Change graceful to opt-in to signal handling

Previously, a set of standard signals would be handled automatically via
an init() function, however that made the package difficult to use in
packages in which an HTTP server would only be spawned some of the times
(perhaps keyed on an environment variable or flag). Now, signals must be
registered manually.

By default, the top-level "goji" package automatically registers
signals with graceful, so this will result in no behavior changes for
most people.

Fixes #35.

goji.go

@@ -59,6 +59,7 @@ func Serve() {
 	listener := bind.Default()
 	log.Println("Starting Goji on", listener.Addr())
 
+	graceful.HandleSignals()
 	bind.Ready()
 
 	err := graceful.Serve(listener, http.DefaultServeMux)

graceful/einhorn.go

@@ -3,7 +3,6 @@
 package graceful
 
 import (
-	"log"
 	"os"
 	"strconv"
 	"syscall"
@@ -18,7 +17,5 @@ func init() {
 	if err != nil || mpid != os.Getppid() {
 		return
 	}
-
-	log.Print("graceful: Einhorn detected, adding SIGUSR2 handler")
-	AddSignal(syscall.SIGUSR2)
+	stdSignals = append(stdSignals, syscall.SIGUSR2)
 }

graceful/signal.go

@@ -24,28 +24,28 @@ var hookLock sync.Mutex
 var prehooks = make([]func(), 0)
 var posthooks = make([]func(), 0)
 
+var stdSignals = []os.Signal{os.Interrupt}
 var sigchan = make(chan os.Signal, 1)
 
 func init() {
-	AddSignal(os.Interrupt)
 	go waitForSignal()
 }
 
+// HandleSignals installs signal handlers for a set of standard signals. By
+// default, this set only includes keyboard interrupts, however when the package
+// detects that it is running under Einhorn, a SIGUSR2 handler is installed as
+// well.
+func HandleSignals() {
+	AddSignal(stdSignals...)
+}
+
 // AddSignal adds the given signal to the set of signals that trigger a graceful
-// shutdown. Note that for convenience the default interrupt (SIGINT) handler is
-// installed at package load time, and unless you call ResetSignals() will be
-// listened for in addition to any signals you provide by calling this function.
+// shutdown.
 func AddSignal(sig ...os.Signal) {
 	signal.Notify(sigchan, sig...)
 }
 
 // ResetSignals resets the list of signals that trigger a graceful shutdown.
-// Useful if, for instance, you don't want to use the default interrupt (SIGINT)
-// handler. Since we necessarily install the SIGINT handler before you have a
-// chance to call ResetSignals(), there will be a brief window during which the
-// set of signals this package listens for will not be as you intend. Therefore,
-// if you intend on using this function, we encourage you to call it as soon as
-// possible.
 func ResetSignals() {
 	signal.Stop(sigchan)
 }