//
// When none is found the tool says that no graphical session of the account is running, and does
// nothing.

import (
	"fmt"
	"os"
	"path/filepath"
	"sort"
	"strconv"
	"strings"
	"syscall"
)

// Session is the operator's X session as a tool reaches it.
type Session struct {
	Display    string `json:"display"`
	XAuthority string `json:"xauthority,omitempty"`
	// FoundBy says how: "environment", "process <pid> (<name>)" or "socket".
	FoundBy string `json:"found_by"`
}

// Env is what a command needs to reach the session.
func (s Session) Env() []string {
	env := []string{"DISPLAY=" + s.Display}
	if s.XAuthority != "" {
		env = append(env, "XAUTHORITY="+s.XAuthority)
	}
	return env
}

// Where the session is looked for. Tests point these at a tree of their own.
var (
	procRoot   = "/proc"
	x11Sockets = "/tmp/.X11-unix"
	getenv     = os.Getenv
	myUID      = os.Getuid
)

// sessionWMs are the programs whose environment is the session's own, preferred over any other
// process's (a terminal's child may carry a stale or forwarded DISPLAY).
var sessionWMs = map[string]bool{"i3": true, "sway": true, "xinit": true, "i3bar": true, "picom": true, "dunst": true}

func accountHome() string {
	if h := strings.TrimSpace(getenv("MESH_OPERATOR_HOME")); h != "" {
		return h
	}
	if h := strings.TrimSpace(getenv("HOME")); h != "" {
		return h
	}
	h, _ := os.UserHomeDir()
	return h
}

// socketOf is the local socket of a display such as ":1" or ":1.0", or "" for a remote one.
func socketOf(display string) string {
	if !strings.HasPrefix(display, ":") {
		return ""
	}
	n := strings.TrimPrefix(display, ":")
	if i := strings.IndexByte(n, '.'); i >= 0 {
		n = n[:i]
	}
	if _, err := strconv.Atoi(n); err != nil {
		return ""
	}
	return filepath.Join(x11Sockets, "X"+n)
}

func exists(p string) bool {
	_, err := os.Stat(p)
	return err == nil
}

// findSession answers the account's X session, or an error saying there is none.
func findSession() (Session, error) {
	if d := strings.TrimSpace(getenv("DISPLAY")); d != "" {
		if s := socketOf(d); s == "" || exists(s) {
			return Session{Display: d, XAuthority: getenv("XAUTHORITY"), FoundBy: "environment"}, nil
		}
	}
	type seen struct {
		Session
		wm    bool
		count int
	}
	found := map[string]*seen{}
	entries, _ := os.ReadDir(procRoot)
	for _, e := range entries {
		pid, err := strconv.Atoi(e.Name())
		if err != nil || !e.IsDir() {
			continue
		}
		dir := filepath.Join(procRoot, e.Name())
		info, err := os.Stat(dir)
		if err != nil {
			continue
		}
		if st, ok := info.Sys().(*syscall.Stat_t); !ok || int(st.Uid) != myUID() {
			continue
		}
		raw, err := os.ReadFile(filepath.Join(dir, "environ"))
		if err != nil {
			continue
		}
		var display, auth string
		for _, kv := range strings.Split(string(raw), "\x00") {
			switch {
			case strings.HasPrefix(kv, "DISPLAY="):
				display = strings.TrimPrefix(kv, "DISPLAY=")
			case strings.HasPrefix(kv, "XAUTHORITY="):
				auth = strings.TrimPrefix(kv, "XAUTHORITY=")
			}
		}
		if display == "" {
			continue
		}
		if s := socketOf(display); s == "" || !exists(s) {
			continue
		}
		comm, _ := os.ReadFile(filepath.Join(dir, "comm"))
		name := strings.TrimSpace(string(comm))
		key := display + "\x00" + auth
		if found[key] == nil {
			found[key] = &seen{Session: Session{Display: display, XAuthority: auth, FoundBy: fmt.Sprintf("process %d (%s)", pid, name)}}
		}
		f := found[key]
		f.count++
		if sessionWMs[name] && !f.wm {
			f.wm = true
			f.FoundBy = fmt.Sprintf("process %d (%s)", pid, name)
		}
	}
	if len(found) > 0 {
		all := make([]*seen, 0, len(found))
		for _, f := range found {
			all = append(all, f)
		}
		sort.Slice(all, func(i, k int) bool {
			if all[i].wm != all[k].wm {
				return all[i].wm
			}
			if all[i].count != all[k].count {
				return all[i].count > all[k].count
			}
			return all[i].Display < all[k].Display
		})
		return all[0].Session, nil
	}
	sockets, _ := filepath.Glob(filepath.Join(x11Sockets, "X*"))
	if len(sockets) == 1 {
		s := Session{Display: ":" + strings.TrimPrefix(filepath.Base(sockets[0]), "X"), FoundBy: "socket"}
		if a := filepath.Join(accountHome(), ".Xauthority"); exists(a) {
			s.XAuthority = a
		}
		return s, nil
	}
	if len(sockets) > 1 {
		return Session{}, fmt.Errorf("no process of this account names its X display, and there are %d X sockets in %s: which one is the operator's session cannot be told", len(sockets), x11Sockets)
	}
	return Session{}, fmt.Errorf("no graphical session of this account is running on this machine: no process of the account has DISPLAY set, and there is no X socket in %s. A desktop tool acts only while the operator is logged in to the graphical session", x11Sockets)
}