synctest

This example demonstrates testing the context.AfterFunc function.

AfterFunc registers a function to execute in a new goroutine after a context is canceled.

The test verifies that the function is not run before the context is canceled, and is run after the context is canceled.

package main

import (
	"context"
	"fmt"
	"testing/synctest"
)

func main() {
	synctest.Run(func() {
		// Create a context.Context which can be canceled.
		ctx, cancel := context.WithCancel(context.Background())

		// context.AfterFunc registers a function to be called
		// when a context is canceled.
		afterFuncCalled := false
		context.AfterFunc(ctx, func() {
			afterFuncCalled = true
		})

		// The context has not been canceled, so the AfterFunc is not called.
		synctest.Wait()
		fmt.Printf("before context is canceled: afterFuncCalled=%v\n", afterFuncCalled)

		// Cancel the context and wait for the AfterFunc to finish executing.
		// Verify that the AfterFunc ran.
		cancel()
		synctest.Wait()
		fmt.Printf("after context is canceled:  afterFuncCalled=%v\n", afterFuncCalled)

	})
}

Output:

before context is canceled: afterFuncCalled=false
after context is canceled:  afterFuncCalled=true

This example demonstrates testing the context.WithTimeout function.

WithTimeout creates a context which is canceled after a timeout.

The test verifies that the context is not canceled before the timeout expires, and is canceled after the timeout expires.

package main

import (
	"context"
	"fmt"
	"testing/synctest"
	"time"
)

func main() {
	synctest.Run(func() {
		// Create a context.Context which is canceled after a timeout.
		const timeout = 5 * time.Second
		ctx, cancel := context.WithTimeout(context.Background(), timeout)
		defer cancel()

		// Wait just less than the timeout.
		time.Sleep(timeout - time.Nanosecond)
		synctest.Wait()
		fmt.Printf("before timeout: ctx.Err() = %v\n", ctx.Err())

		// Wait the rest of the way until the timeout.
		time.Sleep(time.Nanosecond)
		synctest.Wait()
		fmt.Printf("after timeout:  ctx.Err() = %v\n", ctx.Err())

	})
}

Output:

before timeout: ctx.Err() = <nil>
after timeout:  ctx.Err() = context deadline exceeded