wiki/content/20200901141141-goroutines.md

date	id	title
2020-09-01	56b6e0d5-090d-4859-9f06-c54a1a116515	Goroutines

Basics

A goroutine is a lightweight thread managed by the Go runtime.

package main

import (
    "fmt"
    "time"
)

func say(s string) {
    for i := 0; i < 5; i++ {
        time.Sleep(100 * time.Millisecond)
        fmt.Println(s)
    }
}

func main() {
    go say("world")
    say("hello")
}

Channels

Channels are a typed conduit through whichyou can send and receive values with the channel operator `<-`. By default, sends and receives block until the other side is ready. This allows goroutines to synchronize without explicit locks or condition variables. Channels should generally be used for passing ownership of data.

package main

import "fmt"

func sum(s []int, c chan int) {
    sum := 0
    for _, v := range s {
        sum += v
    }
    c <- sum // send sum to c
}

func main() {
    s := []int{7, 2, 8, -9, 4, 0}

    c := make(chan int)
    go sum(s[:len(s)/2], c)
    go sum(s[len(s)/2:], c)
    x, y := <-c, <-c // receive from c

    fmt.Println(x, y, x+y)
}

Channels can also be buffered. Provide the buffer length as the second argument to `make` to initialize a buffered channel:

package main

import "fmt"

func main() {
    ch := make(chan int, 2)
    ch <- 1
    ch <- 2
    fmt.Println(<-ch)
    fmt.Println(<-ch)
}

Sends to a buffered channel block only when the buffer is full. Receives block when the buffer is empty.

Range and close

A sender can close a channel to indicate that no more values will be sent. Receivers can test whether a channel has been closed by assigning a second parameter to the receive expression.

Only the sender should close a channel, never the receiver. Sending on a closed channel will cause a panic. Channels aren't like files; you don't usually need to close them. Closing is only necessary when the receiver must be told there are no more values coming, such as to terminate a range loop.

package main

import (
    "fmt"
)

func fibonacci(n int, c chan int) {
    x, y := 0, 1
    for i := 0; i < n; i++ {
        c <- x
        x, y = y, x+y
    }
    close(c)
}

func main() {
    c := make(chan int, 10)
    go fibonacci(cap(c), c)
    for i := range c {
        fmt.Println(i)
    }
}

Select

The select statement lets a goroutine wait on multiple communication operations.

A select blocks until one of its cases can run, then it executes that case. It chooses one at random if multiple are ready.

package main

import "fmt"

func fibonacci(c, quit chan int) {
    x, y := 0, 1
    for {
        select {
        case c <- x:
            x, y = y, x+y
        case <-quit:
            fmt.Println("quit")
            return
        }
    }
}

func main() {
    c := make(chan int)
    quit := make(chan int)
    go func() {
        for i := 0; i < 10; i++ {
            fmt.Println(<-c)
        }
        quit <- 0
    }()
    fibonacci(c, quit)
}

`default` case in `select` is run if no other case is ready, as one would expect

package main

import (
    "fmt"
    "time"
)

func main() {
    tick := time.Tick(100 * time.Millisecond)
    boom := time.After(500 * time.Millisecond)
    for {
        select {
        case <-tick:
            fmt.Println("tick.")
        case <-boom:
            fmt.Println("BOOM!")
            return
        default:
            fmt.Println("    .")
            time.Sleep(50 * time.Millisecond)
        }
    }
}

Timeout

Often you want to set a timeout value for select so it won't run forver. time.After is a good way of doing this:

package main

import (
    "fmt"
    "time"
)

var c chan int

func handle(int) {}

func main() {
    select {
    case m := <-c:
        handle(m)
    case <-time.After(10 * time.Second):
        fmt.Println("timed out")
    }
}

sync.Mutex

TO make sure only one goroutine at a time can access a variable we can use `sync.Mutex`

package main

import (
    "fmt"
    "sync"
    "time"
)

// SafeCounter is safe to use concurrently.
type SafeCounter struct {
    mu sync.Mutex
    v  map[string]int
}

// Inc increments the counter for the given key.
func (c *SafeCounter) Inc(key string) {
    c.mu.Lock()
    // Lock so only one goroutine at a time can access the map c.v.
    c.v[key]++
    c.mu.Unlock()
}

// Value returns the current value of the counter for the given key.
func (c *SafeCounter) Value(key string) int {
    c.mu.Lock()
    // Lock so only one goroutine at a time can access the map c.v.
    defer c.mu.Unlock()
    return c.v[key]
}

func main() {
    c := SafeCounter{v: make(map[string]int)}
    for i := 0; i < 1000; i++ {
        go c.Inc("somekey")
    }

    time.Sleep(time.Second)
    fmt.Println(c.Value("somekey"))
}