testtime

How to use

testtime package provides testtime.Now() and testtime.SetTime(). testtime.SetTime() stores a fixed time to a map with goroutine ID of testtime.SetTime() as a key. When goroutine ID of testtime.Now() is related to a fixed time by testtime.SetTime(), it returns the fixed time otherwise it returns current time which is returned by time.Now().

package main

import (
	"fmt"
	"time"
	"testing"

	"github.com/tenntenn/testtime"
)

func Test(t *testing.T) {

	t.Run("A", func(t *testing.T) {
		// set zero value
		testtime.SetTime(t, time.Time{})
		// true
		if time.Now().IsZero {
			t.Error("error")
		}
	})

	t.Run("B", func(t *testing.T) {
		// set func which return zero value
		f := func() time.Time {
			return time.Time{}
		}
		testtime.SetFunc(t, f)
		// true
		if time.Now().IsZero {
			t.Error("error")
		}
	})

	// false
	if !time.Now().IsZero {
		t.Error("error")
	}
}

The testtime command replace time.Now to testtime.Now. It prints a file path of overlay JSON which can be given to -overlay flag of go test command.

$ go test -overlay=$(go run github.com/tenntenn/testtime/cmd/testtime@latest)
PASS
ok  	main	0.156s

You can install testtime with the go install command. And don't forget to add the installed directory to the PATH environment variable.

$ go install github.com/tenntenn/testtime/cmd/testtime@latest

The testtime command creates an overlay JSON file and time.go which is replaced time.Now in $GOPATH/pkg/testtime directory. The testtime command does not update these files without -u flag.

$ cat `testtime` | jq
{
  "Replace": {
    "/usr/local/go/src/time/time.go": "/Users/tenntenn/go/pkg/testtime/time_go1.23.1.go"
  }
}

$ diff /usr/local/go/src/time/time.go /Users/tenntenn/go/pkg/testtime/time_go1.23.1.go
94a95,96
> 	"runtime"
> 	"sync"
1159c1161
< func Now() Time {
---
> func _Now() Time {
1695a1698,1726
> 
> // It will be added to GOROOT/src/time/time.go.
> 
> //go:linkname timeMap
> var timeMap sync.Map
> 
> // Now returns a fixed time which is related with the goroutine by SetTime or SetFunc.
> // If the current goroutine is not related with any fixed time or function, Now calls time.Now and returns its returned value.
> func Now() Time {
> 	v, ok := timeMap.Load(goroutineID())
> 	if ok {
> 		return v.(func() Time)()
> 	}
> 	return _Now()
> }
> 
> func goroutineID() string {
> 	var buf [64]byte
> 	n := runtime.Stack(buf[:], false)
> 	// 10: len("goroutine ")
> 	for i := 10; i < n; i++ {
> 		if buf[i] == ' ' {
> 			return string(buf[10:i])
> 		}
> 	}
> 	return ""
> }
> 
> // End of testtime's code

Examples

See _examples directory.