feat(jsonrpc2): add manual receive mode to prevent message loss

In the default 'on-start' receive mode, signal-cli's daemon
auto-pulls inbound messages and pushes them to its JSON-RPC clients
the moment they arrive from the Signal servers. If no websocket
subscriber is currently attached to /v1/receive/{number} — for
example during a brief subscriber redeploy — the receiver loop in
signal-cli-rest-api drops the message via a non-blocking channel
send (see the 'no receiver' debug log in ReceiveData). signal-cli
has already acknowledged the message to Signal, so it is never
redelivered.

Add support for signal-cli's --receive-mode=manual, opt-in via the
new JSON_RPC_RECEIVE_MODE=manual environment variable. In manual
mode signal-cli does NOT auto-receive; signal-cli-rest-api now
issues subscribeReceive when a websocket subscriber attaches and
unsubscribeReceive when the last subscriber detaches. While no
subscriber is attached, signal-cli does not pull messages from the
Signal servers, so they remain buffered server-side under Signal's
normal retention rules and are delivered to the next subscriber
that attaches.

Implementation:

* jsonrpc2-helper.go: thread JSON_RPC_RECEIVE_MODE through to the
  daemon launch command line.
* jsonrpc2.go: add per-account subscription tracking with
  refcounting; subscribeReceive/unsubscribeReceive helpers; an
  unwrap step in ReceiveData so manual-mode notifications
  ({subscription,result}) and auto-mode notifications (envelope
  directly) reach downstream consumers in the same shape.
* client.go, api.go: thread the account number through
  GetReceiveChannel so the JsonRpc2Client can attach the right
  subscription.

The change is fully backward compatible: when JSON_RPC_RECEIVE_MODE
is unset or set to 'on-start', signal-cli runs in auto mode and the
new subscribeReceive code path is bypassed (account is empty, no
RPC issued).

Closes #255 (the same root cause: no subscriber → 'no receiver'
drop).

src/api/api.go

@@ -572,7 +572,7 @@ func (a *Api) SendV2(c *gin.Context) {
 }
 
 func (a *Api) handleSignalReceive(ws *websocket.Conn, number string, stop chan struct{}) {
-	receiveChannel, channelUuid, err := a.signalClient.GetReceiveChannel()
+	receiveChannel, channelUuid, err := a.signalClient.GetReceiveChannel(number)
 	if err != nil {
 		log.Error("Couldn't get receive channel: ", err.Error())
 		return

src/client/client.go

@@ -1059,12 +1059,12 @@ func (s *SignalClient) Receive(number string, timeout int64, ignoreAttachments b
 	}
 }
 
-func (s *SignalClient) GetReceiveChannel() (chan JsonRpc2ReceivedMessage, string, error) {
+func (s *SignalClient) GetReceiveChannel(number string) (chan JsonRpc2ReceivedMessage, string, error) {
 	jsonRpc2Client, err := s.getJsonRpc2Client()
 	if err != nil {
 		return nil, "", err
 	}
-	return jsonRpc2Client.GetReceiveChannel()
+	return jsonRpc2Client.GetReceiveChannel(number)
 }
 
 func (s *SignalClient) RemoveReceiveChannel(channelUuid string) {

src/client/jsonrpc2.go

@@ -5,6 +5,7 @@ import (
 	"bytes"
 	"encoding/json"
 	"errors"
+	"fmt"
 	"net"
 	"net/http"
 	"strconv"
@@ -56,15 +57,26 @@ func (r *RateLimitErrorType) Error() string {
 	return r.Err.Error()
 }
 
+// receiveSubscription tracks the state of a manual-mode signal-cli
+// subscribeReceive call: the subscription id assigned by signal-cli and
+// a refcount of websocket subscribers attached to that account.
+type receiveSubscription struct {
+	id       int64
+	refcount int
+}
+
 type JsonRpc2Client struct {
-	conn                     net.Conn
-	receivedResponsesById    map[string]chan JsonRpc2MessageResponse
-	receivedMessagesChannels map[string]chan JsonRpc2ReceivedMessage
-	signalCliApiConfig       *utils.SignalCliApiConfig
-	number                   string
-	receivedMessagesMutex    sync.Mutex
-	receivedResponsesMutex   sync.Mutex
-	address                  string
+	conn                       net.Conn
+	receivedResponsesById      map[string]chan JsonRpc2MessageResponse
+	receivedMessagesChannels   map[string]chan JsonRpc2ReceivedMessage
+	receiveSubscriptions       map[string]*receiveSubscription // account -> sub state
+	channelAccountByUuid       map[string]string               // channelUuid -> account
+	signalCliApiConfig         *utils.SignalCliApiConfig
+	number                     string
+	receivedMessagesMutex      sync.Mutex
+	receivedResponsesMutex     sync.Mutex
+	receiveSubscriptionsMutex  sync.Mutex
+	address                    string
 }
 
 func NewJsonRpc2Client(signalCliApiConfig *utils.SignalCliApiConfig, number string) *JsonRpc2Client {
@@ -73,6 +85,8 @@ func NewJsonRpc2Client(signalCliApiConfig *utils.SignalCliApiConfig, number stri
 		number:                   number,
 		receivedResponsesById:    make(map[string]chan JsonRpc2MessageResponse),
 		receivedMessagesChannels: make(map[string]chan JsonRpc2ReceivedMessage),
+		receiveSubscriptions:     make(map[string]*receiveSubscription),
+		channelAccountByUuid:     make(map[string]string),
 	}
 }
 
@@ -236,6 +250,19 @@ func (r *JsonRpc2Client) ReceiveData(number string, receiveWebhookUrl string) {
 		var resp1 JsonRpc2ReceivedMessage
 		json.Unmarshal([]byte(str), &resp1)
 		if resp1.Method == "receive" {
+			// In manual receive-mode signal-cli wraps the envelope in
+			// {"subscription":N,"result":{...}}; in auto mode it sends
+			// the envelope directly. Unwrap so the broadcast format is
+			// the same in both modes and downstream consumers (e.g. the
+			// websocket handler) don't have to know which mode is in use.
+			var manualWrapper struct {
+				Subscription int64           `json:"subscription"`
+				Result       json.RawMessage `json:"result"`
+			}
+			if err := json.Unmarshal(resp1.Params, &manualWrapper); err == nil && len(manualWrapper.Result) > 0 {
+				resp1.Params = manualWrapper.Result
+			}
+
 			r.receivedMessagesMutex.Lock()
 			for _, c := range r.receivedMessagesChannels {
 				select {
@@ -244,7 +271,6 @@ func (r *JsonRpc2Client) ReceiveData(number string, receiveWebhookUrl string) {
 				default:
 					log.Debug("Couldn't send message to golang channel, as there's no receiver")
 				}
-				continue
 			}
 			r.receivedMessagesMutex.Unlock()
 
@@ -270,16 +296,102 @@ func (r *JsonRpc2Client) ReceiveData(number string, receiveWebhookUrl string) {
 	}
 }
 
-func (r *JsonRpc2Client) GetReceiveChannel() (chan JsonRpc2ReceivedMessage, string, error) {
-	c := make(chan JsonRpc2ReceivedMessage)
+// subscribeReceive starts receiving messages for an account by calling
+// signal-cli's subscribeReceive JSON-RPC method. Only relevant when
+// signal-cli was launched with --receive-mode=manual; in auto mode the
+// daemon pushes notifications without an explicit subscribe call.
+// Returns the subscription id assigned by signal-cli.
+func (r *JsonRpc2Client) subscribeReceive(account string) (int64, error) {
+	type subscribeReceiveArgs struct{}
+	resultStr, err := r.getRaw("subscribeReceive", &account, subscribeReceiveArgs{})
+	if err != nil {
+		return 0, err
+	}
+	var subscriptionId int64
+	if err := json.Unmarshal([]byte(resultStr), &subscriptionId); err != nil {
+		return 0, fmt.Errorf("subscribeReceive: couldn't parse subscription id from %q: %w", resultStr, err)
+	}
+	return subscriptionId, nil
+}
+
+// unsubscribeReceive cancels a manual-mode subscription previously
+// returned by subscribeReceive.
+func (r *JsonRpc2Client) unsubscribeReceive(account string, subscriptionId int64) error {
+	type unsubscribeReceiveArgs struct {
+		Subscription int64 `json:"subscription"`
+	}
+	_, err := r.getRaw("unsubscribeReceive", &account, unsubscribeReceiveArgs{Subscription: subscriptionId})
+	return err
+}
+
+// acquireReceiveSubscription ensures an active manual-mode subscription
+// exists for the given account, refcounting concurrent websocket
+// subscribers. The first caller for an account triggers a real
+// subscribeReceive RPC; subsequent callers just bump the refcount.
+func (r *JsonRpc2Client) acquireReceiveSubscription(account string) error {
+	r.receiveSubscriptionsMutex.Lock()
+	defer r.receiveSubscriptionsMutex.Unlock()
+
+	if sub, ok := r.receiveSubscriptions[account]; ok {
+		sub.refcount++
+		return nil
+	}
+	id, err := r.subscribeReceive(account)
+	if err != nil {
+		return err
+	}
+	r.receiveSubscriptions[account] = &receiveSubscription{id: id, refcount: 1}
+	log.Infof("Subscribed to receive notifications for account %s (subscription=%d)", account, id)
+	return nil
+}
+
+// releaseReceiveSubscription decrements the per-account refcount and
+// cancels the subscription with signal-cli if it drops to zero.
+func (r *JsonRpc2Client) releaseReceiveSubscription(account string) {
+	r.receiveSubscriptionsMutex.Lock()
+	defer r.receiveSubscriptionsMutex.Unlock()
+
+	sub, ok := r.receiveSubscriptions[account]
+	if !ok {
+		return
+	}
+	sub.refcount--
+	if sub.refcount > 0 {
+		return
+	}
+	if err := r.unsubscribeReceive(account, sub.id); err != nil {
+		log.Warnf("unsubscribeReceive failed for account %s (subscription=%d): %s", account, sub.id, err.Error())
+	} else {
+		log.Infof("Unsubscribed from receive notifications for account %s (subscription=%d)", account, sub.id)
+	}
+	delete(r.receiveSubscriptions, account)
+}
+
+// GetReceiveChannel returns a channel that will receive messages for the
+// given account. If signal-cli is in manual receive-mode, it also acquires
+// a subscription so that signal-cli starts forwarding messages for the
+// account; in auto mode, account is unused (notifications flow regardless).
+//
+// account may be empty when the caller does not need a subscription
+// (e.g. legacy callers in auto mode); in that case no subscribeReceive
+// RPC is issued.
+func (r *JsonRpc2Client) GetReceiveChannel(account string) (chan JsonRpc2ReceivedMessage, string, error) {
+	c := make(chan JsonRpc2ReceivedMessage, 64)
 
 	channelUuid, err := uuid.NewV4()
 	if err != nil {
 		return c, "", err
 	}
 
+	if account != "" {
+		if err := r.acquireReceiveSubscription(account); err != nil {
+			return c, "", fmt.Errorf("subscribeReceive failed for account %s: %w", account, err)
+		}
+	}
+
 	r.receivedMessagesMutex.Lock()
 	r.receivedMessagesChannels[channelUuid.String()] = c
+	r.channelAccountByUuid[channelUuid.String()] = account
 	r.receivedMessagesMutex.Unlock()
 
 	return c, channelUuid.String(), nil
@@ -288,5 +400,11 @@ func (r *JsonRpc2Client) GetReceiveChannel() (chan JsonRpc2ReceivedMessage, stri
 func (r *JsonRpc2Client) RemoveReceiveChannel(channelUuid string) {
 	r.receivedMessagesMutex.Lock()
 	delete(r.receivedMessagesChannels, channelUuid)
+	account := r.channelAccountByUuid[channelUuid]
+	delete(r.channelAccountByUuid, channelUuid)
 	r.receivedMessagesMutex.Unlock()
+
+	if account != "" {
+		r.releaseReceiveSubscription(account)
+	}
 }

src/scripts/jsonrpc2-helper.go

@@ -13,7 +13,7 @@ import (
 const supervisorctlConfigTemplate = `
 [program:%s]
 process_name=%s
-command=%s --output=json --config %s%s daemon %s%s%s%s --tcp 127.0.0.1:%d
+command=%s --output=json --config %s%s daemon%s %s%s%s%s --tcp 127.0.0.1:%d
 autostart=true
 autorestart=true
 startretries=10
@@ -75,6 +75,22 @@ func main() {
 		signalCliIgnoreStickers = " --ignore-stickers"
 	}
 
+	// Receive mode: by default signal-cli auto-receives messages on the
+	// daemon and pushes them as JSON-RPC notifications, regardless of
+	// whether any websocket subscriber is connected. In a deploy where
+	// the subscribing client briefly disconnects, those notifications
+	// have nowhere to go and are silently dropped. With manual receive
+	// mode, signal-cli only fetches messages when explicitly requested
+	// via subscribeReceive — see jsonrpc2.go — so messages stay buffered
+	// on the Signal servers until a subscriber re-attaches.
+	signalCliReceiveMode := ""
+	receiveMode := utils.GetEnv("JSON_RPC_RECEIVE_MODE", "")
+	if receiveMode == "manual" {
+		signalCliReceiveMode = " --receive-mode=manual"
+	} else if receiveMode != "" && receiveMode != "on-start" {
+		log.Fatal("Invalid JSON_RPC_RECEIVE_MODE environment variable set! Must be 'manual' or 'on-start'.")
+	}
+
 	supervisorctlProgramName := "signal-cli-json-rpc-1"
 	supervisorctlLogFolder := "/var/log/" + supervisorctlProgramName
 	_, err := exec.Command("mkdir", "-p", supervisorctlLogFolder).Output()
@@ -100,7 +116,7 @@ func main() {
 	supervisorctlConfigFilename := "/etc/supervisor/conf.d/" + "signal-cli-json-rpc-1.conf"
 
 	supervisorctlConfig := fmt.Sprintf(supervisorctlConfigTemplate, supervisorctlProgramName, supervisorctlProgramName, signalCliBinary,
-		signalCliConfigDir, trustNewIdentities, signalCliIgnoreAttachments, signalCliIgnoreStories,
+		signalCliConfigDir, trustNewIdentities, signalCliReceiveMode, signalCliIgnoreAttachments, signalCliIgnoreStories,
 		signalCliIgnoreAvatars, signalCliIgnoreStickers, tcpPort,
 		supervisorctlProgramName, supervisorctlProgramName)