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// Copyright 2010 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

/*
Package inotify implements a wrapper for the Linux inotify system.

Example:
    watcher, err := inotify.NewWatcher()
    if err != nil {
        log.Fatal(err)
    }
    err = watcher.Watch("/tmp")
    if err != nil {
        log.Fatal(err)
    }
    for {
        select {
        case ev := <-watcher.Event:
            log.Println("event:", ev)
        case err := <-watcher.Error:
            log.Println("error:", err)
        }
    }

*/
package inotify

import (
        "errors"
        "fmt"
        "os"
        "strings"
        "syscall"
        "unsafe"
)

type Event struct {
        Mask   uint32 // Mask of events
        Cookie uint32 // Unique cookie associating related events (for rename(2))
        Name   string // File name (optional)
}

type watch struct {
        wd    uint32 // Watch descriptor (as returned by the inotify_add_watch() syscall)
        flags uint32 // inotify flags of this watch (see inotify(7) for the list of valid flags)
}

type Watcher struct {
        fd       int               // File descriptor (as returned by the inotify_init() syscall)
        watches  map[string]*watch // Map of inotify watches (key: path)
        paths    map[int]string    // Map of watched paths (key: watch descriptor)
        Error    chan error        // Errors are sent on this channel
        Event    chan *Event       // Events are returned on this channel
        done     chan bool         // Channel for sending a "quit message" to the reader goroutine
        isClosed bool              // Set to true when Close() is first called
}

// NewWatcher creates and returns a new inotify instance using inotify_init(2)
func NewWatcher() (*Watcher, error) {
        fd, errno := syscall.InotifyInit()
        if fd == -1 {
                return nil, os.NewSyscallError("inotify_init", errno)
        }
        w := &Watcher{
                fd:      fd,
                watches: make(map[string]*watch),
                paths:   make(map[int]string),
                Event:   make(chan *Event),
                Error:   make(chan error),
                done:    make(chan bool, 1),
        }

        go w.readEvents()
        return w, nil
}

// Close closes an inotify watcher instance
// It sends a message to the reader goroutine to quit and removes all watches
// associated with the inotify instance
func (w *Watcher) Close() error {
        if w.isClosed {
                return nil
        }
        w.isClosed = true

        // Send "quit" message to the reader goroutine
        w.done <- true
        for path := range w.watches {
                w.RemoveWatch(path)
        }

        return nil
}

// AddWatch adds path to the watched file set.
// The flags are interpreted as described in inotify_add_watch(2).
func (w *Watcher) AddWatch(path string, flags uint32) error {
        if w.isClosed {
                return errors.New("inotify instance already closed")
        }

        watchEntry, found := w.watches[path]
        if found {
                watchEntry.flags |= flags
                flags |= syscall.IN_MASK_ADD
        }
        wd, err := syscall.InotifyAddWatch(w.fd, path, flags)
        if err != nil {
                return &os.PathError{
                        Op:   "inotify_add_watch",
                        Path: path,
                        Err:  err,
                }
        }

        if !found {
                w.watches[path] = &watch{wd: uint32(wd), flags: flags}
                w.paths[wd] = path
        }
        return nil
}

// Watch adds path to the watched file set, watching all events.
func (w *Watcher) Watch(path string) error {
        return w.AddWatch(path, IN_ALL_EVENTS)
}

// RemoveWatch removes path from the watched file set.
func (w *Watcher) RemoveWatch(path string) error {
        watch, ok := w.watches[path]
        if !ok {
                return errors.New(fmt.Sprintf("can't remove non-existent inotify watch for: %s", path))
        }
        success, errno := syscall.InotifyRmWatch(w.fd, watch.wd)
        if success == -1 {
                return os.NewSyscallError("inotify_rm_watch", errno)
        }
        delete(w.watches, path)
        return nil
}

// readEvents reads from the inotify file descriptor, converts the
// received events into Event objects and sends them via the Event channel
func (w *Watcher) readEvents() {
        var buf [syscall.SizeofInotifyEvent * 4096]byte

        for {
                n, err := syscall.Read(w.fd, buf[0:])
                // See if there is a message on the "done" channel
                var done bool
                select {
                case done = <-w.done:
                default:
                }

                // If EOF or a "done" message is received
                if n == 0 || done {
                        err := syscall.Close(w.fd)
                        if err != nil {
                                w.Error <- os.NewSyscallError("close", err)
                        }
                        close(w.Event)
                        close(w.Error)
                        return
                }
                if n < 0 {
                        w.Error <- os.NewSyscallError("read", err)
                        continue
                }
                if n < syscall.SizeofInotifyEvent {
                        w.Error <- errors.New("inotify: short read in readEvents()")
                        continue
                }

                var offset uint32 = 0
                // We don't know how many events we just read into the buffer
                // While the offset points to at least one whole event...
                for offset <= uint32(n-syscall.SizeofInotifyEvent) {
                        // Point "raw" to the event in the buffer
                        raw := (*syscall.InotifyEvent)(unsafe.Pointer(&buf[offset]))
                        event := new(Event)
                        event.Mask = uint32(raw.Mask)
                        event.Cookie = uint32(raw.Cookie)
                        nameLen := uint32(raw.Len)
                        // If the event happened to the watched directory or the watched file, the kernel
                        // doesn't append the filename to the event, but we would like to always fill the
                        // the "Name" field with a valid filename. We retrieve the path of the watch from
                        // the "paths" map.
                        event.Name = w.paths[int(raw.Wd)]
                        if nameLen > 0 {
                                // Point "bytes" at the first byte of the filename
                                bytes := (*[syscall.PathMax]byte)(unsafe.Pointer(&buf[offset+syscall.SizeofInotifyEvent]))
                                // The filename is padded with NUL bytes. TrimRight() gets rid of those.
                                event.Name += "/" + strings.TrimRight(string(bytes[0:nameLen]), "\000")
                        }
                        // Send the event on the events channel
                        w.Event <- event

                        // Move to the next event in the buffer
                        offset += syscall.SizeofInotifyEvent + nameLen
                }
        }
}

// String formats the event e in the form
// "filename: 0xEventMask = IN_ACCESS|IN_ATTRIB_|..."
func (e *Event) String() string {
        var events string = ""

        m := e.Mask
        for _, b := range eventBits {
                if m&b.Value != 0 {
                        m &^= b.Value
                        events += "|" + b.Name
                }
        }

        if m != 0 {
                events += fmt.Sprintf("|%#x", m)
        }
        if len(events) > 0 {
                events = " == " + events[1:]
        }

        return fmt.Sprintf("%q: %#x%s", e.Name, e.Mask, events)
}

const (
        // Options for inotify_init() are not exported
        // IN_CLOEXEC    uint32 = syscall.IN_CLOEXEC
        // IN_NONBLOCK   uint32 = syscall.IN_NONBLOCK

        // Options for AddWatch
        IN_DONT_FOLLOW uint32 = syscall.IN_DONT_FOLLOW
        IN_ONESHOT     uint32 = syscall.IN_ONESHOT
        IN_ONLYDIR     uint32 = syscall.IN_ONLYDIR

        // The "IN_MASK_ADD" option is not exported, as AddWatch
        // adds it automatically, if there is already a watch for the given path
        // IN_MASK_ADD      uint32 = syscall.IN_MASK_ADD

        // Events
        IN_ACCESS        uint32 = syscall.IN_ACCESS
        IN_ALL_EVENTS    uint32 = syscall.IN_ALL_EVENTS
        IN_ATTRIB        uint32 = syscall.IN_ATTRIB
        IN_CLOSE         uint32 = syscall.IN_CLOSE
        IN_CLOSE_NOWRITE uint32 = syscall.IN_CLOSE_NOWRITE
        IN_CLOSE_WRITE   uint32 = syscall.IN_CLOSE_WRITE
        IN_CREATE        uint32 = syscall.IN_CREATE
        IN_DELETE        uint32 = syscall.IN_DELETE
        IN_DELETE_SELF   uint32 = syscall.IN_DELETE_SELF
        IN_MODIFY        uint32 = syscall.IN_MODIFY
        IN_MOVE          uint32 = syscall.IN_MOVE
        IN_MOVED_FROM    uint32 = syscall.IN_MOVED_FROM
        IN_MOVED_TO      uint32 = syscall.IN_MOVED_TO
        IN_MOVE_SELF     uint32 = syscall.IN_MOVE_SELF
        IN_OPEN          uint32 = syscall.IN_OPEN

        // Special events
        IN_ISDIR      uint32 = syscall.IN_ISDIR
        IN_IGNORED    uint32 = syscall.IN_IGNORED
        IN_Q_OVERFLOW uint32 = syscall.IN_Q_OVERFLOW
        IN_UNMOUNT    uint32 = syscall.IN_UNMOUNT
)

var eventBits = []struct {
        Value uint32
        Name  string
}{
        {IN_ACCESS, "IN_ACCESS"},
        {IN_ATTRIB, "IN_ATTRIB"},
        {IN_CLOSE, "IN_CLOSE"},
        {IN_CLOSE_NOWRITE, "IN_CLOSE_NOWRITE"},
        {IN_CLOSE_WRITE, "IN_CLOSE_WRITE"},
        {IN_CREATE, "IN_CREATE"},
        {IN_DELETE, "IN_DELETE"},
        {IN_DELETE_SELF, "IN_DELETE_SELF"},
        {IN_MODIFY, "IN_MODIFY"},
        {IN_MOVE, "IN_MOVE"},
        {IN_MOVED_FROM, "IN_MOVED_FROM"},
        {IN_MOVED_TO, "IN_MOVED_TO"},
        {IN_MOVE_SELF, "IN_MOVE_SELF"},
        {IN_OPEN, "IN_OPEN"},
        {IN_ISDIR, "IN_ISDIR"},
        {IN_IGNORED, "IN_IGNORED"},
        {IN_Q_OVERFLOW, "IN_Q_OVERFLOW"},
        {IN_UNMOUNT, "IN_UNMOUNT"},
}