iowatcher.go

/*
Copyright 2014 Google Inc. All rights reserved.

Licensed under the Apache License, Version 2.0 (the "License");
you may not use this file except in compliance with the License.
You may obtain a copy of the License at

    http://www.apache.org/licenses/LICENSE-2.0

Unless required by applicable law or agreed to in writing, software
distributed under the License is distributed on an "AS IS" BASIS,
WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
See the License for the specific language governing permissions and
limitations under the License.
*/

package watch

import (
	"io"
	"sync"

	"github.com/GoogleCloudPlatform/kubernetes/pkg/runtime"
	"github.com/GoogleCloudPlatform/kubernetes/pkg/util"
	"github.com/golang/glog"
)

// Decoder allows StreamWatcher to watch any stream for which a Decoder can be written.
type Decoder interface {
	// Decode should return the type of event, the decoded object, or an error.
	// An error will cause StreamWatcher to call Close(). Decode should block until
	// it has data or an error occurs.
	Decode() (action EventType, object runtime.Object, err error)

	// Close should close the underlying io.Reader, signalling to the source of
	// the stream that it is no longer being watched. Close() must cause any
	// outstanding call to Decode() to return with an error of some sort.
	Close()
}

// StreamWatcher turns any stream for which you can write a Decoder interface
// into a watch.Interface.
type StreamWatcher struct {
	source Decoder
	result chan Event
	sync.Mutex
	stopped bool
}

// NewStreamWatcher creates a StreamWatcher from the given decoder.
func NewStreamWatcher(d Decoder) *StreamWatcher {
	sw := &StreamWatcher{
		source: d,
		// It's easy for a consumer to add buffering via an extra
		// goroutine/channel, but impossible for them to remove it,
		// so nonbuffered is better.
		result: make(chan Event),
	}
	go sw.receive()
	return sw
}

// ResultChan implements Interface.
func (sw *StreamWatcher) ResultChan() <-chan Event {
	return sw.result
}

// Stop implements Interface.
func (sw *StreamWatcher) Stop() {
	// Call Close() exactly once by locking and setting a flag.
	sw.Lock()
	defer sw.Unlock()
	if !sw.stopped {
		sw.stopped = true
		sw.source.Close()
	}
}

// receive reads result from the decoder in a loop and sends down the result channel.
func (sw *StreamWatcher) receive() {
	defer close(sw.result)
	defer sw.Stop()
	defer util.HandleCrash()
	for {
		action, obj, err := sw.source.Decode()
		if err != nil {
			if err != io.EOF {
				glog.Errorf("Unable to decode an event from the watch stream: %v", err)
			}
			return
		}
		sw.result <- Event{
			Type:   action,
			Object: obj,
		}
	}
}