os/signal, runtime: implement notes on Plan 9

This change is an implementation of the signal
runtime and os/signal package on Plan 9.

Contrary to Unix, on Plan 9 a signal is called
a note and is represented by a string.

For this reason, the sigsend and signal_recv
functions had to be reimplemented specifically
for Plan 9.

In order to reuse most of the code and internal
interface of the os/signal package, the note
strings are mapped to integers.

Thanks to Russ Cox for the early review.

Change-Id: I95836645efe21942bb1939f43f87fb3c0eaaef1a
Reviewed-on: https://go-review.googlesource.com/2164
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Rob Pike <r@golang.org>

src/runtime/sigqueue_plan9.go

@@ -0,0 +1,115 @@
+// Copyright 2009 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// This file implements runtime support for signal handling.
+
+package runtime
+
+const qsize = 64
+
+var sig struct {
+	q     noteQueue
+	inuse bool
+
+	lock     mutex
+	note     note
+	sleeping bool
+}
+
+type noteQueue struct {
+	lock mutex
+	data [qsize]*byte
+	ri   int
+	wi   int
+	full bool
+}
+
+func (q *noteQueue) push(item *byte) bool {
+	lock(&q.lock)
+	if q.full {
+		unlock(&q.lock)
+		return false
+	}
+	q.data[q.wi] = item
+	q.wi++
+	if q.wi == qsize {
+		q.wi = 0
+	}
+	if q.wi == q.ri {
+		q.full = true
+	}
+	unlock(&q.lock)
+	return true
+}
+
+func (q *noteQueue) pop() *byte {
+	lock(&q.lock)
+	q.full = false
+	if q.ri == q.wi {
+		unlock(&q.lock)
+		return nil
+	}
+	item := q.data[q.ri]
+	q.ri++
+	if q.ri == qsize {
+		q.ri = 0
+	}
+	unlock(&q.lock)
+	return item
+}
+
+// Called from sighandler to send a signal back out of the signal handling thread.
+// Reports whether the signal was sent. If not, the caller typically crashes the program.
+func sendNote(s *byte) bool {
+	if !sig.inuse {
+		return false
+	}
+
+	// Add signal to outgoing queue.
+	if !sig.q.push(s) {
+		return false
+	}
+
+	lock(&sig.lock)
+	if sig.sleeping {
+		sig.sleeping = false
+		notewakeup(&sig.note)
+	}
+	unlock(&sig.lock)
+
+	return true
+}
+
+// Called to receive the next queued signal.
+// Must only be called from a single goroutine at a time.
+func signal_recv() string {
+	for {
+		note := sig.q.pop()
+		if note != nil {
+			return gostring(note)
+		}
+
+		lock(&sig.lock)
+		sig.sleeping = true
+		noteclear(&sig.note)
+		unlock(&sig.lock)
+		notetsleepg(&sig.note, -1)
+	}
+}
+
+// Must only be called from a single goroutine at a time.
+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.
+		sig.inuse = true // enable reception of signals; cannot disable
+		noteclear(&sig.note)
+		return
+	}
+}
+
+// Must only be called from a single goroutine at a time.
+func signal_disable(s uint32) {
+}