package container

import (
	"context"
	"encoding/gob"
	"errors"
	"fmt"
	"os"
	"os/exec"
	"slices"
	"strconv"
	"syscall"
	"time"

	"hakurei.app/container/check"
	"hakurei.app/container/fhs"
)

func init() { gob.Register(new(DaemonOp)) }

const (
	// daemonTimeout is the duration a [DaemonOp] is allowed to block before the
	// [DaemonOp.Target] marker becomes available.
	daemonTimeout = 5 * time.Second
)

// Daemon appends an [Op] that starts a daemon in the container and blocks until
// [DaemonOp.Target] appears.
func (f *Ops) Daemon(target, path *check.Absolute, args ...string) *Ops {
	*f = append(*f, &DaemonOp{target, path, args})
	return f
}

// DaemonOp starts a daemon in the container and blocks until Target appears.
type DaemonOp struct {
	// Pathname indicating readiness of daemon.
	Target *check.Absolute
	// Absolute pathname passed to [exec.Cmd].
	Path *check.Absolute
	// Arguments (excl. first) passed to [exec.Cmd].
	Args []string
}

// earlyTerminationError is returned by [DaemonOp] when a daemon terminates
// before [DaemonOp.Target] appears.
type earlyTerminationError struct {
	// Returned by [DaemonOp.String].
	op string
	// Copied from wait4 loop.
	wstatus syscall.WaitStatus
}

func (e *earlyTerminationError) Error() string {
	res := ""
	switch {
	case e.wstatus.Exited():
		res = "exit status " + strconv.Itoa(e.wstatus.ExitStatus())
	case e.wstatus.Signaled():
		res = "signal: " + e.wstatus.Signal().String()
	case e.wstatus.Stopped():
		res = "stop signal: " + e.wstatus.StopSignal().String()
		if e.wstatus.StopSignal() == syscall.SIGTRAP && e.wstatus.TrapCause() != 0 {
			res += " (trap " + strconv.Itoa(e.wstatus.TrapCause()) + ")"
		}
	case e.wstatus.Continued():
		res = "continued"
	}
	if e.wstatus.CoreDump() {
		res += " (core dumped)"
	}
	return res
}

func (e *earlyTerminationError) Message() string { return e.op + " " + e.Error() }

func (d *DaemonOp) Valid() bool                                { return d != nil && d.Target != nil && d.Path != nil }
func (d *DaemonOp) early(*setupState, syscallDispatcher) error { return nil }
func (d *DaemonOp) apply(*setupState, syscallDispatcher) error { return nil }
func (d *DaemonOp) late(state *setupState, k syscallDispatcher) error {
	cmd := exec.CommandContext(state.Context, d.Path.String(), d.Args...)
	cmd.Env = state.Env
	cmd.Dir = fhs.Root
	if state.IsVerbose() {
		cmd.Stdout, cmd.Stderr = os.Stdout, os.Stderr
	}
	// WaitDelay: left unset because lifetime is bound by AdoptWaitDelay on cancellation
	cmd.Cancel = func() error { return cmd.Process.Signal(syscall.SIGTERM) }

	state.Verbosef("starting %s", d.String())
	if err := k.start(cmd); err != nil {
		return err
	}

	deadline := time.Now().Add(daemonTimeout)
	var wstatusErr error

	for {
		if _, err := k.stat(d.Target.String()); err != nil {
			if !errors.Is(err, os.ErrNotExist) {
				_ = k.signal(cmd, os.Kill)
				return err
			}

			if time.Now().After(deadline) {
				_ = k.signal(cmd, os.Kill)
				return context.DeadlineExceeded
			}

			if wstatusErr != nil {
				return wstatusErr
			}
			if wstatus, ok := state.terminated(cmd.Process.Pid); ok {
				// check once again: process could have satisfied Target between stat and the lookup
				wstatusErr = &earlyTerminationError{d.String(), wstatus}
				continue
			}

			time.Sleep(500 * time.Microsecond)
			continue
		}

		state.Verbosef("daemon process %d ready", cmd.Process.Pid)
		return nil
	}
}

func (d *DaemonOp) Is(op Op) bool {
	vd, ok := op.(*DaemonOp)
	return ok && d.Valid() && vd.Valid() &&
		d.Target.Is(vd.Target) && d.Path.Is(vd.Path) &&
		slices.Equal(d.Args, vd.Args)
}
func (*DaemonOp) prefix() (string, bool) { return zeroString, false }
func (d *DaemonOp) String() string       { return fmt.Sprintf("daemon providing %q", d.Target) }