Upgrade Sonyflake to v2 (#66)

* Add v2

* Introduce staticcheck

* Introduce golangci-lint

* Add error checks

* Add error checks

* Lint v2 code

* Improve CI trigger

* Use io.ReadAll

* Use int64

* Remove NewSonyflake

* Fix errors

* v2: Change MachineID, sequence, and AmazonEC2MachineID to int; update all usage and tests for type consistency

* docs: update Settings struct in README to use int for MachineID and CheckMachineID (v2)

* docs(v2): clarify Settings, StartTime, MachineID, and CheckMachineID comments and update README links and explanations

* docs(v2/mock): improve comments and docstrings for mock implementations

* docs(types): unify and clarify package and type docstrings for types.go in v1 and v2

* test(v2): refactor and modernize tests, improve error assertions, and update mocks for v2

* test(v2): normalize whitespace in pseudoSleep calls for consistency

* feat(v2): add configurable TimeUnit and refactor time handling for So… (#67)

* feat(v2): add configurable TimeUnit and refactor time handling for Sonyflake v2

* test(v2): add ToTime tests, clarify TimeUnit behavior, and update docs for v2

* gofmt

v2/awsutil/awsutil.go

@@ -0,0 +1,64 @@
+// Package awsutil provides utility functions for using Sonyflake on AWS.
+package awsutil
+
+import (
+	"errors"
+	"io"
+	"net"
+	"net/http"
+	"os/exec"
+	"regexp"
+	"strconv"
+	"time"
+)
+
+func amazonEC2PrivateIPv4() (net.IP, error) {
+	res, err := http.Get("http://169.254.169.254/latest/meta-data/local-ipv4")
+	if err != nil {
+		return nil, err
+	}
+	defer res.Body.Close()
+
+	body, err := io.ReadAll(res.Body)
+	if err != nil {
+		return nil, err
+	}
+
+	ip := net.ParseIP(string(body))
+	if ip == nil {
+		return nil, errors.New("invalid ip address")
+	}
+	return ip.To4(), nil
+}
+
+// AmazonEC2MachineID retrieves the private IP address of the Amazon EC2 instance
+// and returns its lower 16 bits.
+// It works correctly on Docker as well.
+func AmazonEC2MachineID() (int, error) {
+	ip, err := amazonEC2PrivateIPv4()
+	if err != nil {
+		return 0, err
+	}
+
+	return int(ip[2])<<8 + int(ip[3]), nil
+}
+
+// TimeDifference returns the time difference between the localhost and the given NTP server.
+func TimeDifference(server string) (time.Duration, error) {
+	output, err := exec.Command("/usr/sbin/ntpdate", "-q", server).CombinedOutput()
+	if err != nil {
+		return time.Duration(0), err
+	}
+
+	re, _ := regexp.Compile("offset (.*) sec")
+	submatched := re.FindSubmatch(output)
+	if len(submatched) != 2 {
+		return time.Duration(0), errors.New("invalid ntpdate output")
+	}
+
+	f, err := strconv.ParseFloat(string(submatched[1]), 64)
+	if err != nil {
+		return time.Duration(0), err
+	}
+	return time.Duration(f*1000) * time.Millisecond, nil
+}

v2/example/Dockerfile

@@ -0,0 +1,6 @@
+FROM ubuntu:24.04
+
+ADD ./sonyflake_server /
+ENTRYPOINT ["/sonyflake_server"]
+
+EXPOSE 8080

v2/example/Dockerrun.aws.json

@@ -0,0 +1,8 @@
+{
+  "AWSEBDockerrunVersion": "1",
+  "Ports": [
+    {
+      "ContainerPort": "8080"
+    }
+  ]
+}

v2/example/README.md

@@ -0,0 +1,21 @@
+Example
+=======
+
+This example runs Sonyflake on AWS Elastic Beanstalk.
+
+Setup
+-----
+
+1. Build the cross compiler for linux/amd64 if using other platforms.
+
+  ```
+  cd $GOROOT/src && GOOS=linux GOARCH=amd64 ./make.bash
+  ```
+
+2. Build sonyflake_server in the example directory.
+
+  ```
+  ./linux64_build.sh
+  ```
+
+3. Upload the example directory to AWS Elastic Beanstalk.

v2/example/linux64_build.sh

@@ -0,0 +1,2 @@
+#!/bin/sh
+GOOS=linux GOARCH=amd64 go build sonyflake_server.go

v2/example/sonyflake_server.go

@@ -0,0 +1,51 @@
+package main
+
+import (
+	"encoding/json"
+	"net/http"
+
+	"github.com/sony/sonyflake/v2"
+	"github.com/sony/sonyflake/v2/awsutil"
+)
+
+var sf *sonyflake.Sonyflake
+
+func init() {
+	var st sonyflake.Settings
+	st.MachineID = awsutil.AmazonEC2MachineID
+
+	var err error
+	sf, err = sonyflake.New(st)
+	if err != nil {
+		panic(err)
+	}
+}
+
+func handler(w http.ResponseWriter, r *http.Request) {
+	id, err := sf.NextID()
+	if err != nil {
+		http.Error(w, err.Error(), http.StatusInternalServerError)
+		return
+	}
+
+	body, err := json.Marshal(sonyflake.Decompose(id))
+	if err != nil {
+		http.Error(w, err.Error(), http.StatusInternalServerError)
+		return
+	}
+
+	w.Header()["Content-Type"] = []string{"application/json; charset=utf-8"}
+	_, err = w.Write(body)
+	if err != nil {
+		http.Error(w, err.Error(), http.StatusInternalServerError)
+		return
+	}
+}
+
+func main() {
+	http.HandleFunc("/", handler)
+	err := http.ListenAndServe(":8080", nil)
+	if err != nil {
+		panic(err)
+	}
+}

v2/go.mod

@@ -0,0 +1,3 @@
+module github.com/sony/sonyflake/v2
+
+go 1.22

v2/mock/sonyflake_mock.go

@@ -0,0 +1,36 @@
+// Package mock offers mock implementations of interfaces defined in types.go.
+// This allows complete control over input / output for any given method that consumes a given type.
+package mock
+
+import (
+	"errors"
+	"net"
+
+	"github.com/sony/sonyflake/v2/types"
+)
+
+// NewSuccessfulInterfaceAddrs returns a single private IP address.
+func NewSuccessfulInterfaceAddrs() types.InterfaceAddrs {
+	ifat := make([]net.Addr, 0, 1)
+	ifat = append(ifat, &net.IPNet{IP: []byte{192, 168, 0, 1}, Mask: []byte{255, 0, 0, 0}})
+
+	return func() ([]net.Addr, error) {
+		return ifat, nil
+	}
+}
+
+var ErrFailedToGetAddresses = errors.New("failed to get addresses")
+
+// NewFailingInterfaceAddrs returns an error.
+func NewFailingInterfaceAddrs() types.InterfaceAddrs {
+	return func() ([]net.Addr, error) {
+		return nil, ErrFailedToGetAddresses
+	}
+}
+
+// NewNilInterfaceAddrs returns an empty slice of addresses.
+func NewNilInterfaceAddrs() types.InterfaceAddrs {
+	return func() ([]net.Addr, error) {
+		return []net.Addr{}, nil
+	}
+}

v2/sonyflake.go

@@ -0,0 +1,232 @@
+// Package sonyflake implements Sonyflake, a distributed unique ID generator inspired by Twitter's Snowflake.
+//
+// A Sonyflake ID is composed of
+//
+//	39 bits for time in units of 10 msec
+//	 8 bits for a sequence number
+//	16 bits for a machine id
+package sonyflake
+
+import (
+	"errors"
+	"net"
+	"sync"
+	"time"
+
+	"github.com/sony/sonyflake/v2/types"
+)
+
+// These constants are the bit lengths of Sonyflake ID parts.
+const (
+	BitLenTime     = 39                               // bit length of time
+	BitLenSequence = 8                                // bit length of sequence number
+	BitLenMachine  = 63 - BitLenTime - BitLenSequence // bit length of machine id
+)
+
+// Settings configures Sonyflake:
+//
+// TimeUnit is the time unit of Sonyflake.
+// If TimeUnit is 0, the default time unit is used, which is 10 msec.
+// TimeUnit must be equal to or greater than 1 msec.
+//
+// StartTime is the time since which the Sonyflake time is defined as the elapsed time.
+// If StartTime is 0, the start time of the Sonyflake instance is set to "2025-01-01 00:00:00 +0000 UTC".
+// StartTime must be before the current time.
+//
+// MachineID returns the unique ID of a Sonyflake instance.
+// If MachineID returns an error, the instance will not be created.
+// If MachineID is nil, the default MachineID is used, which returns the lower 16 bits of the private IP address.
+//
+// CheckMachineID validates the uniqueness of a machine ID.
+// If CheckMachineID returns false, the instance will not be created.
+// If CheckMachineID is nil, no validation is done.
+type Settings struct {
+	TimeUnit       time.Duration
+	StartTime      time.Time
+	MachineID      func() (int, error)
+	CheckMachineID func(int) bool
+}
+
+// Sonyflake is a distributed unique ID generator.
+type Sonyflake struct {
+	mutex       *sync.Mutex
+	timeUnit    int64
+	startTime   int64
+	elapsedTime int64
+	sequence    int
+	machine     int
+}
+
+var (
+	ErrStartTimeAhead   = errors.New("start time is ahead of now")
+	ErrNoPrivateAddress = errors.New("no private ip address")
+	ErrOverTimeLimit    = errors.New("over the time limit")
+	ErrInvalidMachineID = errors.New("invalid machine id")
+	ErrInvalidTimeUnit  = errors.New("invalid time unit")
+)
+
+const defaultTimeUnit = 1e7 // nsec, i.e. 10 msec
+
+var defaultInterfaceAddrs = net.InterfaceAddrs
+
+// New returns a new Sonyflake configured with the given Settings.
+// New returns an error in the following cases:
+// - Settings.StartTime is ahead of the current time.
+// - Settings.MachineID returns an error.
+// - Settings.CheckMachineID returns false.
+func New(st Settings) (*Sonyflake, error) {
+	if st.StartTime.After(time.Now()) {
+		return nil, ErrStartTimeAhead
+	}
+
+	sf := new(Sonyflake)
+	sf.mutex = new(sync.Mutex)
+	sf.sequence = 1<<BitLenSequence - 1
+
+	if st.TimeUnit == 0 {
+		sf.timeUnit = defaultTimeUnit
+	} else if st.TimeUnit >= time.Millisecond {
+		sf.timeUnit = int64(st.TimeUnit)
+	} else {
+		return nil, ErrInvalidTimeUnit
+	}
+
+	if st.StartTime.IsZero() {
+		sf.startTime = sf.toInternalTime(time.Date(2025, 1, 1, 0, 0, 0, 0, time.UTC))
+	} else {
+		sf.startTime = sf.toInternalTime(st.StartTime)
+	}
+
+	var err error
+	if st.MachineID == nil {
+		sf.machine, err = lower16BitPrivateIP(defaultInterfaceAddrs)
+	} else {
+		sf.machine, err = st.MachineID()
+	}
+	if err != nil {
+		return nil, err
+	}
+
+	if st.CheckMachineID != nil && !st.CheckMachineID(sf.machine) {
+		return nil, ErrInvalidMachineID
+	}
+
+	return sf, nil
+}
+
+// NextID generates a next unique ID as int64.
+// After the Sonyflake time overflows, NextID returns an error.
+func (sf *Sonyflake) NextID() (int64, error) {
+	const maskSequence = 1<<BitLenSequence - 1
+
+	sf.mutex.Lock()
+	defer sf.mutex.Unlock()
+
+	current := sf.currentElapsedTime()
+	if sf.elapsedTime < current {
+		sf.elapsedTime = current
+		sf.sequence = 0
+	} else {
+		sf.sequence = (sf.sequence + 1) & maskSequence
+		if sf.sequence == 0 {
+			sf.elapsedTime++
+			overtime := sf.elapsedTime - current
+			sf.sleep(overtime)
+		}
+	}
+
+	return sf.toID()
+}
+
+func (sf *Sonyflake) toInternalTime(t time.Time) int64 {
+	return t.UTC().UnixNano() / sf.timeUnit
+}
+
+func (sf *Sonyflake) currentElapsedTime() int64 {
+	return sf.toInternalTime(time.Now()) - sf.startTime
+}
+
+func (sf *Sonyflake) sleep(overtime int64) {
+	sleepTime := time.Duration(overtime*sf.timeUnit) -
+		time.Duration(time.Now().UTC().UnixNano()%sf.timeUnit)
+	time.Sleep(sleepTime)
+}
+
+func (sf *Sonyflake) toID() (int64, error) {
+	if sf.elapsedTime >= 1<<BitLenTime {
+		return 0, ErrOverTimeLimit
+	}
+
+	return sf.elapsedTime<<(BitLenSequence+BitLenMachine) |
+		int64(sf.sequence)<<BitLenMachine |
+		int64(sf.machine), nil
+}
+
+func privateIPv4(interfaceAddrs types.InterfaceAddrs) (net.IP, error) {
+	as, err := interfaceAddrs()
+	if err != nil {
+		return nil, err
+	}
+
+	for _, a := range as {
+		ipnet, ok := a.(*net.IPNet)
+		if !ok || ipnet.IP.IsLoopback() {
+			continue
+		}
+
+		ip := ipnet.IP.To4()
+		if isPrivateIPv4(ip) {
+			return ip, nil
+		}
+	}
+	return nil, ErrNoPrivateAddress
+}
+
+func isPrivateIPv4(ip net.IP) bool {
+	// Allow private IP addresses (RFC1918) and link-local addresses (RFC3927)
+	return ip != nil &&
+		(ip[0] == 10 || ip[0] == 172 && (ip[1] >= 16 && ip[1] < 32) || ip[0] == 192 && ip[1] == 168 || ip[0] == 169 && ip[1] == 254)
+}
+
+func lower16BitPrivateIP(interfaceAddrs types.InterfaceAddrs) (int, error) {
+	ip, err := privateIPv4(interfaceAddrs)
+	if err != nil {
+		return 0, err
+	}
+
+	return int(ip[2])<<8 + int(ip[3]), nil
+}
+
+func (sf *Sonyflake) ToTime(id int64) time.Time {
+	return time.Unix(0, (sf.startTime+Time(id))*sf.timeUnit)
+}
+
+// Time returns the Sonyflake time when the given ID was generated.
+func Time(id int64) int64 {
+	return id >> (BitLenSequence + BitLenMachine)
+}
+
+// SequenceNumber returns the sequence number of a Sonyflake ID.
+func SequenceNumber(id int64) int {
+	const maskSequence = int64((1<<BitLenSequence - 1) << BitLenMachine)
+	return int((id & maskSequence) >> BitLenMachine)
+}
+
+// MachineID returns the machine ID of a Sonyflake ID.
+func MachineID(id int64) int {
+	const maskMachine = int64(1<<BitLenMachine - 1)
+	return int(id & maskMachine)
+}
+
+// Decompose returns a set of Sonyflake ID parts.
+func Decompose(id int64) map[string]int64 {
+	time := Time(id)
+	sequence := SequenceNumber(id)
+	machine := MachineID(id)
+	return map[string]int64{
+		"id":       id,
+		"time":     time,
+		"sequence": int64(sequence),
+		"machine":  int64(machine),
+	}
+}

v2/sonyflake_test.go

@@ -0,0 +1,331 @@
+package sonyflake
+
+import (
+	"errors"
+	"fmt"
+	"net"
+	"runtime"
+	"testing"
+	"time"
+
+	"github.com/sony/sonyflake/v2/mock"
+	"github.com/sony/sonyflake/v2/types"
+)
+
+func TestNew(t *testing.T) {
+	errGetMachineID := fmt.Errorf("failed to get machine id")
+
+	testCases := []struct {
+		name     string
+		settings Settings
+		err      error
+	}{
+		{
+			name: "invalid time unit",
+			settings: Settings{
+				TimeUnit: time.Microsecond,
+			},
+			err: ErrInvalidTimeUnit,
+		},
+		{
+			name: "start time ahead",
+			settings: Settings{
+				StartTime: time.Now().Add(time.Minute),
+			},
+			err: ErrStartTimeAhead,
+		},
+		{
+			name: "cannot get machine id",
+			settings: Settings{
+				MachineID: func() (int, error) {
+					return 0, errGetMachineID
+				},
+			},
+			err: errGetMachineID,
+		},
+		{
+			name: "invalid machine id",
+			settings: Settings{
+				CheckMachineID: func(int) bool {
+					return false
+				},
+			},
+			err: ErrInvalidMachineID,
+		},
+		{
+			name:     "success",
+			settings: Settings{},
+		},
+	}
+
+	for _, tc := range testCases {
+		t.Run(tc.name, func(t *testing.T) {
+			sf, err := New(tc.settings)
+
+			if !errors.Is(err, tc.err) {
+				t.Fatalf("unexpected error: %v", err)
+			}
+
+			if err == nil && sf == nil {
+				t.Fatal("sonyflake instance must be created")
+			}
+		})
+	}
+}
+
+func newSonyflake(t *testing.T, st Settings) *Sonyflake {
+	sf, err := New(st)
+	if err != nil {
+		t.Fatalf("failed to create sonyflake: %v", err)
+	}
+	return sf
+}
+
+func nextID(t *testing.T, sf *Sonyflake) int64 {
+	id, err := sf.NextID()
+	if err != nil {
+		t.Fatalf("failed to generate id: %v", err)
+	}
+	return id
+}
+
+func defaultMachineID(t *testing.T) int {
+	ip, err := lower16BitPrivateIP(defaultInterfaceAddrs)
+	if err != nil {
+		t.Fatalf("failed to get private ip address: %v", err)
+	}
+	return ip
+}
+
+func TestNextID(t *testing.T) {
+	sf := newSonyflake(t, Settings{StartTime: time.Now()})
+
+	sleepTime := int64(50)
+	time.Sleep(time.Duration(sleepTime * sf.timeUnit))
+
+	id := nextID(t, sf)
+
+	actualTime := Time(id)
+	if actualTime < sleepTime || actualTime > sleepTime+1 {
+		t.Errorf("unexpected time: %d", actualTime)
+	}
+
+	actualSequence := SequenceNumber(id)
+	if actualSequence != 0 {
+		t.Errorf("unexpected sequence: %d", actualSequence)
+	}
+
+	actualMachine := MachineID(id)
+	if actualMachine != defaultMachineID(t) {
+		t.Errorf("unexpected machine: %d", actualMachine)
+	}
+
+	fmt.Println("sonyflake id:", id)
+	fmt.Println("decompose:", Decompose(id))
+}
+
+func TestNextID_InSequence(t *testing.T) {
+	now := time.Now()
+	sf := newSonyflake(t, Settings{
+		TimeUnit:  time.Millisecond,
+		StartTime: now,
+	})
+	startTime := sf.toInternalTime(now)
+	machineID := int64(defaultMachineID(t))
+
+	var numID int
+	var lastID int64
+	var maxSeq int64
+
+	currentTime := startTime
+	for currentTime-startTime < 100 {
+		id := nextID(t, sf)
+		currentTime = sf.toInternalTime(time.Now())
+		numID++
+
+		if id == lastID {
+			t.Fatal("duplicated id")
+		}
+		if id < lastID {
+			t.Fatal("must increase with time")
+		}
+		lastID = id
+
+		parts := Decompose(id)
+
+		actualTime := parts["time"]
+		overtime := startTime + actualTime - currentTime
+		if overtime > 0 {
+			t.Errorf("unexpected overtime: %d", overtime)
+		}
+
+		actualSequence := parts["sequence"]
+		if actualSequence > maxSeq {
+			maxSeq = actualSequence
+		}
+
+		actualMachine := parts["machine"]
+		if actualMachine != machineID {
+			t.Errorf("unexpected machine: %d", actualMachine)
+		}
+	}
+
+	if maxSeq != 1<<BitLenSequence-1 {
+		t.Errorf("unexpected max sequence: %d", maxSeq)
+	}
+	fmt.Println("max sequence:", maxSeq)
+	fmt.Println("number of id:", numID)
+}
+
+func TestNextID_InParallel(t *testing.T) {
+	sf1 := newSonyflake(t, Settings{MachineID: func() (int, error) { return 1, nil }})
+	sf2 := newSonyflake(t, Settings{MachineID: func() (int, error) { return 2, nil }})
+
+	numCPU := runtime.NumCPU()
+	runtime.GOMAXPROCS(numCPU)
+	fmt.Println("number of cpu:", numCPU)
+
+	consumer := make(chan int64)
+
+	const numID = 1000
+	generate := func(sf *Sonyflake) {
+		for i := 0; i < numID; i++ {
+			id := nextID(t, sf)
+			consumer <- id
+		}
+	}
+
+	var numGenerator int
+	for i := 0; i < numCPU/2; i++ {
+		go generate(sf1)
+		go generate(sf2)
+		numGenerator += 2
+	}
+
+	set := make(map[int64]struct{})
+	for i := 0; i < numID*numGenerator; i++ {
+		id := <-consumer
+		if _, ok := set[id]; ok {
+			t.Fatal("duplicated id")
+		}
+		set[id] = struct{}{}
+	}
+	fmt.Println("number of id:", len(set))
+}
+
+func pseudoSleep(sf *Sonyflake, period time.Duration) {
+	sf.startTime -= int64(period) / sf.timeUnit
+}
+
+func TestNextID_ReturnsError(t *testing.T) {
+	sf := newSonyflake(t, Settings{StartTime: time.Now()})
+
+	year := time.Duration(365*24) * time.Hour
+	pseudoSleep(sf, time.Duration(174)*year)
+	nextID(t, sf)
+
+	pseudoSleep(sf, time.Duration(1)*year)
+	_, err := sf.NextID()
+	if err == nil {
+		t.Errorf("time is not over")
+	}
+}
+
+func TestToTime(t *testing.T) {
+	start := time.Now()
+	sf := newSonyflake(t, Settings{
+		TimeUnit:  time.Millisecond,
+		StartTime: start,
+	})
+
+	id := nextID(t, sf)
+
+	tm := sf.ToTime(id)
+	diff := tm.Sub(start)
+	if diff < 0 || diff >= time.Duration(sf.timeUnit) {
+		t.Errorf("unexpected time: %v", tm)
+	}
+}
+
+func TestPrivateIPv4(t *testing.T) {
+	testCases := []struct {
+		description    string
+		expected       net.IP
+		interfaceAddrs types.InterfaceAddrs
+		error          error
+	}{
+		{
+			description:    "returns an error",
+			expected:       nil,
+			interfaceAddrs: mock.NewFailingInterfaceAddrs(),
+			error:          mock.ErrFailedToGetAddresses,
+		},
+		{
+			description:    "empty address list",
+			expected:       nil,
+			interfaceAddrs: mock.NewNilInterfaceAddrs(),
+			error:          ErrNoPrivateAddress,
+		},
+		{
+			description:    "success",
+			expected:       net.IP{192, 168, 0, 1},
+			interfaceAddrs: mock.NewSuccessfulInterfaceAddrs(),
+			error:          nil,
+		},
+	}
+
+	for _, tc := range testCases {
+		t.Run(tc.description, func(t *testing.T) {
+			actual, err := privateIPv4(tc.interfaceAddrs)
+
+			if !errors.Is(err, tc.error) {
+				t.Fatalf("unexpected error: %v", err)
+			}
+
+			if !net.IP.Equal(actual, tc.expected) {
+				t.Errorf("unexpected ip: %s", actual)
+			}
+		})
+	}
+}
+
+func TestLower16BitPrivateIP(t *testing.T) {
+	testCases := []struct {
+		description    string
+		expected       int
+		interfaceAddrs types.InterfaceAddrs
+		error          error
+	}{
+		{
+			description:    "returns an error",
+			expected:       0,
+			interfaceAddrs: mock.NewFailingInterfaceAddrs(),
+			error:          mock.ErrFailedToGetAddresses,
+		},
+		{
+			description:    "empty address list",
+			expected:       0,
+			interfaceAddrs: mock.NewNilInterfaceAddrs(),
+			error:          ErrNoPrivateAddress,
+		},
+		{
+			description:    "success",
+			expected:       1,
+			interfaceAddrs: mock.NewSuccessfulInterfaceAddrs(),
+			error:          nil,
+		},
+	}
+	for _, tc := range testCases {
+		t.Run(tc.description, func(t *testing.T) {
+			actual, err := lower16BitPrivateIP(tc.interfaceAddrs)
+
+			if !errors.Is(err, tc.error) {
+				t.Fatalf("unexpected error: %v", err)
+			}
+
+			if actual != tc.expected {
+				t.Errorf("unexpected ip: %d", actual)
+			}
+		})
+	}
+}

v2/types/types.go

@@ -0,0 +1,8 @@
+// Package types defines type signatures used throughout sonyflake.
+// This provides the ability to mock out imports.
+package types
+
+import "net"
+
+// InterfaceAddrs defines the interface used for retrieving network addresses.
+type InterfaceAddrs func() ([]net.Addr, error)