486 lines
10 KiB
Text
486 lines
10 KiB
Text
Горутины и каналы
|
||
Лекция 4
|
||
|
||
Фёдор Короткий
|
||
|
||
* Горутины
|
||
|
||
f() // call f(); wait for it to return
|
||
go f() // create a new goroutine that calls f(); don't wait
|
||
|
||
* Spinner Example
|
||
|
||
.play spinner/main.go /func main()/,/END OMIT/
|
||
|
||
* Clock Server
|
||
|
||
.play clock1/main.go /func main()/,/^}/
|
||
|
||
* Clock Handler
|
||
|
||
.play clock1/main.go /func handleConn/,/^}/
|
||
|
||
* Concurrent Server
|
||
|
||
.play clock2/main.go /func main()/,/^}/
|
||
|
||
* Channels
|
||
|
||
ch := make(chan int)
|
||
|
||
- Канал - это *reference*type*.
|
||
- Нулевое значение канала - `nil`.
|
||
- Каналы поддерживают две операции: _send_ и _receive_.
|
||
|
||
ch <- x // a send statement
|
||
x = <-ch // a receive expression in an assignment statement
|
||
<-ch // a receive statement; result is discarded
|
||
|
||
- `close` закрывает канал.
|
||
- _send_ будут паниковать.
|
||
- _receive_ будут возвращать значения, которые успели записать в канал, и нулевые значения после этого.
|
||
|
||
close(ch)
|
||
ch <- x // panic
|
||
|
||
* Channel buffer
|
||
|
||
ch = make(chan int) // unbuffered channel
|
||
ch = make(chan int, 0) // unbuffered channel
|
||
ch = make(chan int, 3) // buffered channel
|
||
|
||
- Посылка значения по каналу _happens_before_ получения этого значения в другой горутине.
|
||
|
||
* Netcat example
|
||
|
||
.play netcat3/main.go /func main()/,/^}/
|
||
|
||
* Netcat example
|
||
|
||
.play netcat4/main.go /func main()/,/^}/
|
||
|
||
* Pipeline
|
||
|
||
.play pipeline1/main.go /func main()/,/^}/
|
||
|
||
* Pipeline termination
|
||
|
||
Завершение `Counter`
|
||
|
||
go func() { // Counter
|
||
for x := 0; x < 100; x++ {
|
||
naturals <- x
|
||
}
|
||
close(naturals)
|
||
}()
|
||
|
||
* Pipeline termination
|
||
|
||
Завершение `Squarer`
|
||
|
||
go func() { // Squarer
|
||
for {
|
||
x, ok := <-naturals
|
||
if !ok {
|
||
break
|
||
}
|
||
squares <- x * x
|
||
}
|
||
close(squares)
|
||
}()
|
||
|
||
Используя `range`.
|
||
|
||
go func() { // Squarer
|
||
for x := range naturals {
|
||
squares <- x * x
|
||
}
|
||
close(squares)
|
||
}()
|
||
|
||
* Unidirectional channel types
|
||
|
||
func counter(out chan int)
|
||
func squarer(out, in chan int)
|
||
func printer(in chan int)
|
||
|
||
Явные типы для _send_only_ и _receive_only_ каналов.
|
||
|
||
var ch chan int = make(chan int)
|
||
var send chan<- int = ch
|
||
var recv <-chan int = ch
|
||
|
||
func counter(out chan<- int)
|
||
func squarer(out chan<- int, int <-chan int)
|
||
func printer(in <-chan int)
|
||
|
||
func main() {
|
||
naturals := make(chan int)
|
||
squares := make(chan int)
|
||
go counter(naturals)
|
||
go squarer(squares, naturals)
|
||
printer(squares)
|
||
}
|
||
|
||
* Buffered channels
|
||
|
||
ch := make(chan string, 3)
|
||
|
||
ch <- "A"
|
||
ch <- "B"
|
||
ch <- "C"
|
||
|
||
fmt.Println(<-ch) // "A"
|
||
|
||
// not very useful in concurrent program
|
||
fmt.Println(cap(ch)) // 3
|
||
fmt.Println(len(ch)) // 2
|
||
|
||
* Mirrored query
|
||
|
||
func mirroredQuery() string {
|
||
responses := make(chan string, 3)
|
||
go func() { responses <- request("asia.gopl.io") }()
|
||
go func() { responses <- request("europe.gopl.io") }()
|
||
go func() { responses <- request("americas.gopl.io") }()
|
||
return <-responses // return the quickest response
|
||
}
|
||
func request(hostname string) (response string) { /* ... */ }
|
||
|
||
- Почему нельзя использовать `make(chan string)`?
|
||
|
||
* Parallel loop
|
||
|
||
// ImageFile reads an image from infile and writes
|
||
// a thumbnail-size version of it in the same directory.
|
||
// It returns the generated file name, e.g., "foo.thumb.jpg".
|
||
func ImageFile(infile string) (string, error)
|
||
|
||
// makeThumbnails makes thumbnails of the specified files.
|
||
func makeThumbnails(filenames []string) {
|
||
for _, f := range filenames {
|
||
if _, err := thumbnail.ImageFile(f); err != nil {
|
||
log.Println(err)
|
||
}
|
||
}
|
||
}
|
||
|
||
* Parallel loop
|
||
|
||
// NOTE: incorrect!
|
||
func makeThumbnails2(filenames []string) {
|
||
for _, f := range filenames {
|
||
go thumbnail.ImageFile(f) // NOTE: ignoring errors
|
||
}
|
||
}
|
||
|
||
* Parallel loop
|
||
|
||
// makeThumbnails3 makes thumbnails of the specified files in parallel.
|
||
func makeThumbnails3(filenames []string) {
|
||
ch := make(chan struct{})
|
||
for _, f := range filenames {
|
||
go func(f string) {
|
||
thumbnail.ImageFile(f) // NOTE: ignoring errors
|
||
ch <- struct{}{}
|
||
}(f)
|
||
}
|
||
|
||
// Wait for goroutines to complete.
|
||
for range filenames {
|
||
<-ch
|
||
}
|
||
}
|
||
|
||
* Parallel loop
|
||
|
||
// makeThumbnails4 makes thumbnails for the specified files in parallel.
|
||
// It returns an error if any step failed.
|
||
func makeThumbnails4(filenames []string) error {
|
||
errors := make(chan error)
|
||
for _, f := range filenames {
|
||
go func(f string) {
|
||
_, err := thumbnail.ImageFile(f)
|
||
errors <- err
|
||
}(f)
|
||
}
|
||
|
||
for range filenames {
|
||
if err := <-errors; err != nil {
|
||
return err
|
||
}
|
||
}
|
||
return nil
|
||
}
|
||
|
||
- Найдите баг?)
|
||
|
||
* Parallel loop
|
||
|
||
func makeThumbnails5(filenames []string) (thumbfiles []string, err error) {
|
||
type item struct {
|
||
thumbfile string
|
||
err error
|
||
}
|
||
ch := make(chan item, len(filenames))
|
||
for _, f := range filenames {
|
||
go func(f string) {
|
||
var it item
|
||
it.thumbfile, it.err = thumbnail.ImageFile(f)
|
||
ch <- it
|
||
}(f)
|
||
}
|
||
for range filenames {
|
||
it := <-ch
|
||
if it.err != nil {
|
||
return nil, it.err
|
||
}
|
||
thumbfiles = append(thumbfiles, it.thumbfile)
|
||
}
|
||
return thumbfiles, nil
|
||
}
|
||
|
||
* Parallel loop
|
||
|
||
func makeThumbnails5(filenames []string) {
|
||
var wg sync.WaitGroup
|
||
for _, f := range filenames {
|
||
wg.Add(1)
|
||
go func(f string) {
|
||
defer wg.Done()
|
||
|
||
_, _ = thumbnail.ImageFile(f)
|
||
}(f)
|
||
}
|
||
wg.Wait()
|
||
}
|
||
|
||
* Web Crawler
|
||
|
||
func crawl(url string) []string {
|
||
fmt.Println(url)
|
||
list, err := links.Extract(url)
|
||
if err != nil {
|
||
log.Print(err)
|
||
}
|
||
return list
|
||
}
|
||
|
||
* Web Crawler
|
||
|
||
func main() {
|
||
worklist := make(chan []string)
|
||
|
||
// Start with the command-line arguments.
|
||
go func() { worklist <- os.Args[1:] }()
|
||
|
||
// Crawl the web concurrently.
|
||
seen := make(map[string]bool)
|
||
for list := range worklist {
|
||
for _, link := range list {
|
||
if !seen[link] {
|
||
seen[link] = true
|
||
go func(link string) {
|
||
worklist <- crawl(link)
|
||
}(link)
|
||
}
|
||
}
|
||
}
|
||
}
|
||
|
||
- Почему этот код упадёт?
|
||
|
||
* Web Crawler
|
||
|
||
// tokens is a counting semaphore used to
|
||
// enforce a limit of 20 concurrent requests.
|
||
var tokens = make(chan struct{}, 20)
|
||
func crawl(url string) []string {
|
||
fmt.Println(url)
|
||
tokens <- struct{}{} // acquire a token
|
||
list, err := links.Extract(url)
|
||
<-tokens // release the token
|
||
if err != nil {
|
||
log.Print(err)
|
||
}
|
||
return list
|
||
}
|
||
|
||
* Select
|
||
|
||
func main() {
|
||
fmt.Println("Commencing countdown.")
|
||
tick := time.Tick(1 * time.Second) // just example, use time.NewTicker.
|
||
for countdown := 10; countdown > 0; countdown-- {
|
||
fmt.Println(countdown)
|
||
<-tick
|
||
}
|
||
launch()
|
||
}
|
||
|
||
В другой горутине:
|
||
|
||
abort := make(chan struct{})
|
||
go func() {
|
||
os.Stdin.Read(make([]byte, 1)) // read a single byte
|
||
abort <- struct{}{}
|
||
}()
|
||
|
||
* Select
|
||
|
||
select {
|
||
case <-ch1:
|
||
// ...
|
||
case x := <-ch2:
|
||
// ...use x...
|
||
case ch3 <- y:
|
||
// ...
|
||
default:
|
||
// ...
|
||
}
|
||
|
||
- select блокируется, пока ни один из `case`-ов не может выполниться.
|
||
|
||
* Launch Abort
|
||
|
||
func main() {
|
||
// ...create abort channel...
|
||
fmt.Println("Commencing countdown. Press return to abort.")
|
||
select {
|
||
case <-time.After(10 * time.Second):
|
||
// Do nothing.
|
||
case <-abort:
|
||
fmt.Println("Launch aborted!")
|
||
return
|
||
}
|
||
launch()
|
||
}
|
||
|
||
* Ticker
|
||
|
||
func Tick(d time.Duration) <-chan time.Time {
|
||
ch := make(chan time.Time)
|
||
go func() {
|
||
for {
|
||
time.Sleep(d)
|
||
ch <- time.Now()
|
||
}
|
||
}()
|
||
return ch
|
||
}
|
||
|
||
- `time.Tick` нельзя остановить
|
||
|
||
ticker := time.NewTicker(1 * time.Second)
|
||
defer ticker.Stop() // cause the ticker's goroutine to terminate
|
||
|
||
<-ticker.C // receive from the ticker's channel
|
||
|
||
* Non blocking send & receive
|
||
|
||
select {
|
||
case <-ch:
|
||
// ...
|
||
default:
|
||
// ...
|
||
}
|
||
|
||
select {
|
||
case ch <- "A":
|
||
// ...
|
||
default:
|
||
// ...
|
||
}
|
||
|
||
* Directory Traversal
|
||
|
||
.play du1/main.go /func walkDir()/,/-walkDir/
|
||
|
||
* Directory Traversal
|
||
|
||
.play du1/main.go /func main/,/^}/
|
||
|
||
* Directory Traversal Progress
|
||
|
||
.play du2/main.go /Print the results/,/^\tprint/
|
||
|
||
* Concurrent Directory Traversal
|
||
|
||
.play du3/main.go /fileSizes/,/^\t}\(\)/
|
||
|
||
.play du3/main.go /func walkDir/,/^}/
|
||
|
||
* Concurrent Directory Traversal
|
||
|
||
.play du3/main.go /var sema/,/^}/
|
||
|
||
* Cancelation
|
||
|
||
var done = make(chan struct{})
|
||
|
||
func cancelled() bool {
|
||
select {
|
||
case <-done:
|
||
return true
|
||
default:
|
||
return false
|
||
}
|
||
}
|
||
|
||
// Cancel traversal when input is detected.
|
||
go func() {
|
||
os.Stdin.Read(make([]byte, 1)) // read a single byte
|
||
close(done)
|
||
}()
|
||
|
||
* Cancelation
|
||
|
||
func walkDir(dir string, n *sync.WaitGroup, fileSizes chan<- int64) {
|
||
defer n.Done()
|
||
if cancelled() {
|
||
return
|
||
}
|
||
for _, entry := range dirents(dir) {
|
||
// ...
|
||
}
|
||
}
|
||
|
||
* Cancelation
|
||
|
||
for {
|
||
select {
|
||
case <-done:
|
||
// Drain fileSizes to allow existing goroutines to finish.
|
||
for range fileSizes {
|
||
// Do nothing.
|
||
}
|
||
return
|
||
case size, ok := <-fileSizes:
|
||
// ...
|
||
}
|
||
}
|
||
|
||
* Cancelation
|
||
|
||
func dirents(dir string) []os.FileInfo {
|
||
select {
|
||
case sema <- struct{}{}: // acquire token
|
||
case <-done:
|
||
return nil // cancelled
|
||
}
|
||
defer func() { <-sema }() // release token
|
||
// ...read directory...
|
||
}
|
||
|
||
* Chat server
|
||
|
||
.play chat/chat.go /func main/,/^}/
|
||
|
||
* Chat broadcaster
|
||
|
||
.play chat/chat.go /type client/,/^}/
|
||
|
||
* Chat client
|
||
|
||
.play chat/chat.go /func handle/,/END_WRITER/
|
||
|