Add POC of GitHub Webhook Delivery Forwarder (#682)

* Add POC of GitHub Webhook Delivery Forwarder

* multi-forwarder and ctrl-c existing and fix for non-woring http post

* Rename source files

* Extract signal handling into a dedicated source file

* Faster ctrl-c handling

* Enable automatic creation of repo hook on startup

* Add support for forwarding org hook deliveries

* Set hook secret on hook creation via envvar (HOOK_SECRET)

* Fix org hook support

* Fix HOOK_SECRET for consistency

* Refactor to prepare for custom log position provider

* Refactor to extract inmemory log position provider

* Add configmap-based log position provider

* Rename githubwebhookdeliveryforwarder to hookdeliveryforwarder

* Refactor to rename LogPositionProvider to Checkpointer and extract ConfigMap checkpointer into a dedicated pkg

* Refactor to extract logger initialization

* Add hookdeliveryforwarder README and bump go-github to unreleased ver

pkg/hookdeliveryforwarder/signal.go

@@ -0,0 +1,48 @@
+/*
+Copyright 2017 The Kubernetes 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 hookdeliveryforwarder
+
+import (
+	"context"
+	"os"
+	"os/signal"
+	"syscall"
+)
+
+var onlyOneSignalHandler = make(chan struct{})
+
+var shutdownSignals = []os.Signal{os.Interrupt, syscall.SIGTERM}
+
+// SetupSignalHandler registers for SIGTERM and SIGINT. A stop channel is returned
+// which is closed on one of these signals. If a second signal is caught, the program
+// is terminated with exit code 1.
+func SetupSignalHandler() context.Context {
+	close(onlyOneSignalHandler) // panics when called twice
+
+	ctx, cancel := context.WithCancel(context.Background())
+
+	c := make(chan os.Signal, 2)
+	signal.Notify(c, shutdownSignals...)
+	go func() {
+		<-c
+		cancel()
+		<-c
+		os.Exit(1) // second signal. Exit directly.
+	}()
+
+	return ctx
+}