package runtime

import (
	"encoding/binary"
	"errors"
	"fmt"
	"io/fs"
	"os"
	"path/filepath"
	"syscall"
	"time"

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

// Settle is how long the bundles' directories stay quiet before the runtime acts on what changed there:
// a delivery unpacks a bundle in many writes, and the runtime loads it once, when they are done
// (issue 1945, note 23).
var Settle = 300 * time.Millisecond

// watchMask is what the runtime hears of a watched directory: a file or directory made, written,
// renamed in or out, removed or given another mode, and the directory itself removed or renamed.
const watchMask = unix.IN_CREATE | unix.IN_CLOSE_WRITE | unix.IN_MOVED_TO | unix.IN_MOVED_FROM | unix.IN_DELETE |
	unix.IN_ATTRIB | unix.IN_DELETE_SELF | unix.IN_MOVE_SELF | unix.IN_ONLYDIR

// bundleWatch hears, through inotify, the directory of every entrypoint the runtime serves, or for one
// not there yet the nearest directory above it that is, and calls changed once a burst of changes has
// settled. Nothing is looked at on a timer: between bursts the watch sleeps in poll(2).
type bundleWatch struct {
	fd      int
	wake    [2]int // a pipe: a byte written to wake[1] ends the loop
	paths   []string
	byWD    map[int]string
	settle  time.Duration
	changed func()
	logf    func(string, ...any)
	done    chan struct{}
}

// WatchFile hears, as the bundles are heard, the directory of the file at path, or the nearest directory above it that
// exists, and calls changed once at its start and once after each burst of changes there has settled: a runtime waiting
// for its broker credential reads it again when the node-engine writes it, never on a clock (issue 1496). It answers its
// stop.
func WatchFile(path string, changed func(), logf func(string, ...any)) (func(), error) {
	return watchBundles([]string{path}, changed, logf)
}

// watchBundles starts the watch of the entrypoints at paths. It answers its stop.
func watchBundles(paths []string, changed func(), logf func(string, ...any)) (func(), error) {
	fd, err := unix.InotifyInit1(unix.IN_CLOEXEC | unix.IN_NONBLOCK)
	if err != nil {
		return nil, fmt.Errorf("inotify: %w", err)
	}
	w := &bundleWatch{fd: fd, paths: paths, byWD: map[int]string{}, settle: Settle, changed: changed, logf: logf,
		done: make(chan struct{})}
	if err := unix.Pipe2(w.wake[:], unix.O_CLOEXEC|unix.O_NONBLOCK); err != nil {
		unix.Close(fd)
		return nil, fmt.Errorf("pipe: %w", err)
	}
	w.arm()
	go w.loop()
	return func() {
		_, _ = unix.Write(w.wake[1], []byte{1})
		<-w.done
		unix.Close(w.fd)
		unix.Close(w.wake[0])
		unix.Close(w.wake[1])
	}, nil
}

// arm watches, for every entrypoint, its directory, or the nearest directory above it that exists. A
// watch already held on the same directory is kept (inotify answers its descriptor again). A directory
// made below the watched one between its stat and its watch would be heard by nobody, so after each watch
// the path is looked at again, and a deeper directory that came to be is watched in turn.
func (w *bundleWatch) arm() {
	for _, p := range w.paths {
		target := filepath.Dir(p)
		watched := ""
		for {
			dir := nearestDir(target)
			if dir == watched {
				break
			}
			wd, err := unix.InotifyAddWatch(w.fd, dir, watchMask)
			if err != nil {
				w.logf("[mesh-tools] cannot watch %s for %s's bundle: %v; a bundle placed or changed there is served only after a restart", dir, p, err)
				break
			}
			w.byWD[wd] = dir
			watched = dir
		}
	}
}

// nearestDir is dir when it is a directory, else the nearest directory above it.
func nearestDir(dir string) string {
	for {
		if info, err := os.Stat(dir); err == nil && info.IsDir() {
			return dir
		}
		up := filepath.Dir(dir)
		if up == dir {
			return dir
		}
		dir = up
	}
}

func (w *bundleWatch) loop() {
	defer close(w.done)
	buf := make([]byte, 64*1024)
	// Looked at once as the watch begins: a bundle that came between its load and the watch is not missed.
	pending := true
	quietAt := time.Now()
	for {
		timeout := -1
		if pending {
			timeout = int((time.Until(quietAt) + time.Millisecond - 1) / time.Millisecond)
			if timeout < 0 {
				timeout = 0
			}
		}
		fds := []unix.PollFd{{Fd: int32(w.fd), Events: unix.POLLIN}, {Fd: int32(w.wake[0]), Events: unix.POLLIN}}
		n, err := unix.Poll(fds, timeout)
		if errors.Is(err, unix.EINTR) {
			continue
		}
		if err != nil {
			w.logf("[mesh-tools] the watch of the bundles stopped: %v; a bundle placed or changed later is served only after a restart", err)
			return
		}
		if fds[1].Revents != 0 {
			return
		}
		if n == 0 {
			// Quiet for the settle: watch the directories that came to be, then act on what changed.
			pending = false
			w.arm()
			w.changed()
			continue
		}
		if fds[0].Revents&unix.POLLIN != 0 {
			w.drain(buf)
			pending = true
			quietAt = time.Now().Add(w.settle)
		}
	}
}

// drain reads every event queued, dropping the watches of directories that went away or moved: the
// next arm watches what is at their path now.
func (w *bundleWatch) drain(buf []byte) {
	for {
		n, err := unix.Read(w.fd, buf)
		if err != nil || n <= 0 {
			return
		}
		for off := 0; off+unix.SizeofInotifyEvent <= n; {
			wd := int(int32(binary.NativeEndian.Uint32(buf[off:])))
			mask := binary.NativeEndian.Uint32(buf[off+4:])
			length := int(binary.NativeEndian.Uint32(buf[off+12:]))
			switch {
			case mask&unix.IN_IGNORED != 0:
				delete(w.byWD, wd)
			case mask&unix.IN_MOVE_SELF != 0:
				_, _ = unix.InotifyRmWatch(w.fd, uint32(wd))
				delete(w.byWD, wd)
			}
			off += unix.SizeofInotifyEvent + length
		}
	}
}

// signatureOf is what the runtime compares to tell that an entrypoint changed: the file's identity,
// mode, size and times; "" is a file that does not exist. Only the entrypoint: a delivery puts every file
// of a bundle in place anew (a directory swapped in, or each file renamed in), so the entrypoint is new
// whenever its build is, while a file beside it may be one the bundle itself writes (a cache, a socket),
// which would load it again after every quiet spell, and modules sharing a directory each other.
func signatureOf(path string) string {
	info, err := os.Stat(path)
	if errors.Is(err, fs.ErrNotExist) {
		return ""
	}
	if err != nil {
		return "unreadable: " + err.Error()
	}
	return statSignature(info)
}

func statSignature(info fs.FileInfo) string {
	sig := fmt.Sprintf("%s %d %d", info.Mode(), info.Size(), info.ModTime().UnixNano())
	if st, ok := info.Sys().(*syscall.Stat_t); ok {
		sig += fmt.Sprintf(" %d:%d %d.%d", st.Dev, st.Ino, st.Ctim.Sec, st.Ctim.Nsec)
	}
	return sig
}

// notStartable says why the runtime cannot start an entrypoint, in its real reason, or "" when it can
// (issue 1945, note 23: a missing file was said as "not executable").
func notStartable(path string) string {
	info, err := os.Stat(path)
	switch {
	case errors.Is(err, fs.ErrNotExist):
		return path + " does not exist; it is served as soon as it is in place"
	case err != nil:
		return path + " cannot be read: " + err.Error()
	case info.IsDir():
		return path + " is a directory, not a bundle's entrypoint"
	case info.Mode().Perm()&0o111 == 0:
		return fmt.Sprintf("%s is not executable (mode %s); a bundle the runtime serves is started, never imported, "+
			"and its build makes it executable (novox/hq ADR 0193)", path, info.Mode().Perm())
	}
	return ""
}