add "once" package

R=r
DELTA=79  (79 added, 0 deleted, 0 changed)
OCL=15656
CL=15656

src/lib/make.bash

@@ -25,6 +25,7 @@ for i in \
 	io.go\
 	bufio.go\
 	strings.go\
+	once.go\
 
 do
 	base=$(basename $i .go)

src/lib/once.go

@@ -0,0 +1,79 @@
+// 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.
+
+// For one-time initialization that is not done during init.
+// Wrap the initialization in a niladic function f() and call
+//	once.Do(&f)
+// If multiple processes call once.Do(&f) simultaneously
+// with the same f argument, only one will call f, and the
+// others will block until f finishes running.
+
+package once
+
+type Job struct {
+	done bool;
+	doit *chan bool;	// buffer of 1
+}
+
+type Request struct {
+	f *();
+	reply *chan *Job
+}
+
+// TODO: Would like to use chan Request but 6g rejects it.
+var service = new(chan *Request)
+var jobmap = new(map[*()]*Job)
+
+// Moderate access to the jobmap.
+// Even if accesses were thread-safe (they should be but are not)
+// something needs to serialize creation of new jobs.
+// That's what the Server does.
+func Server() {
+	for {
+		req := <-service;
+		job, present := jobmap[req.f]
+		if !present {
+			job = new(Job);
+			job.doit = new(chan bool, 1);
+			job.doit <- true;
+			jobmap[req.f] = job
+		}
+		req.reply <- job
+	}
+}
+
+export func Do(f *()) {
+	// Look for job in map (avoids channel communication).
+	// If not there, ask map server to make one.
+	// TODO: Uncomment use of jobmap[f] once
+	// maps are thread-safe.
+	var job *Job
+	var present bool
+	// job, present = jobmap[f]
+	if !present {
+		c := new(chan *Job);
+		req := Request{f, c};
+		service <- &req;
+		job = <-c
+	}
+
+	// Optimization
+	if job.done {
+		return
+	}
+
+	// If we're the first one, job.doit has a true waiting.
+	if <-job.doit {
+		f();
+		job.done = true
+	}
+
+	// Leave a false waiting for the next guy.
+	job.doit <- false
+}
+
+func init() {
+	go Server()
+}
+