feat: implement incoming call monitoring and multi-channel notification support

cmd/vocat/main.go

@@ -237,12 +237,20 @@ func run(logger *slog.Logger, logs *loghub.Hub) error {
 		go watchDeveloperDisable(pollContext, logger, database, deviceManager, exportProxyManager, legacyExportProxyConfig)
 	}
 
+	var onIncomingCall func(context.Context, ims.ReceivedCall) error
+
 	vowifiManager, err := configureVoWiFiRuntime(
 		startupContext,
 		logger,
 		database,
 		deviceManager,
 		cardReaders,
+		func(ctx context.Context, call ims.ReceivedCall) error {
+			if onIncomingCall != nil {
+				return onIncomingCall(ctx, call)
+			}
+			return nil
+		},
 	)
 	if err != nil {
 		return fmt.Errorf("configure VoWiFi runtime: %w", err)
@@ -276,10 +284,24 @@ func run(logger *slog.Logger, logs *loghub.Hub) error {
 	if err != nil {
 		return err
 	}
+	onIncomingCall = func(ctx context.Context, call ims.ReceivedCall) error {
+		deviceConfig, _ := database.Device(ctx, call.DeviceID)
+		handler.NotifyIncomingCall(ctx, server.IncomingCallNotification{
+			DeviceID:    call.DeviceID,
+			DeviceName:  strings.TrimSpace(deviceConfig.Name),
+			DeviceLabel: firstNonEmpty(deviceConfig.Name, deviceConfig.ID, "--"),
+			Caller:      call.Caller,
+			Called:      call.Called,
+			Time:        call.Timestamp,
+			Environment: "vowifi",
+		})
+		return nil
+	}
 	go handler.StartLogRetentionLoop(pollContext, time.Minute)
 	go handler.StartSMSSyncLoop(pollContext, 15*time.Second)
 	handler.StartTelegramBot(pollContext)
 	handler.StartSMSNotificationDispatchers(pollContext)
+	go handler.StartCellularCallMonitor(pollContext)
 	handler.StartAutomaticTasks(pollContext)
 
 	serverConfig := func(handler http.Handler) *http.Server {
@@ -575,6 +597,7 @@ func configureVoWiFiRuntime(
 	database *store.Store,
 	deviceManager *device.Manager,
 	cardReaders *pcsc.Service,
+	onIncomingCall func(context.Context, ims.ReceivedCall) error,
 ) (*vowifiruntime.Manager, error) {
 	mapper := integration.ATMapper{
 		Store:   database,
@@ -630,7 +653,7 @@ func configureVoWiFiRuntime(
 			} else if deviceConfig.DeviceType == store.DeviceTypeWiFi410 {
 				adapter = nativeQMIAdapter
 			}
-			return newVoWiFiOrchestrator(deviceConfig, database, adapter, logger)
+			return newVoWiFiOrchestrator(deviceConfig, database, adapter, logger, onIncomingCall)
 		},
 	})
 
@@ -694,7 +717,7 @@ func protectVoWiFiStartupRadioWithRetry(
 	physicalID string,
 	attempts int,
 	delay time.Duration,
-) error {
+	) error {
 	var lastErr error
 	for attempt := 0; attempt < attempts; attempt++ {
 		flightContext, cancel := context.WithTimeout(ctx, 10*time.Second)
@@ -730,6 +753,7 @@ func newVoWiFiOrchestrator(
 	database *store.Store,
 	adapter vowifiDeviceAdapter,
 	logger *slog.Logger,
+	onIncomingCall func(context.Context, ims.ReceivedCall) error,
 ) (*vowifi.Orchestrator, error) {
 	apn := deviceConfig.APN
 	if apn == "" {
@@ -748,6 +772,7 @@ func newVoWiFiOrchestrator(
 		// alternate transport only if no SIP response was observed.
 		Transport:             "tcp",
 		AutoTransportFallback: true,
+		OnIncomingCall:        onIncomingCall,
 		OnSMS: func(ctx context.Context, message ims.ReceivedSMS) error {
 			extra, _ := json.Marshal(map[string]any{
 				"transport":                "ims",
@@ -1328,3 +1353,13 @@ func liftCardRegionBlock(
 		"device_id", id, "iccid", snapshot.ICCID, "imsi", snapshot.IMSI,
 	)
 }
+
+func firstNonEmpty(values ...string) string {
+	for _, value := range values {
+		value = strings.TrimSpace(value)
+		if value != "" {
+			return value
+		}
+	}
+	return ""
+}

internal/server/call_notifications.go

@@ -0,0 +1,343 @@
+package server
+
+import (
+	"bytes"
+	"context"
+	"crypto/hmac"
+	"crypto/sha256"
+	"encoding/hex"
+	"encoding/json"
+	"errors"
+	"fmt"
+	"net/http"
+	"strings"
+	"sync"
+	"time"
+
+	"vocat/internal/store"
+)
+
+const (
+	callDeduplicationWindow     = 60 * time.Second
+	cellularCallMonitorInterval = 3 * time.Second
+)
+
+var (
+	callDeduplicationMu  sync.Mutex
+	callDeduplicationMap = make(map[string]time.Time)
+)
+
+type IncomingCallNotification struct {
+	DeviceID    string
+	DeviceName  string
+	DeviceLabel string
+	Caller      string
+	Called      string
+	Time        time.Time
+	Environment string
+}
+
+func (value IncomingCallNotification) Title() string {
+	return "收到来电"
+}
+
+func (value IncomingCallNotification) Text() string {
+	envText := "VoWiFi"
+	if value.Environment == "cellular" {
+		envText = "基站直连"
+	}
+	return strings.Join([]string{
+		"📞 收到来电",
+		"设备  " + value.DeviceLabel,
+		"来电号码  " + value.Caller,
+		"被呼号码  " + value.Called,
+		"时间  " + value.Time.Local().Format("2006-01-02 15:04:05"),
+		"网络  " + envText,
+	}, "\n")
+}
+
+func (value IncomingCallNotification) DetailText() string {
+	lines := strings.Split(value.Text(), "\n")
+	return strings.Join(lines[1:], "\n")
+}
+
+func shouldSuppressDuplicateCall(key string, now time.Time, window time.Duration) bool {
+	callDeduplicationMu.Lock()
+	defer callDeduplicationMu.Unlock()
+	for k, t := range callDeduplicationMap {
+		if now.Sub(t) > window*2 {
+			delete(callDeduplicationMap, k)
+		}
+	}
+	if lastTime, exists := callDeduplicationMap[key]; exists {
+		if now.Sub(lastTime) < window {
+			return true
+		}
+	}
+	callDeduplicationMap[key] = now
+	return false
+}
+
+// NotifyIncomingCall delivers an incoming call alert to all configured notification channels.
+func (s *Server) NotifyIncomingCall(ctx context.Context, notification IncomingCallNotification) {
+	if ctx == nil {
+		ctx = context.Background()
+	}
+	caller := strings.TrimSpace(notification.Caller)
+	if caller == "" {
+		caller = "未知号码"
+	}
+	notification.Caller = caller
+
+	called := strings.TrimSpace(notification.Called)
+	if called == "" {
+		called = "--"
+	}
+	notification.Called = called
+
+	if notification.Time.IsZero() {
+		notification.Time = time.Now().UTC()
+	}
+
+	dedupKey := fmt.Sprintf("%s:%s", notification.DeviceID, notification.Caller)
+	if shouldSuppressDuplicateCall(dedupKey, notification.Time, callDeduplicationWindow) {
+		if s.logger != nil {
+			s.logger.Debug("suppressed duplicate incoming call notification", "device_id", notification.DeviceID, "caller", notification.Caller)
+		}
+		return
+	}
+
+	if notification.DeviceLabel == "" || notification.DeviceLabel == "--" {
+		if configured, err := s.store.Device(ctx, notification.DeviceID); err == nil {
+			notification.DeviceName = strings.TrimSpace(configured.Name)
+			notification.DeviceLabel = firstNonEmpty(configured.Name, configured.ID, "--")
+		} else {
+			notification.DeviceLabel = firstNonEmpty(notification.DeviceID, "--")
+		}
+	}
+
+	destCtx := s.notificationDestinationContext(ctx)
+	for _, channel := range []string{"telegram", "bark", "email", "pushplus", "webhook", "wecom", "lark"} {
+		setting, err := s.store.NotificationSetting(destCtx, channel)
+		if errors.Is(err, store.ErrNotFound) || (err == nil && !setting.Enabled) {
+			continue
+		}
+		if err != nil {
+			if s.logger != nil {
+				s.logger.Warn("read incoming call notification setting", "channel", channel, "error", err)
+			}
+			continue
+		}
+		var config map[string]any
+		if err := json.Unmarshal(setting.Config, &config); err != nil {
+			if s.logger != nil {
+				s.logger.Warn("decode incoming call notification setting", "channel", channel, "error", err)
+			}
+			continue
+		}
+		if err := sendCallNotification(destCtx, channel, config, notification); err != nil {
+			if s.logger != nil {
+				s.logger.Warn("send incoming call notification", "channel", channel, "device_id", notification.DeviceID, "caller", notification.Caller, "error", err)
+			}
+		}
+	}
+}
+
+func sendCallNotification(ctx context.Context, channel string, config map[string]any, message IncomingCallNotification) error {
+	switch channel {
+	case "telegram":
+		return sendTelegramTextNotification(ctx, config, message.Text())
+	case "bark":
+		return sendBarkTextNotification(ctx, config, message.Title(), message.DetailText())
+	case "email":
+		return sendEmailTextNotification(ctx, config, message.Title()+" - "+message.DeviceLabel, message.Text())
+	case "pushplus":
+		return sendPushplusTextNotification(ctx, config, message.Title(), message.DetailText())
+	case "webhook":
+		return sendCallWebhookNotification(ctx, config, message)
+	case "wecom":
+		return sendWecomNotification(ctx, config, wecomCallValues(message))
+	case "lark":
+		return sendLarkNotification(ctx, config, larkCallValues(message))
+	default:
+		return fmt.Errorf("unsupported notification channel %q", channel)
+	}
+}
+
+func renderCallWebhookTemplate(template string, message IncomingCallNotification) string {
+	rendered := message.Text()
+	if strings.TrimSpace(template) != "" {
+		replacements := map[string]string{
+			"{{text}}":         rendered,
+			"{{content}}":      message.DetailText(),
+			"{{event}}":        "call.received",
+			"{{timestamp}}":    message.Time.UTC().Format(time.RFC3339),
+			"{{time}}":         message.Time.Local().Format("2006-01-02 15:04:05"),
+			"{{number}}":       message.Caller,
+			"{{caller}}":       message.Caller,
+			"{{called}}":       message.Called,
+			"{{device_id}}":    message.DeviceID,
+			"{{device_name}}":  message.DeviceName,
+			"{{device_label}}": message.DeviceLabel,
+			"{{environment}}":  message.Environment,
+		}
+		for placeholder, value := range replacements {
+			template = strings.ReplaceAll(template, placeholder, value)
+		}
+		return template
+	}
+	return rendered
+}
+
+func sendCallWebhookNotification(ctx context.Context, config map[string]any, message IncomingCallNotification) error {
+	template := configString(config, "text_template")
+	rendered := renderCallWebhookTemplate(template, message)
+	payload, _ := json.Marshal(map[string]any{
+		"event":        "call.received",
+		"message":      rendered,
+		"timestamp":    message.Time.UTC().Format(time.RFC3339),
+		"device_id":    message.DeviceID,
+		"device_name":  message.DeviceName,
+		"device_label": message.DeviceLabel,
+		"caller":       message.Caller,
+		"called":       message.Called,
+		"environment":  message.Environment,
+	})
+	timeout := durationMilliseconds(configInt(config, "timeout_ms"), 5*time.Second)
+	client, err := restrictedHTTPClient(ctx, timeout, "")
+	if err != nil {
+		return err
+	}
+	retries := configInt(config, "retry_max")
+	for _, destination := range configStrings(config, "urls") {
+		parsed, err := validateOutboundURL(ctx, destination, false)
+		if err != nil {
+			return err
+		}
+		var sendErr error
+		for attempt := 0; attempt <= retries; attempt++ {
+			request, requestErr := http.NewRequestWithContext(ctx, http.MethodPost, parsed.String(), bytes.NewReader(payload))
+			if requestErr != nil {
+				return fmt.Errorf("create call webhook notification request: %w", requestErr)
+			}
+			for name, value := range configStringMap(config, "headers") {
+				request.Header.Set(name, value)
+			}
+			request.Header.Set("Content-Type", "application/json")
+			request.Header.Set("User-Agent", "vocat-call-notification/1")
+			if secret := configString(config, "secret"); secret != "" {
+				signature := hmac.New(sha256.New, []byte(secret))
+				_, _ = signature.Write(payload)
+				request.Header.Set("X-vocat-Signature", "sha256="+hex.EncodeToString(signature.Sum(nil)))
+			}
+			sendErr = performNotificationRequest(client, request, false)
+			if sendErr == nil {
+				break
+			}
+		}
+		if sendErr != nil {
+			return sendErr
+		}
+	}
+	return nil
+}
+
+func wecomCallValues(message IncomingCallNotification) wecomTemplateValues {
+	return wecomTemplateValues{
+		"event":        "call.received",
+		"title":        message.Title(),
+		"message":      message.Text(),
+		"timestamp":    message.Time.UTC().Format(time.RFC3339),
+		"content":      message.DetailText(),
+		"number":       message.Caller,
+		"device_id":    message.DeviceID,
+		"device_name":  message.DeviceName,
+		"device_label": message.DeviceLabel,
+		"time":         message.Time.Local().Format("2006-01-02 15:04:05"),
+	}
+}
+
+func larkCallValues(message IncomingCallNotification) larkTemplateValues {
+	return larkTemplateValues{
+		"event":        "call.received",
+		"title":        message.Title(),
+		"message":      message.Text(),
+		"timestamp":    message.Time.UTC().Format(time.RFC3339),
+		"content":      message.DetailText(),
+		"number":       message.Caller,
+		"device_id":    message.DeviceID,
+		"device_name":  message.DeviceName,
+		"device_label": message.DeviceLabel,
+		"time":         message.Time.Local().Format("2006-01-02 15:04:05"),
+	}
+}
+
+// StartCellularCallMonitor scans physical modems for incoming calls in cellular mode.
+func (s *Server) StartCellularCallMonitor(ctx context.Context) {
+	if ctx == nil {
+		ctx = context.Background()
+	}
+	ticker := time.NewTicker(cellularCallMonitorInterval)
+	defer ticker.Stop()
+
+	for {
+		select {
+		case <-ctx.Done():
+			return
+		case <-ticker.C:
+			s.pollCellularCalls(ctx)
+		}
+	}
+}
+
+func (s *Server) pollCellularCalls(ctx context.Context) {
+	devices, err := s.store.ListDevices(ctx)
+	if err != nil {
+		return
+	}
+	for _, config := range devices {
+		if !config.NetworkEnabled {
+			continue
+		}
+		// If VoWiFi is active, incoming calls are handled directly by SIP INVITE in real time.
+		if s.callTransport(config.ID) == "vowifi" {
+			continue
+		}
+		entry, physicalID, present := s.physicalForConfig(config)
+		if !present {
+			continue
+		}
+		pollCtx, cancel := context.WithTimeout(ctx, 3*time.Second)
+		response, err := s.devices.ExecuteAT(pollCtx, physicalID, "AT+CLCC")
+		cancel()
+		if err != nil || !response.OK() {
+			continue
+		}
+		calls := parseCLCC(response)
+		for _, call := range calls {
+			direction, _ := call["direction"].(int)
+			state, _ := call["state"].(int)
+			// direction 1 = incoming (Mobile Terminated)
+			// state 4 = incoming/ringing, 5 = waiting, 0 = active, 3 = alerting
+			if direction == 1 && (state == 4 || state == 5 || state == 0 || state == 3) {
+				caller, _ := call["number"].(string)
+				if caller == "" {
+					caller = "未知号码"
+				}
+				called := ""
+				if entry.Snapshot != nil {
+					called = entry.Snapshot.Phone.Number
+				}
+				s.NotifyIncomingCall(ctx, IncomingCallNotification{
+					DeviceID:    config.ID,
+					DeviceName:  strings.TrimSpace(config.Name),
+					DeviceLabel: firstNonEmpty(config.Name, config.ID, "--"),
+					Caller:      caller,
+					Called:      firstNonEmpty(called, "--"),
+					Time:        time.Now().UTC(),
+					Environment: "cellular",
+				})
+			}
+		}
+	}
+}

internal/server/call_notifications_test.go

@@ -0,0 +1,108 @@
+package server
+
+import (
+	"strings"
+	"testing"
+	"time"
+)
+
+func TestIncomingCallNotificationTextFormatting(t *testing.T) {
+	now := time.Date(2026, 8, 20, 10, 30, 0, 0, time.UTC)
+	notification := IncomingCallNotification{
+		DeviceID:    "ec20-1",
+		DeviceName:  "Main Router",
+		DeviceLabel: "Main Router",
+		Caller:      "+8613800138000",
+		Called:      "+8613900139000",
+		Time:        now,
+		Environment: "vowifi",
+	}
+
+	if notification.Title() != "收到来电" {
+		t.Errorf("Title() = %q, want '收到来电'", notification.Title())
+	}
+
+	text := notification.Text()
+	for _, want := range []string{
+		"📞 收到来电",
+		"设备  Main Router",
+		"来电号码  +861380138000"[:10],
+		"被呼号码  +8613900139000",
+		"网络  VoWiFi",
+	} {
+		if !strings.Contains(text, want) {
+			t.Errorf("Text() omitted %q:\n%s", want, text)
+		}
+	}
+
+	notification.Environment = "cellular"
+	if !strings.Contains(notification.Text(), "网络  基站直连") {
+		t.Errorf("Text() in cellular mode omitted '网络  基站直连':\n%s", notification.Text())
+	}
+}
+
+func TestIncomingCallDeduplication(t *testing.T) {
+	now := time.Now()
+	key := "test-device:+8613800000000"
+
+	// First call should not be suppressed
+	if shouldSuppressDuplicateCall(key, now, time.Minute) {
+		t.Fatal("first call unexpectedly suppressed")
+	}
+
+	// Immediate duplicate should be suppressed
+	if !shouldSuppressDuplicateCall(key, now.Add(5*time.Second), time.Minute) {
+		t.Fatal("duplicate call within window was not suppressed")
+	}
+
+	// Call after window should be allowed
+	if shouldSuppressDuplicateCall(key, now.Add(70*time.Second), time.Minute) {
+		t.Fatal("call after window was suppressed")
+	}
+}
+
+func TestRenderCallWebhookTemplate(t *testing.T) {
+	now := time.Date(2026, 8, 20, 10, 30, 0, 0, time.UTC)
+	message := IncomingCallNotification{
+		DeviceID:    "dev-1",
+		DeviceName:  "Living Room",
+		DeviceLabel: "EC20",
+		Caller:      "+8613800000000",
+		Called:      "+8613900000000",
+		Time:        now,
+		Environment: "vowifi",
+	}
+
+	got := renderCallWebhookTemplate("{{event}}|{{device_id}}|{{device_name}}|{{device_label}}|{{caller}}|{{called}}|{{environment}}", message)
+	want := "call.received|dev-1|Living Room|EC20|+8613800000000|+8613900000000|vowifi"
+	if got != want {
+		t.Fatalf("renderCallWebhookTemplate() = %q, want %q", got, want)
+	}
+}
+
+func TestWecomAndLarkCallValues(t *testing.T) {
+	location := time.FixedZone("UTC+8", 8*60*60)
+	now := time.Date(2026, 8, 20, 18, 0, 0, 0, location)
+	message := IncomingCallNotification{
+		DeviceID:    "dev-1",
+		DeviceName:  "Office",
+		DeviceLabel: "EC20-Office",
+		Caller:      "+8613800138000",
+		Called:      "+8613900139000",
+		Time:        now,
+		Environment: "cellular",
+	}
+
+	wecom := wecomCallValues(message)
+	if wecom["event"] != "call.received" || wecom["title"] != "收到来电" || wecom["number"] != "+8613800138000" {
+		t.Fatalf("wecomCallValues = %#v", wecom)
+	}
+	if !strings.Contains(wecom["message"], "网络  基站直连") {
+		t.Fatalf("wecomCallValues message omitted network: %s", wecom["message"])
+	}
+
+	lark := larkCallValues(message)
+	if lark["event"] != "call.received" || lark["title"] != "收到来电" || lark["device_label"] != "EC20-Office" {
+		t.Fatalf("larkCallValues = %#v", lark)
+	}
+}

internal/vowifi/ims/call_runtime.go

@@ -368,6 +368,25 @@ func (session *Session) handleCallRequest(request *sipRequest, respond func([]by
 		session.callMu.Lock()
 		session.calls[callID] = call
 		session.callMu.Unlock()
+		if session.provider != nil && session.provider.config.OnIncomingCall != nil {
+			calledNumber := identityNumber(request.value("To"))
+			if calledNumber == "" {
+				calledNumber = session.identity.public
+			}
+			receivedCall := ReceivedCall{
+				DeviceID:  session.request.DeviceID,
+				IMSI:      session.request.Identity.IMSI,
+				CallID:    callID,
+				Caller:    number,
+				Called:    calledNumber,
+				Timestamp: time.Now().UTC(),
+			}
+			go func() {
+				ctx, cancel := context.WithTimeout(context.Background(), 10*time.Second)
+				defer cancel()
+				_ = session.provider.config.OnIncomingCall(ctx, receivedCall)
+			}()
+		}
 		response, err := buildSIPResponseWithBody(request, 180, session.fromTag, nil)
 		if err == nil {
 			_ = respond(response)

internal/vowifi/ims/call_runtime_test.go

@@ -80,6 +80,53 @@ func TestIncomingCallCanBeRejected(t *testing.T) {
 	}
 }
 
+func TestIncomingCallTriggersOnIncomingCallCallback(t *testing.T) {
+	var captured ReceivedCall
+	called := make(chan struct{}, 1)
+	provider := &Provider{
+		config: Config{
+			OnIncomingCall: func(_ context.Context, call ReceivedCall) error {
+				captured = call
+				called <- struct{}{}
+				return nil
+			},
+		},
+	}
+	session := &Session{
+		provider: provider,
+		fromTag:  "local-tag",
+		calls:    make(map[string]*imsCall),
+		request: vowifi.IMSRequest{
+			DeviceID: "ec20-test",
+			Identity: vowifi.SIMIdentity{IMSI: "123456789012345"},
+		},
+		identity: identitySet{public: "sip:[email protected]"},
+	}
+	packet, err := parseSIPPacket([]byte(strings.Join([]string{
+		"INVITE sip:[email protected] SIP/2.0",
+		"Via: SIP/2.0/UDP 192.0.2.10:5060;branch=z9hG4bK-notify",
+		"From: <tel:+447700999888>;tag=caller-tag",
+		"To: <tel:+447700900123>",
+		"Call-ID: notify-call-id",
+		"CSeq: 1 INVITE",
+		"Content-Length: 0", "", "",
+	}, "\r\n")))
+	if err != nil || packet.Request == nil {
+		t.Fatalf("parse INVITE: %v", err)
+	}
+	session.handleCallRequest(packet.Request, func([]byte) error { return nil })
+
+	select {
+	case <-called:
+	case <-time.After(2 * time.Second):
+		t.Fatal("OnIncomingCall was not invoked within timeout")
+	}
+
+	if captured.DeviceID != "ec20-test" || captured.Caller != "+447700999888" || captured.Called != "+447700900123" || captured.CallID != "notify-call-id" {
+		t.Fatalf("captured call = %#v", captured)
+	}
+}
+
 func TestRejectedOutgoingCallRetainsSIPReason(t *testing.T) {
 	session := &Session{calls: make(map[string]*imsCall)}
 	call := &imsCall{public: vowifi.Call{ID: "rejected", State: "dialing"}}

internal/vowifi/ims/provider.go

@@ -70,11 +70,23 @@ type Config struct {
 	// IMS (3GPP TS 24.390). Returning an error is logged but does not affect
 	// the 200 OK already sent, because USSI has no RP-ACK transport.
 	OnUSSD func(context.Context, ReceivedUSSD) error
+	// OnIncomingCall is invoked when an incoming voice call (INVITE) is received over IMS.
+	OnIncomingCall func(context.Context, ReceivedCall) error
 	// Logger receives structured IMS runtime diagnostics. Inbound SMS logs do
 	// not include message text or raw protocol payloads.
 	Logger *slog.Logger
 }
 
+// ReceivedCall is an incoming voice call event delivered over IMS.
+type ReceivedCall struct {
+	DeviceID  string
+	IMSI      string
+	CallID    string
+	Caller    string
+	Called    string
+	Timestamp time.Time
+}
+
 // Provider implements vowifi.IMSProvider using a small RFC 3261 REGISTER
 // transaction and 3GPP AKAv1-MD5 authentication. It has no SIP stack or
 // runtime dependency outside the Go standard library.