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fswatcher.go
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fswatcher.go
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package entrypoint
import (
"context"
"fmt"
"io/ioutil"
"os"
"path"
"path/filepath"
"strings"
"sync"
"time"
"github.com/datawire/dlib/dlog"
"github.com/fsnotify/fsnotify"
)
// FSWatcher is a thing that can watch the filesystem for us, and
// call handler functions when things change.
//
// The core of an FSWatcher is fsnotify/fsnotify, but we wrap some
// state around it.
//
// First, fsnotify tries to mark the operation associated with a
// change -- however, these are not always accurate, since the
// filesystem tries to coalesce events that are close in time.
// Therefore FSWatcher doesn't actually look at the operation:
// everything is just "a change happened".
//
// This causes one interesting problem: given a touch of
// temporal separation between Create and Write, we may decide
// to trigger a reconfigure on the Create, before the data have
// been written. To mitigate against that, we'll wait up to half
// a second after an event to see if any other events will be
// happening (with the idea that if you've come within half a
// second of your cert expiring before renewing it, uh, yeah,
// maybe you _will_ have some transient errors).
//
// Second, when we start watching a directory, we make sure that
// "update" events get posted for every file in the directory.
// These are marked as "bootstrap" events.
//
// Finally, rather than posting things to channels, we call a
// handler function whenever anything interesting happens,
// where "interesting" is one of the events above, or an error.
type FSWatcher struct {
FSW *fsnotify.Watcher
mutex sync.Mutex
handlers map[string]FSWEventHandler
handleError FSWErrorHandler
cTimer *time.Timer
marker chan time.Time
outstanding map[string]bool
}
// FSWEventHandler is a handler function for an interesting
// event.
type FSWEventHandler func(ctx context.Context, event FSWEvent)
// FSWErrorHandler is a handler function for an error.
type FSWErrorHandler func(ctx context.Context, err error)
// FSWOp specifies the operation for an event.
type FSWOp string
const (
// FSWUpdate is an update operation
FSWUpdate FSWOp = "update"
// FSWDelete is a delete operation
FSWDelete FSWOp = "delete"
)
// FSWEvent represents a single interesting event.
type FSWEvent struct {
// Path is the fully-qualified path of the file that changed.
Path string
// Op is the operation for this event.
Op FSWOp
// Bootstrap is true IFF this is a synthesized event noting
// that a file existed at the moment we started watching a
// directory.
Bootstrap bool
// Time is when this event happened
Time time.Time
}
// String returns a string representation of an FSEvent.
func (event FSWEvent) String() string {
bstr := ""
if event.Bootstrap {
bstr = "B|"
}
return fmt.Sprintf("%s%s %s", bstr, event.Op, event.Path)
}
// NewFSWatcher instantiates an FSWatcher. At instantiation time,
// no directories are being watched.
func NewFSWatcher(ctx context.Context) (*FSWatcher, error) {
watcher, err := fsnotify.NewWatcher()
if err != nil {
dlog.Errorf(ctx, "FSW: could not initialize FSWatcher: %v", err)
return nil, err
}
dlog.Debugf(ctx, "FSW: initialized FSWatcher!")
fsw := &FSWatcher{
FSW: watcher,
handlers: make(map[string]FSWEventHandler),
outstanding: make(map[string]bool),
marker: make(chan time.Time),
}
// Start with the default error handler...
fsw.handleError = fsw.defaultErrorHandler
return fsw, nil
}
// SetErrorHandler sets the function that will be used to respond to errors.
func (fsw *FSWatcher) SetErrorHandler(handler FSWErrorHandler) {
fsw.handleError = handler
}
// WatchDir starts watching a directory, using a specific handler function.
// You'll need to separately call WatchDir for subdirectories if you want
// recursive watches.
func (fsw *FSWatcher) WatchDir(ctx context.Context, dir string, handler FSWEventHandler) error {
fsw.mutex.Lock()
defer fsw.mutex.Unlock()
dlog.Infof(ctx, "FSW: watching %s", dir)
if err := fsw.FSW.Add(dir); err != nil {
return err
}
fsw.handlers[dir] = handler
fileinfos, err := ioutil.ReadDir(dir)
if err != nil {
return err
}
for _, info := range fileinfos {
fswevent := FSWEvent{
Path: path.Join(dir, info.Name()),
Op: FSWUpdate,
Bootstrap: true,
Time: info.ModTime(),
}
dlog.Debugf(ctx, "FSWatcher: synthesizing %s", fswevent)
handler(ctx, fswevent)
}
return nil
}
// The default error handler just logs the error.
func (fsw *FSWatcher) defaultErrorHandler(ctx context.Context, err error) {
dlog.Errorf(ctx, "FSW: FSWatcher error: %s", err)
}
// Watch for events, and handle them.
func (fsw *FSWatcher) Run(ctx context.Context) {
for {
select {
case event := <-fsw.FSW.Events:
fsw.mutex.Lock()
dlog.Debugf(ctx, "FSW: raw event %s", event)
// Note that this path is outstanding.
fsw.outstanding[event.Name] = true
// Coalesce events for up to half a second.
if fsw.cTimer != nil {
dlog.Debugf(ctx, "FSW: stopping cTimer")
if !fsw.cTimer.Stop() {
<-fsw.cTimer.C
}
}
dlog.Debugf(ctx, "FSW: starting cTimer")
fsw.cTimer = time.AfterFunc(500*time.Millisecond, func() {
fsw.marker <- time.Now()
})
dlog.Debugf(ctx, "FSW: unlocking")
fsw.mutex.Unlock()
case <-fsw.marker:
fsw.mutex.Lock()
dlog.Debugf(ctx, "FSW: MARKER LOCK")
fsw.cTimer = nil
keys := make([]string, 0, len(fsw.outstanding))
for key := range fsw.outstanding {
keys = append(keys, key)
}
fsw.outstanding = make(map[string]bool)
fsw.mutex.Unlock()
dlog.Debugf(ctx, "FSW: updates! %s", strings.Join(keys, ", "))
for _, evtPath := range keys {
dirname := filepath.Dir(evtPath)
handler, handlerExists := fsw.handlers[dirname]
if handlerExists {
op := FSWUpdate
info, err := os.Stat(evtPath)
eventTime := time.Now()
if err != nil {
op = FSWDelete
} else {
eventTime = info.ModTime()
}
fswevent := FSWEvent{
Path: evtPath,
Op: op,
Bootstrap: false,
Time: eventTime,
}
dlog.Debugf(ctx, "FSW: handling %s", fswevent)
handler(ctx, fswevent)
} else {
dlog.Debugf(ctx, "FSW: drop, no handler for dir %s", dirname)
}
}
case err := <-fsw.FSW.Errors:
dlog.Errorf(ctx, "FSW: filesystem watch error %s", err)
fsw.handleError(ctx, err)
case <-ctx.Done():
dlog.Infof(ctx, "FSW: ctx shutdown, exiting")
return
}
}
}