os/signal, runtime: remove runtime sigqueue initialization

We can initialize the runtime sigqueue packages on first use.
We don't require an explicit initialization step. So, remove it.

Change-Id: I484e02dc2c67395fd5584f35ecda2e28b37168df
Reviewed-on: https://go-review.googlesource.com/c/go/+/226540
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>

src/os/signal/signal.go

@@ -122,12 +122,6 @@ func Notify(c chan<- os.Signal, sig ...os.Signal) {
 		panic("os/signal: Notify using nil channel")
 	}
 
-	watchSignalLoopOnce.Do(func() {
-		if watchSignalLoop != nil {
-			go watchSignalLoop()
-		}
-	})
-
 	handlers.Lock()
 	defer handlers.Unlock()
 
@@ -148,6 +142,14 @@ func Notify(c chan<- os.Signal, sig ...os.Signal) {
 			h.set(n)
 			if handlers.ref[n] == 0 {
 				enableSignal(n)
+
+				// The runtime requires that we enable a
+				// signal before starting the watcher.
+				watchSignalLoopOnce.Do(func() {
+					if watchSignalLoop != nil {
+						go watchSignalLoop()
+					}
+				})
 			}
 			handlers.ref[n]++
 		}

src/os/signal/signal_plan9.go

@@ -11,7 +11,7 @@ import (
 
 var sigtab = make(map[os.Signal]int)
 
-// In sig.s; jumps to runtime.
+// Defined by the runtime package.
 func signal_disable(uint32)
 func signal_enable(uint32)
 func signal_ignore(uint32)
@@ -19,8 +19,6 @@ func signal_ignored(uint32) bool
 func signal_recv() string
 
 func init() {
-	signal_enable(0) // first call - initialize
-
 	watchSignalLoop = loop
 }
 

src/os/signal/signal_unix.go

@@ -25,8 +25,6 @@ func loop() {
 }
 
 func init() {
-	signal_enable(0) // first call - initialize
-
 	watchSignalLoop = loop
 }
 

src/runtime/sigqueue.go

@@ -192,16 +192,13 @@ func signalWaitUntilIdle() {
 //go:linkname signal_enable os/signal.signal_enable
 func signal_enable(s uint32) {
 	if !sig.inuse {
-		// The first call to signal_enable is for us
-		// to use for initialization. It does not pass
-		// signal information in m.
+		// This is the first call to signal_enable. Initialize.
 		sig.inuse = true // enable reception of signals; cannot disable
 		if GOOS == "darwin" {
 			sigNoteSetup(&sig.note)
-			return
+		} else {
+			noteclear(&sig.note)
 		}
-		noteclear(&sig.note)
-		return
 	}
 
 	if s >= uint32(len(sig.wanted)*32) {

src/runtime/sigqueue_plan9.go

@@ -134,12 +134,9 @@ func signalWaitUntilIdle() {
 //go:linkname signal_enable os/signal.signal_enable
 func signal_enable(s uint32) {
 	if !sig.inuse {
-		// The first call to signal_enable is for us
-		// to use for initialization. It does not pass
-		// signal information in m.
+		// This is the first call to signal_enable. Initialize.
 		sig.inuse = true // enable reception of signals; cannot disable
 		noteclear(&sig.note)
-		return
 	}
 }