To use golang Mutex

ulid.go

package main

import (
	"fmt"
	"sync"
)

func main() {
	InitUlidGenerator(nil)

	var wg sync.WaitGroup
	fn := func(idx int) {
		id := GenUlid()
		fmt.Println(id)
		defer wg.Done()
	}

	for i := 0; i < 100; i++ {
		wg.Add(1)
		idx := i
		go fn(idx)
	}
	wg.Wait()
}

ulidgen.go

package main

import (
	"io"
	"math/rand"
	"sync"
	"time"

	"github.com/oklog/ulid"
)

type Ulid struct {
	mu      sync.Mutex
	t       time.Time
	entropy io.Reader
}

var ulidGen *Ulid

func InitUlidGenerator(now func() time.Time) {
	var currentTime time.Time
	if now != nil {
		currentTime = now()
	} else {
		currentTime = time.Now()
	}
	ulidGen = new(Ulid)
	ulidGen.t = time.Unix(currentTime.Unix(), 0)
	ulidGen.entropy = ulid.Monotonic(rand.New(rand.NewSource(ulidGen.t.UnixNano())), 0)
}

// genUlid generates ULID.
// see: https://qiita.com/kai_kou/items/b4ac2d316920e08ac75a
func GenUlid() string {
	if ulidGen == nil {
		InitUlidGenerator(nil)
	}
	ulidGen.mu.Lock()
	defer ulidGen.mu.Unlock()
	id := ulid.MustNew(ulid.Timestamp(ulidGen.t), ulidGen.entropy).String()
	return id
}

はい，テスト用に時間を注入できるようにしていますが，ここでは使ってないですね・・