Add Time Trigger API and client (#153) (#161)

This change adds a timer trigger API, client, and implementation.  Users can trigger a function with a cron-compatible string. 

This change also moves the publisher interface and webhook-based publisher implementation out of kubewatcher and into a separate `publisher` package.

publisher/webhookPublisher.go

@@ -0,0 +1,121 @@
+/*
+Copyright 2017 The Fission Authors.
+
+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 publisher
+
+import (
+	"bytes"
+	"io/ioutil"
+	"log"
+	"net/http"
+	"strings"
+	"time"
+)
+
+type (
+	// A webhook publisher for a single URL. Satisifies the Publisher interface.
+	WebhookPublisher struct {
+		requestChannel chan *publishRequest
+
+		maxRetries int
+		retryDelay time.Duration
+
+		baseUrl string
+	}
+	publishRequest struct {
+		body       string
+		headers    map[string]string
+		target     string
+		retries    int
+		retryDelay time.Duration
+	}
+)
+
+func MakeWebhookPublisher(baseUrl string) *WebhookPublisher {
+	p := &WebhookPublisher{
+		baseUrl:        baseUrl,
+		requestChannel: make(chan *publishRequest, 32), // buffered channel
+		// TODO make this configurable
+		maxRetries: 10,
+		retryDelay: 500 * time.Millisecond,
+	}
+	go p.svc()
+	return p
+}
+
+func (p *WebhookPublisher) Publish(body string, headers map[string]string, target string) {
+	p.requestChannel <- &publishRequest{
+		body:       body,
+		headers:    headers,
+		target:     target,
+		retries:    p.maxRetries,
+		retryDelay: p.retryDelay,
+	}
+}
+
+func (p *WebhookPublisher) svc() {
+	for {
+		r := <-p.requestChannel
+		p.makeHttpRequest(r)
+	}
+}
+
+func (p *WebhookPublisher) makeHttpRequest(r *publishRequest) {
+	url := p.baseUrl + "/" + strings.TrimPrefix(r.target, "/")
+	log.Printf("Making HTTP request to %v", url)
+
+	var buf bytes.Buffer
+	buf.WriteString(r.body)
+
+	// Create request
+	req, err := http.NewRequest("POST", url, &buf)
+	for k, v := range r.headers {
+		req.Header.Add(k, v)
+	}
+
+	// Make the request
+	resp, err := http.DefaultClient.Do(req)
+
+	// All done if the request succeeded with 200 OK.
+	if err == nil && resp.StatusCode == 200 {
+		resp.Body.Close()
+		return
+	}
+
+	// Log errors
+	if err != nil {
+		log.Printf("Request failed: %v", r)
+	} else if resp.StatusCode != 200 {
+		log.Printf("Request returned failure: %v", resp.StatusCode)
+		body, err := ioutil.ReadAll(resp.Body)
+		resp.Body.Close()
+		if err == nil {
+			log.Printf("request error: %v", string(body))
+		}
+	}
+
+	// Schedule a retry, or give up if out of retries
+	r.retries--
+	if r.retries > 0 {
+		r.retryDelay *= time.Duration(2)
+		time.AfterFunc(r.retryDelay, func() {
+			p.requestChannel <- r
+		})
+	} else {
+		log.Printf("Final retry failed, giving up on %v", url)
+		// Event dropped
+	}
+}