Cleanup: refactor shutdown and signal handling facility

This disentangles the following functions, which were previously all mixed together:

* 1) Waiting for jobs to terminate when shutting down
* 2) Handling signals in the Docker daemon
* 3) Per-subsystem cleanup handlers
* 4) pidfile management

Responsibilities are dispatched as follows:

* Signal traps are set in `main`, and trigger `engine.Shutdown`
* `engine.Shutdown` coordinates cleanup by waiting for jobs to complete, and calling shutdown handlers
* To perform cleanup at shutdown, each subsystem registers handlers with `engine.OnShutdown`
* `daemon` is one subsystem, so it registers cleanup via `engine.OnShutdown`.
* `daemon` owns the pidfile, which is used to lock access to `/var/lib/docker`. Part of its cleanup is to remove the pidfile.

Signed-off-by: Solomon Hykes <solomon@docker.com>
Upstream-commit: c9f3fd3fc7a4beb97de40ef8da7330b23397d9d3
Component: engine

components/engine/docker/daemon.go

@@ -10,6 +10,7 @@ import (
 	"github.com/docker/docker/dockerversion"
 	"github.com/docker/docker/engine"
 	flag "github.com/docker/docker/pkg/mflag"
+	"github.com/docker/docker/pkg/signal"
 	"github.com/docker/docker/sysinit"
 )
 
@@ -39,6 +40,7 @@ func mainDaemon() {
 	}
 
 	eng := engine.New()
+	signal.Trap(eng.Shutdown)
 	// Load builtins
 	if err := builtins.Register(eng); err != nil {
 		log.Fatal(err)