Added service wrapper code (#6220)

This is the basic code to add the Windows Service Manager hooks to Nomad.

Includes vendoring golang.org/x/sys/windows/svc and added Docs:
* guide for installing as a windows service.
* configuration for logging to file from PR #6429

CHANGELOG.md

@@ -14,7 +14,7 @@ BUG FIXES:
  * scheduler: Changes to devices in resource stanza should cause rescheduling [[GH-6644](https://github.com/hashicorp/nomad/issues/6644)]
  * api: Decompress web socket response body if gzipped on error responses [[GH-6650](https://github.com/hashicorp/nomad/issues/6650)]
  * api: Return a 404 if endpoint not found instead of redirecting to /ui/ [[GH-6658](https://github.com/hashicorp/nomad/issues/6658)]
- 
+
 ## 0.10.1 (November 4, 2019)
 
 BUG FIXES:

command/agent/command.go

@@ -29,6 +29,7 @@ import (
 	flaghelper "github.com/hashicorp/nomad/helper/flag-helpers"
 	gatedwriter "github.com/hashicorp/nomad/helper/gated-writer"
 	"github.com/hashicorp/nomad/helper/logging"
+	"github.com/hashicorp/nomad/helper/winsvc"
 	"github.com/hashicorp/nomad/nomad/structs/config"
 	"github.com/hashicorp/nomad/version"
 	"github.com/mitchellh/cli"
@@ -777,6 +778,8 @@ WAIT:
 	select {
 	case s := <-signalCh:
 		sig = s
+	case <-winsvc.ShutdownChannel():
+		sig = os.Interrupt
 	case <-c.ShutdownCh:
 		sig = os.Interrupt
 	case <-c.retryJoinErrCh:

helper/winsvc/service.go

@@ -0,0 +1,9 @@
+package winsvc
+
+var chanGraceExit = make(chan int)
+
+// ShutdownChannel returns a channel that sends a message that a shutdown
+// signal has been received for the service.
+func ShutdownChannel() <-chan int {
+	return chanGraceExit
+}

helper/winsvc/service_windows.go

@@ -0,0 +1,42 @@
+//+build windows
+
+package winsvc
+
+import (
+	wsvc "golang.org/x/sys/windows/svc"
+)
+
+type serviceWindows struct{}
+
+func init() {
+	interactive, err := wsvc.IsAnInteractiveSession()
+	if err != nil {
+		panic(err)
+	}
+	// Cannot run as a service when running interactively
+	if interactive {
+		return
+	}
+	go wsvc.Run("", serviceWindows{})
+}
+
+// Execute implements the Windows service Handler type. It will be
+// called at the start of the service, and the service will exit
+// once Execute completes.
+func (serviceWindows) Execute(args []string, r <-chan wsvc.ChangeRequest, s chan<- wsvc.Status) (svcSpecificEC bool, exitCode uint32) {
+	const accCommands = wsvc.AcceptStop | wsvc.AcceptShutdown
+	s <- wsvc.Status{State: wsvc.Running, Accepts: accCommands}
+	for {
+		c := <-r
+		switch c.Cmd {
+		case wsvc.Interrogate:
+			s <- c.CurrentStatus
+		case wsvc.Stop, wsvc.Shutdown:
+			s <- wsvc.Status{State: wsvc.StopPending}
+			chanGraceExit <- 1
+			return false, 0
+		}
+	}
+
+	return false, 0
+}