doc: add Go Concurrency Patterns: Timing out, moving on article

Originally published on The Go Programming Language Blog, September 23, 2010.

http://blog.golang.org/2010/09/go-concurrency-patterns-timing-out-and.html

Update #2547.

R=golang-dev, adg
CC=golang-dev
https://golang.org/cl/5815044

doc/Makefile

@@ -7,6 +7,8 @@ RAWHTML=\
 	articles/error_handling.rawhtml\
 	articles/slices_usage_and_internals.rawhtml\
 	articles/laws_of_reflection.rawhtml\
+	articles/c_go_cgo.rawhtml\
+	articles/go_concurrency_patterns_timing_out_moving_on.rawhtml\
 	articles/image_draw.rawhtml\
 	effective_go.rawhtml\
 	go1.rawhtml\
@@ -17,4 +19,4 @@ all: $(RAWHTML)
 	godoc -url /doc/$< >$@
 
 clean:
-	rm -f $(RAWHTML) 
+	rm -f $(RAWHTML)

doc/articles/go_concurrency_patterns_timing_out_moving_on.html

+<!--{
+"Title": "Go Concurrency Patterns: Timing out, moving on",
+"Template": true
+}-->
+
+<p>
+Concurrent programming has its own idioms. A good example is timeouts. Although
+Go's channels do not support them directly, they are easy to implement. Say we
+want to receive from the channel <code>ch</code>, but want to wait at most one
+second for the value to arrive. We would start by creating a signalling channel
+and launching a goroutine that sleeps before sending on the channel:
+</p>
+
+{{code "/doc/progs/timeout1.go" `/timeout :=/` `/STOP/`}}
+
+<p>
+We can then use a <code>select</code> statement to receive from either
+<code>ch</code> or <code>timeout</code>. If nothing arrives on <code>ch</code>
+after one second, the timeout case is selected and the attempt to read from
+<cde>ch</cde> is abandoned.
+</p>
+
+{{code "/doc/progs/timeout1.go" `/select {/` `/STOP/`}}
+
+<p>
+The <code>timeout</code> channel is buffered with space for 1 value, allowing
+the timeout goroutine to send to the channel and then exit. The goroutine
+doesn't know (or care) whether the value is received. This means the goroutine
+won't hang around forever if the <code>ch</code> receive happens before the
+timeout is reached. The <code>timeout</code> channel will eventually be
+deallocated by the garbage collector.
+</p>
+
+<p>
+(In this example we used <code>time.Sleep</code> to demonstrate the mechanics
+of goroutines and channels. In real programs you should use <code>
+<a href="/pkg/time/#After">time.After</a></code>, a function that returns
+a channel and sends on that channel after the specified duration.)
+</p>
+
+<p>
+Let's look at another variation of this pattern. In this example we have a
+program that reads from multiple replicated databases simultaneously. The
+program needs only one of the answers, and it should accept the answer that
+arrives first.
+</p>
+
+<p>
+The function <code>Query</code> takes a slice of database connections and a
+<code>query</code> string. It queries each of the databases in parallel and
+returns the first response it receives:
+</p>
+
+{{code "/doc/progs/timeout2.go" `/func Query/` `/STOP/`}}
+
+<p>
+In this example, the closure does a non-blocking send, which it achieves by
+using the send operation in <code>select</code> statement with a
+<code>default</code> case. If the send cannot go through immediately the
+default case will be selected. Making the send non-blocking guarantees that
+none of the goroutines launched in the loop will hang around. However, if the
+result arrives before the main function has made it to the receive, the send
+could fail since no one is ready.
+</p>
+
+<p>
+This problem is a textbook of example of what is known as a
+<a href="https://en.wikipedia.org/wiki/Race_condition">race condition</a>, but
+the fix is trivial. We just make sure to buffer the channel <code>ch</code> (by
+adding the buffer length as the second argument to <a href="/pkg/builtin/#make">make</a>),
+guaranteeing that the first send has a place to put the value. This ensures the
+send will always succeed, and the first value to arrive will be retrieved
+regardless of the order of execution.
+</p>
+
+<p>
+These two examples demonstrate the simplicity with which Go can express complex
+interactions between goroutines.
+</p>

doc/docs.html

@@ -105,7 +105,7 @@ Guided tours of Go programs.
 <li><a href="http://blog.golang.org/2010/04/json-rpc-tale-of-interfaces.html">JSON-RPC: a tale of interfaces</a></li>
 <li><a href="/doc/articles/gos_declaration_syntax.html">Go's Declaration Syntax</a></li>
 <li><a href="/doc/articles/defer_panic_recover.html">Defer, Panic, and Recover</a></li>
-<li><a href="http://blog.golang.org/2010/09/go-concurrency-patterns-timing-out-and.html">Go Concurrency Patterns: Timing out, moving on</a></li>
+<li><a href="/doc/articles/go_concurrency_patterns_timing_out_moving_on.html">Go Concurrency Patterns: Timing out, moving on</a></li>
 <li><a href="/doc/articles/slices_usage_and_internals.html">Go Slices: usage and internals</a></li>
 <li><a href="http://blog.golang.org/2011/05/gif-decoder-exercise-in-go-interfaces.html">A GIF decoder: an exercise in Go interfaces</a></li>
 <li><a href="/doc/articles/error_handling.html">Error Handling and Go</a></li>

doc/progs/run

@@ -41,7 +41,12 @@ if [ "$goos" == "freebsd" ]; then
 	c_go_cgo="cgo3 cgo4"
 fi
 
-all=$(echo $defer_panic_recover $effective_go $error_handling $law_of_reflection $c_go_cgo slices go1)
+timeout="
+	timeout1
+	timeout2
+"
+
+all=$(echo $defer_panic_recover $effective_go $error_handling $law_of_reflection $c_go_cgo $timeout slices go1)
 
 for i in $all; do
 	go build $i.go

doc/progs/timeout1.go

+// Copyright 2012 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.
+package timeout
+
+import (
+	"time"
+)
+
+func Timeout() {
+	ch := make(chan bool, 1)
+	timeout := make(chan bool, 1)
+	go func() {
+		time.Sleep(1e9) // one second
+		timeout <- true
+	}()
+
+	// STOP OMIT
+
+	select {
+	case <-ch:
+		// a read from ch has occurred
+	case <-timeout:
+		// the read from ch has timed out
+	}
+
+	// STOP OMIT
+}

doc/progs/timeout2.go

+// Copyright 2012 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.
+package query
+
+type Conn string
+
+func (c Conn) DoQuery(query string) Result {
+	return Result("result")
+}
+
+type Result string
+
+func Query(conns []Conn, query string) Result {
+	ch := make(chan Result, 1)
+	for _, conn := range conns {
+		go func(c Conn) {
+			select {
+			case ch <- c.DoQuery(query):
+			default:
+			}
+		}(conn)
+	}
+	return <-ch
+}
+
+// STOP OMIT