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watcher.go
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watcher.go
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package watcher
import (
"context"
"fmt"
"github.com/fsnotify/fsnotify"
"github.com/onionltd/go-oniontree"
"path"
"path/filepath"
"strings"
)
type Watcher struct {
ot *oniontree.OnionTree
}
func (w *Watcher) watchTagged(
ctx context.Context,
eventCh chan<- Event,
) error {
defer func() {
if r := recover(); r != nil {
fmt.Printf("recovered: %s\n", r)
}
}()
filenameToTagName := func(f string) string {
return path.Base(f)
}
fsEventToTagEvent := func(e fsnotify.Event) Event {
tagName := filenameToTagName(e.Name)
switch e.Op {
case fsnotify.Create:
return tagCreated{
Name: tagName,
}
}
return nil
}
emitEvent := func(e Event) {
select {
case eventCh <- e:
}
}
taggedWatcher, err := fsnotify.NewWatcher()
if err != nil {
return err
}
defer taggedWatcher.Close()
if err := taggedWatcher.Add(w.ot.TaggedDir()); err != nil {
return err
}
for {
select {
case e := <-taggedWatcher.Events:
if event := fsEventToTagEvent(e); event != nil {
emitEvent(event)
}
case err := <-taggedWatcher.Errors:
return err
case <-ctx.Done():
return nil
}
}
}
func (w *Watcher) watchTags(
ctx context.Context,
taggedEventCh <-chan Event,
eventCh chan<- Event,
) error {
defer func() {
if r := recover(); r != nil {
fmt.Printf("recovered: %s\n", r)
}
}()
emitEvent := func(e Event) {
select {
case eventCh <- e:
}
}
filenameToServiceID := func(f string) string {
f = path.Base(f)
return strings.TrimSuffix(f, filepath.Ext(f))
}
filenameToTagName := func(f string) string {
return path.Base(path.Dir(f))
}
fsEventToServiceEvent := func(e fsnotify.Event) Event {
tagName := filenameToTagName(e.Name)
serviceID := filenameToServiceID(e.Name)
// If tag name is named "tagged", it means the tag directory itself
// was removed. Such event must not be forwarded.
if tagName == "tagged" {
return nil
}
switch e.Op {
case fsnotify.Create:
return ServiceTagged{
ID: serviceID,
Tag: tagName,
}
case fsnotify.Remove:
return ServiceUntagged{
ID: serviceID,
Tag: tagName,
}
}
return nil
}
tagsWatcher, err := fsnotify.NewWatcher()
if err != nil {
return err
}
defer tagsWatcher.Close()
tags, err := w.ot.ListTags()
if err != nil {
return err
}
for i := range tags {
pth := path.Join(w.ot.TaggedDir(), tags[i].String())
if err := tagsWatcher.Add(pth); err != nil {
return err
}
}
for {
select {
case e := <-taggedEventCh:
switch t := e.(type) {
case tagCreated:
// WARNING: There's a lingering race condition!
//
// If a service is tagged with a tag that didn't exist before,
// a new directory `tagged/{foo}` is created, followed by a symbolic link
// `tagged/{foo}/{bar}.yaml`. The newly created directory is added to the watch list,
// however, the symbolic link may be created long before that happens, and create
// event gets lost.
//
// A duct-tape solution is to look into the directory immediately after it's created
// and treat all the files found there as a new tag and emit the event for each one of them.
// Only after that add the directory to the watch list. This doesn't prevent the race
// condition but lowers the likelihood.
//
// The race condition may manifest itself as:
//
// * Emitted duplicate ServiceTagged events.
// * Lost ServiceTagged event.
serviceIDs, err := w.ot.ListServicesWithTag(oniontree.Tag(t.Name))
if err != nil {
return err
}
for i := range serviceIDs {
emitEvent(ServiceTagged{
ID: serviceIDs[i],
Tag: t.Name,
})
}
// Start watching newly created tag directory
pth := path.Join(w.ot.TaggedDir(), t.Name)
if err := tagsWatcher.Add(pth); err != nil {
return err
}
}
case e := <-tagsWatcher.Events:
if event := fsEventToServiceEvent(e); event != nil {
emitEvent(event)
}
case err := <-tagsWatcher.Errors:
return err
case <-ctx.Done():
return nil
}
}
}
func (w *Watcher) watchUnsorted(
ctx context.Context,
eventCh chan<- Event,
) error {
defer func() {
if r := recover(); r != nil {
fmt.Printf("recovered: %s\n", r)
}
}()
emitEvent := func(e Event) {
select {
case eventCh <- e:
}
}
filenameToServiceID := func(f string) string {
f = path.Base(f)
return strings.TrimSuffix(f, filepath.Ext(f))
}
fsEventToServiceEvent := func(e fsnotify.Event) Event {
serviceID := filenameToServiceID(e.Name)
switch e.Op {
case fsnotify.Create:
return ServiceAdded{
ID: serviceID,
}
case fsnotify.Remove:
return ServiceRemoved{
ID: serviceID,
}
case fsnotify.Write:
return ServiceUpdated{
ID: serviceID,
}
}
return nil
}
unsortedWatcher, err := fsnotify.NewWatcher()
if err != nil {
return err
}
defer unsortedWatcher.Close()
if err := unsortedWatcher.Add(w.ot.UnsortedDir()); err != nil {
return err
}
for {
select {
case e := <-unsortedWatcher.Events:
if event := fsEventToServiceEvent(e); event != nil {
emitEvent(event)
}
case err := <-unsortedWatcher.Errors:
return err
case <-ctx.Done():
return nil
}
}
}
// Watch watches for events, emitting them to channel `eventCh`.
// The call to Watch blocks until there's an error, or the context `ctx` is canceled.
func (w *Watcher) Watch(
ctx context.Context,
eventCh chan<- Event,
) error {
ctx, cancel := context.WithCancel(ctx)
defer cancel()
defer close(eventCh)
errCh := make(chan error, 3)
taggedEventCh := make(chan Event)
go func() {
if err := w.watchTagged(ctx, taggedEventCh); err != nil {
errCh <- err
}
}()
go func() {
if err := w.watchTags(ctx, taggedEventCh, eventCh); err != nil {
errCh <- err
}
}()
go func() {
if err := w.watchUnsorted(ctx, eventCh); err != nil {
errCh <- err
}
}()
return <-errCh
}
// NewWatcher returns a new Watcher.
func NewWatcher(ot *oniontree.OnionTree) *Watcher {
return &Watcher{
ot: ot,
}
}