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+---
+tags:
+  - golang
+  - go-weekly
+authors:
+  - fuatto
+title: 'Go Commentary #24: Coming in Go 1.24: testing/synctest experiment for time and concurrency testing'
+short_title: '#24 Go 1.24 testing/synctest experiment for time and concurrency testing'
+description: Go 1.24 testing/synctest experiment for time and concurrency testing
+
+date: 2024-12-13
+---
+
+## [Coming in Go 1.24: testing/synctest experiment for time and concurrency testing](https://danp.net/posts/synctest-experiment/)
+
+### Context
+
+```go
+func Test(t *testing.T) {
+    before := time.Now()
+    time.Sleep(time.Second)
+    after := time.Now()
+    if d := after.Sub(before); d != time.Second {
+        t.Fatalf("took %v", d)
+    }
+}
+```
+
+- Traditional hack
+
+```go
+func Test(t *testing.T) {
+    before := time.Now()
+    time.Sleep(time.Second)
+    after := time.Now()
+    if d := after.Sub(before); d >= 2*time.Second {
+        t.Fatalf("took %v", d)
+    }
+}
+```
+
+- It's still flaky because it depends on the system clock.
+
+
+
+### Solution
+
+- The `testing/synctest` package is an experiment to provide a more deterministic way to test time and concurrency in Go.
+
+```go
+import (
+	"testing"
+	"testing/synctest"
+	"time"
+)
+
+func Test(t *testing.T) {
+	synctest.Run(func() {
+		before := time.Now()
+		time.Sleep(time.Second)
+		after := time.Now()
+		if d := after.Sub(before); d != time.Second {
+			t.Fatalf("took %v", d)
+		}
+	})
+}
+```
+
+- And then use [gotip](https://pkg.go.dev/golang.org/dl/gotip) with `GOEXPERIMENT=synctest`
+
+### Extending to concurrency
+
+```go
+func Test(t *testing.T) {
+	ctx := context.Background()
+
+	ctx, cancel := context.WithCancel(ctx)
+
+	var hits atomic.Int32
+	go func() {
+		tick := time.NewTicker(time.Millisecond)
+		defer tick.Stop()
+		for {
+			select {
+			case <-ctx.Done():
+				return
+			case <-tick.C:
+				hits.Add(1)
+			}
+		}
+	}()
+
+	time.Sleep(3 * time.Millisecond)
+	cancel()
+
+	got := int(hits.Load())
+	if want := 3; got != want {
+		t.Fatalf("got %v, want %v", got, want)
+	}
+}
+```
+
+- It's flaky because of the initial delay of the Ticker
+
+- Wrap the test in `synctest.Run` to make it deterministic
+
+```go
+func Test(t *testing.T) {
+	synctest.Run(func() {
+		ctx := context.Background()
+
+		ctx, cancel := context.WithCancel(ctx)
+
+		var hits atomic.Int32
+		go func() {
+			tick := time.NewTicker(time.Millisecond)
+			defer tick.Stop()
+			for {
+				select {
+				case <-ctx.Done():
+					return
+				case <-tick.C:
+					hits.Add(1)
+				}
+			}
+		}()
+
+		time.Sleep(4 * time.Millisecond)
+		cancel()
+
+		got := int(hits.Load())
+		if want := 3; got != want {
+			t.Fatalf("got %v, want %v", got, want)
+		}
+	})
+}
+```
+
+### Conclusion
+
+- It seems that `testing/synctest` will significantly improve testing code that involves time or concurrency. Example in go source: [https://go-review.googlesource.com/c/go/+/630382](https://go-review.googlesource.com/c/go/+/630382)
+
+- You can try it yourself now by using `gotip` and setting `GOEXPERIMENT=synctest`. When Go 1.24 comes out GOEXPERIMENT=synctest will still be required.
+
+- Review the [main proposal](https://github.com/golang/go/issues/67434) and share any experience you have.
+
+---
+
+https://danp.net/posts/synctest-experiment/
+
+https://go-review.googlesource.com/c/go/+/630382
+
+https://github.com/golang/go/issues/67434
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