messaging.go

// Copyright 2018 Google Inc. All Rights Reserved.
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//      http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.

// Package messaging contains functions for sending messages and managing
// device subscriptions with Firebase Cloud Messaging (FCM).
package messaging // import "firebase.google.com/go/messaging"

import (
	"context"
	"encoding/json"
	"errors"
	"fmt"
	"net/http"
	"regexp"
	"strconv"
	"strings"
	"time"

	"firebase.google.com/go/internal"
	"google.golang.org/api/transport"
)

const (
	messagingEndpoint = "https://fcm.googleapis.com/v1"
	batchEndpoint     = "https://fcm.googleapis.com/batch"

	firebaseClientHeader   = "X-Firebase-Client"
	apiFormatVersionHeader = "X-GOOG-API-FORMAT-VERSION"
	apiFormatVersion       = "2"

	internalError                  = "internal-error"
	invalidAPNSCredentials         = "invalid-apns-credentials"
	invalidArgument                = "invalid-argument"
	messageRateExceeded            = "message-rate-exceeded"
	mismatchedCredential           = "mismatched-credential"
	registrationTokenNotRegistered = "registration-token-not-registered"
	serverUnavailable              = "server-unavailable"
	tooManyTopics                  = "too-many-topics"
	unknownError                   = "unknown-error"
)

var (
	topicNamePattern = regexp.MustCompile("^(/topics/)?(private/)?[a-zA-Z0-9-_.~%]+$")

	fcmErrorCodes = map[string]struct{ Code, Msg string }{
		// FCM v1 canonical error codes
		"NOT_FOUND": {
			registrationTokenNotRegistered,
			"app instance has been unregistered; code: " + registrationTokenNotRegistered,
		},
		"PERMISSION_DENIED": {
			mismatchedCredential,
			"sender id does not match registration token; code: " + mismatchedCredential,
		},
		"RESOURCE_EXHAUSTED": {
			messageRateExceeded,
			"messaging service quota exceeded; code: " + messageRateExceeded,
		},
		"UNAUTHENTICATED": {
			invalidAPNSCredentials,
			"apns certificate or auth key was invalid; code: " + invalidAPNSCredentials,
		},

		// FCM v1 new error codes
		"APNS_AUTH_ERROR": {
			invalidAPNSCredentials,
			"apns certificate or auth key was invalid; code: " + invalidAPNSCredentials,
		},
		"INTERNAL": {
			internalError,
			"backend servers encountered an unknown internl error; code: " + internalError,
		},
		"INVALID_ARGUMENT": {
			invalidArgument,
			"request contains an invalid argument; code: " + invalidArgument,
		},
		"SENDER_ID_MISMATCH": {
			mismatchedCredential,
			"sender id does not match registration token; code: " + mismatchedCredential,
		},
		"QUOTA_EXCEEDED": {
			messageRateExceeded,
			"messaging service quota exceeded; code: " + messageRateExceeded,
		},
		"UNAVAILABLE": {
			serverUnavailable,
			"backend servers are temporarily unavailable; code: " + serverUnavailable,
		},
		"UNREGISTERED": {
			registrationTokenNotRegistered,
			"app instance has been unregistered; code: " + registrationTokenNotRegistered,
		},
	}
)

// Message to be sent via Firebase Cloud Messaging.
//
// Message contains payload data, recipient information and platform-specific configuration
// options. A Message must specify exactly one of Token, Topic or Condition fields. Apart from
// that a Message may specify any combination of Data, Notification, Android, Webpush and APNS
// fields. See https://firebase.google.com/docs/reference/fcm/rest/v1/projects.messages for more
// details on how the backend FCM servers handle different message parameters.
type Message struct {
	Data         map[string]string `json:"data,omitempty"`
	Notification *Notification     `json:"notification,omitempty"`
	Android      *AndroidConfig    `json:"android,omitempty"`
	Webpush      *WebpushConfig    `json:"webpush,omitempty"`
	APNS         *APNSConfig       `json:"apns,omitempty"`
	FCMOptions   *FCMOptions       `json:"fcm_options,omitempty"`
	Token        string            `json:"token,omitempty"`
	Topic        string            `json:"-"`
	Condition    string            `json:"condition,omitempty"`
}

// MarshalJSON marshals a Message into JSON (for internal use only).
func (m *Message) MarshalJSON() ([]byte, error) {
	// Create a new type to prevent infinite recursion. We use this technique whenever it is needed
	// to customize how a subset of the fields in a struct should be serialized.
	type messageInternal Message
	temp := &struct {
		BareTopic string `json:"topic,omitempty"`
		*messageInternal
	}{
		BareTopic:       strings.TrimPrefix(m.Topic, "/topics/"),
		messageInternal: (*messageInternal)(m),
	}
	return json.Marshal(temp)
}

// UnmarshalJSON unmarshals a JSON string into a Message (for internal use only).
func (m *Message) UnmarshalJSON(b []byte) error {
	type messageInternal Message
	s := struct {
		BareTopic string `json:"topic,omitempty"`
		*messageInternal
	}{
		messageInternal: (*messageInternal)(m),
	}
	if err := json.Unmarshal(b, &s); err != nil {
		return err
	}
	m.Topic = s.BareTopic
	return nil
}

// Notification is the basic notification template to use across all platforms.
type Notification struct {
	Title    string `json:"title,omitempty"`
	Body     string `json:"body,omitempty"`
	ImageURL string `json:"image,omitempty"`
}

// AndroidConfig contains messaging options specific to the Android platform.
type AndroidConfig struct {
	CollapseKey           string               `json:"collapse_key,omitempty"`
	Priority              string               `json:"priority,omitempty"` // one of "normal" or "high"
	TTL                   *time.Duration       `json:"-"`
	RestrictedPackageName string               `json:"restricted_package_name,omitempty"`
	Data                  map[string]string    `json:"data,omitempty"` // if specified, overrides the Data field on Message type
	Notification          *AndroidNotification `json:"notification,omitempty"`
	FCMOptions            *AndroidFCMOptions   `json:"fcm_options,omitempty"`
}

// MarshalJSON marshals an AndroidConfig into JSON (for internal use only).
func (a *AndroidConfig) MarshalJSON() ([]byte, error) {
	var ttl string
	if a.TTL != nil {
		seconds := int64(*a.TTL / time.Second)
		nanos := int64((*a.TTL - time.Duration(seconds)*time.Second) / time.Nanosecond)
		if nanos > 0 {
			ttl = fmt.Sprintf("%d.%09ds", seconds, nanos)
		} else {
			ttl = fmt.Sprintf("%ds", seconds)
		}
	}

	type androidInternal AndroidConfig
	temp := &struct {
		TTL string `json:"ttl,omitempty"`
		*androidInternal
	}{
		TTL:             ttl,
		androidInternal: (*androidInternal)(a),
	}
	return json.Marshal(temp)
}

// UnmarshalJSON unmarshals a JSON string into an AndroidConfig (for internal use only).
func (a *AndroidConfig) UnmarshalJSON(b []byte) error {
	type androidInternal AndroidConfig
	temp := struct {
		TTL string `json:"ttl,omitempty"`
		*androidInternal
	}{
		androidInternal: (*androidInternal)(a),
	}
	if err := json.Unmarshal(b, &temp); err != nil {
		return err
	}
	if temp.TTL != "" {
		segments := strings.Split(strings.TrimSuffix(temp.TTL, "s"), ".")
		if len(segments) != 1 && len(segments) != 2 {
			return fmt.Errorf("incorrect number of segments in ttl: %q", temp.TTL)
		}
		seconds, err := strconv.ParseInt(segments[0], 10, 64)
		if err != nil {
			return err
		}
		ttl := time.Duration(seconds) * time.Second
		if len(segments) == 2 {
			nanos, err := strconv.ParseInt(strings.TrimLeft(segments[1], "0"), 10, 64)
			if err != nil {
				return err
			}
			ttl += time.Duration(nanos) * time.Nanosecond
		}
		a.TTL = &ttl
	}
	return nil
}

// AndroidNotification is a notification to send to Android devices.
type AndroidNotification struct {
	Title        string   `json:"title,omitempty"` // if specified, overrides the Title field of the Notification type
	Body         string   `json:"body,omitempty"`  // if specified, overrides the Body field of the Notification type
	Icon         string   `json:"icon,omitempty"`
	Color        string   `json:"color,omitempty"` // notification color in #RRGGBB format
	Sound        string   `json:"sound,omitempty"`
	Tag          string   `json:"tag,omitempty"`
	ClickAction  string   `json:"click_action,omitempty"`
	BodyLocKey   string   `json:"body_loc_key,omitempty"`
	BodyLocArgs  []string `json:"body_loc_args,omitempty"`
	TitleLocKey  string   `json:"title_loc_key,omitempty"`
	TitleLocArgs []string `json:"title_loc_args,omitempty"`
	ChannelID    string   `json:"channel_id,omitempty"`
	ImageURL     string   `json:"image,omitempty"`
}

// AndroidFCMOptions contains additional options for features provided by the FCM Android SDK.
type AndroidFCMOptions struct {
	AnalyticsLabel string `json:"analytics_label,omitempty"`
}

// WebpushConfig contains messaging options specific to the WebPush protocol.
//
// See https://tools.ietf.org/html/rfc8030#section-5 for additional details, and supported
// headers.
type WebpushConfig struct {
	Headers      map[string]string    `json:"headers,omitempty"`
	Data         map[string]string    `json:"data,omitempty"`
	Notification *WebpushNotification `json:"notification,omitempty"`
	FcmOptions   *WebpushFcmOptions   `json:"fcm_options,omitempty"`
}

// WebpushNotificationAction represents an action that can be performed upon receiving a WebPush notification.
type WebpushNotificationAction struct {
	Action string `json:"action,omitempty"`
	Title  string `json:"title,omitempty"`
	Icon   string `json:"icon,omitempty"`
}

// WebpushNotification is a notification to send via WebPush protocol.
//
// See https://developer.mozilla.org/en-US/docs/Web/API/notification/Notification for additional
// details.
type WebpushNotification struct {
	Actions            []*WebpushNotificationAction `json:"actions,omitempty"`
	Title              string                       `json:"title,omitempty"` // if specified, overrides the Title field of the Notification type
	Body               string                       `json:"body,omitempty"`  // if specified, overrides the Body field of the Notification type
	Icon               string                       `json:"icon,omitempty"`
	Badge              string                       `json:"badge,omitempty"`
	Direction          string                       `json:"dir,omitempty"` // one of 'ltr' or 'rtl'
	Data               interface{}                  `json:"data,omitempty"`
	Image              string                       `json:"image,omitempty"`
	Language           string                       `json:"lang,omitempty"`
	Renotify           bool                         `json:"renotify,omitempty"`
	RequireInteraction bool                         `json:"requireInteraction,omitempty"`
	Silent             bool                         `json:"silent,omitempty"`
	Tag                string                       `json:"tag,omitempty"`
	TimestampMillis    *int64                       `json:"timestamp,omitempty"`
	Vibrate            []int                        `json:"vibrate,omitempty"`
	CustomData         map[string]interface{}
}

// standardFields creates a map containing all the fields except the custom data.
//
// We implement a standardFields function whenever we want to add custom and arbitrary
// fields to an object during its serialization. This helper function also comes in
// handy during validation of the message (to detect duplicate specifications of
// fields), and also during deserialization.
func (n *WebpushNotification) standardFields() map[string]interface{} {
	m := make(map[string]interface{})
	addNonEmpty := func(key, value string) {
		if value != "" {
			m[key] = value
		}
	}
	addTrue := func(key string, value bool) {
		if value {
			m[key] = value
		}
	}
	if len(n.Actions) > 0 {
		m["actions"] = n.Actions
	}
	addNonEmpty("title", n.Title)
	addNonEmpty("body", n.Body)
	addNonEmpty("icon", n.Icon)
	addNonEmpty("badge", n.Badge)
	addNonEmpty("dir", n.Direction)
	addNonEmpty("image", n.Image)
	addNonEmpty("lang", n.Language)
	addTrue("renotify", n.Renotify)
	addTrue("requireInteraction", n.RequireInteraction)
	addTrue("silent", n.Silent)
	addNonEmpty("tag", n.Tag)
	if n.Data != nil {
		m["data"] = n.Data
	}
	if n.TimestampMillis != nil {
		m["timestamp"] = *n.TimestampMillis
	}
	if len(n.Vibrate) > 0 {
		m["vibrate"] = n.Vibrate
	}
	return m
}

// MarshalJSON marshals a WebpushNotification into JSON (for internal use only).
func (n *WebpushNotification) MarshalJSON() ([]byte, error) {
	m := n.standardFields()
	for k, v := range n.CustomData {
		m[k] = v
	}
	return json.Marshal(m)
}

// UnmarshalJSON unmarshals a JSON string into a WebpushNotification (for internal use only).
func (n *WebpushNotification) UnmarshalJSON(b []byte) error {
	type webpushNotificationInternal WebpushNotification
	var temp = (*webpushNotificationInternal)(n)
	if err := json.Unmarshal(b, temp); err != nil {
		return err
	}
	allFields := make(map[string]interface{})
	if err := json.Unmarshal(b, &allFields); err != nil {
		return err
	}
	for k := range n.standardFields() {
		delete(allFields, k)
	}
	if len(allFields) > 0 {
		n.CustomData = allFields
	}
	return nil
}

// WebpushFcmOptions contains additional options for features provided by the FCM web SDK.
type WebpushFcmOptions struct {
	Link string `json:"link,omitempty"`
}

// APNSConfig contains messaging options specific to the Apple Push Notification Service (APNS).
//
// See https://developer.apple.com/library/content/documentation/NetworkingInternet/Conceptual/RemoteNotificationsPG/CommunicatingwithAPNs.html
// for more details on supported headers and payload keys.
type APNSConfig struct {
	Headers    map[string]string `json:"headers,omitempty"`
	Payload    *APNSPayload      `json:"payload,omitempty"`
	FCMOptions *APNSFCMOptions   `json:"fcm_options,omitempty"`
}

// APNSPayload is the payload that can be included in an APNS message.
//
// The payload mainly consists of the aps dictionary. Additionally it may contain arbitrary
// key-values pairs as custom data fields.
//
// See https://developer.apple.com/library/content/documentation/NetworkingInternet/Conceptual/RemoteNotificationsPG/PayloadKeyReference.html
// for a full list of supported payload fields.
type APNSPayload struct {
	Aps        *Aps                   `json:"aps,omitempty"`
	CustomData map[string]interface{} `json:"-"`
}

// standardFields creates a map containing all the fields except the custom data.
func (p *APNSPayload) standardFields() map[string]interface{} {
	return map[string]interface{}{"aps": p.Aps}
}

// MarshalJSON marshals an APNSPayload into JSON (for internal use only).
func (p *APNSPayload) MarshalJSON() ([]byte, error) {
	m := p.standardFields()
	for k, v := range p.CustomData {
		m[k] = v
	}
	return json.Marshal(m)
}

// UnmarshalJSON unmarshals a JSON string into an APNSPayload (for internal use only).
func (p *APNSPayload) UnmarshalJSON(b []byte) error {
	type apnsPayloadInternal APNSPayload
	var temp = (*apnsPayloadInternal)(p)
	if err := json.Unmarshal(b, temp); err != nil {
		return err
	}
	allFields := make(map[string]interface{})
	if err := json.Unmarshal(b, &allFields); err != nil {
		return err
	}
	for k := range p.standardFields() {
		delete(allFields, k)
	}
	if len(allFields) > 0 {
		p.CustomData = allFields
	}
	return nil
}

// Aps represents the aps dictionary that may be included in an APNSPayload.
//
// Alert may be specified as a string (via the AlertString field), or as a struct (via the Alert
// field).
type Aps struct {
	AlertString      string                 `json:"-"`
	Alert            *ApsAlert              `json:"-"`
	Badge            *int                   `json:"badge,omitempty"`
	Sound            string                 `json:"-"`
	CriticalSound    *CriticalSound         `json:"-"`
	ContentAvailable bool                   `json:"-"`
	MutableContent   bool                   `json:"-"`
	Category         string                 `json:"category,omitempty"`
	ThreadID         string                 `json:"thread-id,omitempty"`
	CustomData       map[string]interface{} `json:"-"`
}

// standardFields creates a map containing all the fields except the custom data.
func (a *Aps) standardFields() map[string]interface{} {
	m := make(map[string]interface{})
	if a.Alert != nil {
		m["alert"] = a.Alert
	} else if a.AlertString != "" {
		m["alert"] = a.AlertString
	}
	if a.ContentAvailable {
		m["content-available"] = 1
	}
	if a.MutableContent {
		m["mutable-content"] = 1
	}
	if a.Badge != nil {
		m["badge"] = *a.Badge
	}
	if a.CriticalSound != nil {
		m["sound"] = a.CriticalSound
	} else if a.Sound != "" {
		m["sound"] = a.Sound
	}
	if a.Category != "" {
		m["category"] = a.Category
	}
	if a.ThreadID != "" {
		m["thread-id"] = a.ThreadID
	}
	return m
}

// MarshalJSON marshals an Aps into JSON (for internal use only).
func (a *Aps) MarshalJSON() ([]byte, error) {
	m := a.standardFields()
	for k, v := range a.CustomData {
		m[k] = v
	}
	return json.Marshal(m)
}

// UnmarshalJSON unmarshals a JSON string into an Aps (for internal use only).
func (a *Aps) UnmarshalJSON(b []byte) error {
	type apsInternal Aps
	temp := struct {
		AlertObject         *json.RawMessage `json:"alert,omitempty"`
		SoundObject         *json.RawMessage `json:"sound,omitempty"`
		ContentAvailableInt int              `json:"content-available,omitempty"`
		MutableContentInt   int              `json:"mutable-content,omitempty"`
		*apsInternal
	}{
		apsInternal: (*apsInternal)(a),
	}
	if err := json.Unmarshal(b, &temp); err != nil {
		return err
	}
	a.ContentAvailable = (temp.ContentAvailableInt == 1)
	a.MutableContent = (temp.MutableContentInt == 1)
	if temp.AlertObject != nil {
		if err := json.Unmarshal(*temp.AlertObject, &a.Alert); err != nil {
			a.Alert = nil
			if err := json.Unmarshal(*temp.AlertObject, &a.AlertString); err != nil {
				return fmt.Errorf("failed to unmarshal alert as a struct or a string: %v", err)
			}
		}
	}
	if temp.SoundObject != nil {
		if err := json.Unmarshal(*temp.SoundObject, &a.CriticalSound); err != nil {
			a.CriticalSound = nil
			if err := json.Unmarshal(*temp.SoundObject, &a.Sound); err != nil {
				return fmt.Errorf("failed to unmarshal sound as a struct or a string")
			}
		}
	}

	allFields := make(map[string]interface{})
	if err := json.Unmarshal(b, &allFields); err != nil {
		return err
	}
	for k := range a.standardFields() {
		delete(allFields, k)
	}
	if len(allFields) > 0 {
		a.CustomData = allFields
	}
	return nil
}

// CriticalSound is the sound payload that can be included in an Aps.
type CriticalSound struct {
	Critical bool    `json:"-"`
	Name     string  `json:"name,omitempty"`
	Volume   float64 `json:"volume,omitempty"`
}

// MarshalJSON marshals a CriticalSound into JSON (for internal use only).
func (cs *CriticalSound) MarshalJSON() ([]byte, error) {
	type criticalSoundInternal CriticalSound
	temp := struct {
		CriticalInt int `json:"critical,omitempty"`
		*criticalSoundInternal
	}{
		criticalSoundInternal: (*criticalSoundInternal)(cs),
	}
	if cs.Critical {
		temp.CriticalInt = 1
	}
	return json.Marshal(temp)
}

// UnmarshalJSON unmarshals a JSON string into a CriticalSound (for internal use only).
func (cs *CriticalSound) UnmarshalJSON(b []byte) error {
	type criticalSoundInternal CriticalSound
	temp := struct {
		CriticalInt int `json:"critical,omitempty"`
		*criticalSoundInternal
	}{
		criticalSoundInternal: (*criticalSoundInternal)(cs),
	}
	if err := json.Unmarshal(b, &temp); err != nil {
		return err
	}
	cs.Critical = (temp.CriticalInt == 1)
	return nil
}

// ApsAlert is the alert payload that can be included in an Aps.
//
// See https://developer.apple.com/library/content/documentation/NetworkingInternet/Conceptual/RemoteNotificationsPG/PayloadKeyReference.html
// for supported fields.
type ApsAlert struct {
	Title           string   `json:"title,omitempty"` // if specified, overrides the Title field of the Notification type
	SubTitle        string   `json:"subtitle,omitempty"`
	Body            string   `json:"body,omitempty"` // if specified, overrides the Body field of the Notification type
	LocKey          string   `json:"loc-key,omitempty"`
	LocArgs         []string `json:"loc-args,omitempty"`
	TitleLocKey     string   `json:"title-loc-key,omitempty"`
	TitleLocArgs    []string `json:"title-loc-args,omitempty"`
	SubTitleLocKey  string   `json:"subtitle-loc-key,omitempty"`
	SubTitleLocArgs []string `json:"subtitle-loc-args,omitempty"`
	ActionLocKey    string   `json:"action-loc-key,omitempty"`
	LaunchImage     string   `json:"launch-image,omitempty"`
}

// APNSFCMOptions contains additional options for features provided by the FCM Aps SDK.
type APNSFCMOptions struct {
	AnalyticsLabel string `json:"analytics_label,omitempty"`
	ImageURL       string `json:"image,omitempty"`
}

// FCMOptions contains additional options to use across all platforms.
type FCMOptions struct {
	AnalyticsLabel string `json:"analytics_label,omitempty"`
}

// ErrorInfo is a topic management error.
type ErrorInfo struct {
	Index  int
	Reason string
}

// Client is the interface for the Firebase Cloud Messaging (FCM) service.
type Client struct {
	*fcmClient
	*iidClient
}

// NewClient creates a new instance of the Firebase Cloud Messaging Client.
//
// This function can only be invoked from within the SDK. Client applications should access the
// the messaging service through firebase.App.
func NewClient(ctx context.Context, c *internal.MessagingConfig) (*Client, error) {
	if c.ProjectID == "" {
		return nil, errors.New("project ID is required to access Firebase Cloud Messaging client")
	}

	hc, _, err := transport.NewHTTPClient(ctx, c.Opts...)
	if err != nil {
		return nil, err
	}

	return &Client{
		fcmClient: newFCMClient(hc, c),
		iidClient: newIIDClient(hc),
	}, nil
}

type fcmClient struct {
	fcmEndpoint   string
	batchEndpoint string
	project       string
	version       string
	httpClient    *internal.HTTPClient
}

func newFCMClient(hc *http.Client, conf *internal.MessagingConfig) *fcmClient {
	client := internal.WithDefaultRetryConfig(hc)
	client.CreateErrFn = handleFCMError
	client.SuccessFn = internal.HasSuccessStatus

	version := fmt.Sprintf("fire-admin-go/%s", conf.Version)
	client.Opts = []internal.HTTPOption{
		internal.WithHeader(apiFormatVersionHeader, apiFormatVersion),
		internal.WithHeader(firebaseClientHeader, version),
	}

	return &fcmClient{
		fcmEndpoint:   messagingEndpoint,
		batchEndpoint: batchEndpoint,
		project:       conf.ProjectID,
		version:       version,
		httpClient:    client,
	}
}

// Send sends a Message to Firebase Cloud Messaging.
//
// The Message must specify exactly one of Token, Topic and Condition fields. FCM will
// customize the message for each target platform based on the arguments specified in the
// Message.
func (c *fcmClient) Send(ctx context.Context, message *Message) (string, error) {
	payload := &fcmRequest{
		Message: message,
	}
	return c.makeSendRequest(ctx, payload)
}

// SendDryRun sends a Message to Firebase Cloud Messaging in the dry run (validation only) mode.
//
// This function does not actually deliver the message to target devices. Instead, it performs all
// the SDK-level and backend validations on the message, and emulates the send operation.
func (c *fcmClient) SendDryRun(ctx context.Context, message *Message) (string, error) {
	payload := &fcmRequest{
		ValidateOnly: true,
		Message:      message,
	}
	return c.makeSendRequest(ctx, payload)
}

func (c *fcmClient) makeSendRequest(ctx context.Context, req *fcmRequest) (string, error) {
	if err := validateMessage(req.Message); err != nil {
		return "", err
	}

	request := &internal.Request{
		Method: http.MethodPost,
		URL:    fmt.Sprintf("%s/projects/%s/messages:send", c.fcmEndpoint, c.project),
		Body:   internal.NewJSONEntity(req),
	}

	var result fcmResponse
	_, err := c.httpClient.DoAndUnmarshal(ctx, request, &result)
	return result.Name, err
}

// IsInternal checks if the given error was due to an internal server error.
func IsInternal(err error) bool {
	return internal.HasErrorCode(err, internalError)
}

// IsInvalidAPNSCredentials checks if the given error was due to invalid APNS certificate or auth
// key.
func IsInvalidAPNSCredentials(err error) bool {
	return internal.HasErrorCode(err, invalidAPNSCredentials)
}

// IsInvalidArgument checks if the given error was due to an invalid argument in the request.
func IsInvalidArgument(err error) bool {
	return internal.HasErrorCode(err, invalidArgument)
}

// IsMessageRateExceeded checks if the given error was due to the client exceeding a quota.
func IsMessageRateExceeded(err error) bool {
	return internal.HasErrorCode(err, messageRateExceeded)
}

// IsMismatchedCredential checks if the given error was due to an invalid credential or permission
// error.
func IsMismatchedCredential(err error) bool {
	return internal.HasErrorCode(err, mismatchedCredential)
}

// IsRegistrationTokenNotRegistered checks if the given error was due to a registration token that
// became invalid.
func IsRegistrationTokenNotRegistered(err error) bool {
	return internal.HasErrorCode(err, registrationTokenNotRegistered)
}

// IsServerUnavailable checks if the given error was due to the backend server being temporarily
// unavailable.
func IsServerUnavailable(err error) bool {
	return internal.HasErrorCode(err, serverUnavailable)
}

// IsTooManyTopics checks if the given error was due to the client exceeding the allowed number
// of topics.
func IsTooManyTopics(err error) bool {
	return internal.HasErrorCode(err, tooManyTopics)
}

// IsUnknown checks if the given error was due to unknown error returned by the backend server.
func IsUnknown(err error) bool {
	return internal.HasErrorCode(err, unknownError)
}

type fcmRequest struct {
	ValidateOnly bool     `json:"validate_only,omitempty"`
	Message      *Message `json:"message,omitempty"`
}

type fcmResponse struct {
	Name string `json:"name"`
}

type fcmError struct {
	Error struct {
		Status  string `json:"status"`
		Message string `json:"message"`
		Details []struct {
			Type      string `json:"@type"`
			ErrorCode string `json:"errorCode"`
		}
	} `json:"error"`
}

func handleFCMError(resp *internal.Response) error {
	var fe fcmError
	json.Unmarshal(resp.Body, &fe) // ignore any json parse errors at this level
	var serverCode string
	for _, d := range fe.Error.Details {
		if d.Type == "type.googleapis.com/google.firebase.fcm.v1.FcmError" {
			serverCode = d.ErrorCode
			break
		}
	}
	if serverCode == "" {
		serverCode = fe.Error.Status
	}

	var clientCode, msg string
	info, ok := fcmErrorCodes[serverCode]
	if ok {
		clientCode, msg = info.Code, info.Msg
	} else {
		clientCode = unknownError
		msg = fmt.Sprintf("server responded with an unknown error; response: %s", string(resp.Body))
	}
	if fe.Error.Message != "" {
		msg += "; details: " + fe.Error.Message
	}
	return internal.Errorf(clientCode, "http error status: %d; reason: %s", resp.Status, msg)
}