Add SIGUSR1 handler for dumping stack/goroutine traces

Add handler for SIGUSR1 based on feedback regarding when to dump
goroutine stacks.  This will also dump goroutine stack traces on SIGQUIT
followed by a hard-exit from the daemon.

Docker-DCO-1.1-Signed-off-by: Phil Estes <estesp@linux.vnet.ibm.com> (github: estesp)

daemon/daemon.go

@@ -747,6 +747,9 @@ func NewDaemon(config *Config, registryService *registry.Service) (daemon *Daemo
 		return nil, err
 	}
 
+	// set up SIGUSR1 handler to dump Go routine stacks
+	setupSigusr1Trap()
+
 	// set up the tmpDir to use a canonical path
 	tmp, err := tempDir(config.Root)
 	if err != nil {

daemon/debugtrap.go

@@ -0,0 +1,21 @@
+// +build !windows
+
+package daemon
+
+import (
+	"os"
+	"os/signal"
+	"syscall"
+
+	psignal "github.com/docker/docker/pkg/signal"
+)
+
+func setupSigusr1Trap() {
+	c := make(chan os.Signal, 1)
+	signal.Notify(c, syscall.SIGUSR1)
+	go func() {
+		for range c {
+			psignal.DumpStacks()
+		}
+	}()
+}

daemon/debugtrap_unsupported.go

@@ -0,0 +1,7 @@
+// +build !linux,!darwin,!freebsd
+
+package signal
+
+func setupSigusr1Trap() {
+	return
+}