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* 1. all statements and declarations are terminated by semicolons * 2. semicolons can be omitted at top level. * 3. semicolons can be omitted before and after the closing ) or } * on a list of statements or declarations. /home/rsc/bin/addsemi and then diff+tweak. R=r,gri OCL=16620 CL=16643
80 lines
1.7 KiB
Go
80 lines
1.7 KiB
Go
// Copyright 2009 The Go Authors. All rights reserved.
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// Use of this source code is governed by a BSD-style
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// license that can be found in the LICENSE file.
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// For one-time initialization that is not done during init.
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// Wrap the initialization in a niladic function f() and call
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// once.Do(&f)
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// If multiple processes call once.Do(&f) simultaneously
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// with the same f argument, only one will call f, and the
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// others will block until f finishes running.
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package once
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type Job struct {
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done bool;
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doit *chan bool; // buffer of 1
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}
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type Request struct {
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f *();
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reply *chan *Job
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}
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// TODO: Would like to use chan Request but 6g rejects it.
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var service = new(chan *Request)
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var jobmap = new(map[*()]*Job)
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// Moderate access to the jobmap.
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// Even if accesses were thread-safe (they should be but are not)
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// something needs to serialize creation of new jobs.
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// That's what the Server does.
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func Server() {
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for {
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req := <-service;
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job, present := jobmap[req.f];
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if !present {
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job = new(Job);
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job.doit = new(chan bool, 1);
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job.doit <- true;
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jobmap[req.f] = job
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}
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req.reply <- job
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}
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}
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export func Do(f *()) {
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// Look for job in map (avoids channel communication).
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// If not there, ask map server to make one.
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// TODO: Uncomment use of jobmap[f] once
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// maps are thread-safe.
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var job *Job;
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var present bool;
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// job, present = jobmap[f]
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if !present {
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c := new(chan *Job);
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req := Request{f, c};
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service <- &req;
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job = <-c
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}
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// Optimization
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if job.done {
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return
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}
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// If we're the first one, job.doit has a true waiting.
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if <-job.doit {
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f();
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job.done = true
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}
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// Leave a false waiting for the next guy.
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job.doit <- false
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}
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func init() {
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go Server()
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}
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