Protect against dropped databases

sync3/extensions/todevice.go

@@ -65,6 +65,30 @@ func (r *ToDeviceRequest) ProcessInitial(ctx context.Context, res *Response, ext
 		r.Limit = 100 // default to 100
 	}
 	l := logger.With().Str("user", extCtx.UserID).Str("device", extCtx.DeviceID).Logger()
+
+	mapMu.Lock()
+	lastSentPos, exists := deviceIDToSinceDebugOnly[extCtx.DeviceID]
+	internal.Logf(ctx, "to_device", "since=%v limit=%v last_sent=%v", r.Since, r.Limit, lastSentPos)
+	isFirstRequest := !exists
+	mapMu.Unlock()
+
+	// If this is the first time we've seen this device ID since starting up, ignore the client-provided 'since'
+	// value. This is done to protect against dropped postgres sequences. Consider:
+	//   - 5 to-device messages arrive for Alice
+	//   - Alice requests all messages, gets them and acks them so since=5, and the nextval() sequence is 6.
+	//   - the server admin drops the DB and starts over again. The DB sequence starts back at 1.
+	//   - 2 to-device messages arrive for Alice
+	//   - Alice requests messages from since=5. No messages are returned as the 2 new messages have a lower sequence number.
+	//   - Even worse, those 2 messages are deleted because sending since=5 ACKNOWLEDGES all messages <=5.
+	// By ignoring the first `since` on startup, we effectively force the client into sending since=0. In this scenario,
+	// it will then A) not delete anything as since=0 acknowledges nothing, B) return the 2 to-device events.
+	//
+	// The cost to this is that it is possible to send duplicate to-device events if the server restarts before the client
+	// has time to send the ACK to the server. This isn't fatal as clients do suppress duplicate to-device events.
+	if isFirstRequest {
+		r.Since = ""
+	}
+
 	var from int64
 	var err error
 	if r.Since != "" {
@@ -82,10 +106,7 @@ func (r *ToDeviceRequest) ProcessInitial(ctx context.Context, res *Response, ext
 			internal.GetSentryHubFromContextOrDefault(ctx).CaptureException(err)
 		}
 	}
-	mapMu.Lock()
-	lastSentPos := deviceIDToSinceDebugOnly[extCtx.DeviceID]
-	internal.Logf(ctx, "to_device", "since=%v limit=%v last_sent=%v", r.Since, r.Limit, lastSentPos)
-	mapMu.Unlock()
+
 	if from < lastSentPos {
 		// we told the client about a newer position, but yet they are using an older position, yell loudly
 		l.Warn().Int64("last_sent", lastSentPos).Int64("recv", from).Bool("initial", extCtx.IsInitial).Msg(

tests-integration/extensions_test.go

@@ -2,6 +2,8 @@ package syncv3
 
 import (
 	"encoding/json"
+	"fmt"
+	"strconv"
 	"testing"
 	"time"
 
@@ -443,6 +445,57 @@ func TestExtensionToDevice(t *testing.T) {
 	m.MatchResponse(t, res, m.MatchList("a", m.MatchV3Count(0)), m.MatchToDeviceMessages(newToDeviceMsgs))
 }
 
+// Test that if you sync with a very very high numbered since value, we return lower numbered entries.
+// This guards against dropped databases.
+func TestExtensionToDeviceSequence(t *testing.T) {
+	pqString := testutils.PrepareDBConnectionString()
+	// setup code
+	v2 := runTestV2Server(t)
+	v3 := runTestServer(t, v2, pqString)
+	defer v2.close()
+	defer v3.close()
+	alice := "@TestExtensionToDeviceSequence_alice:localhost"
+	aliceToken := "ALICE_BEARER_TOKEN_TestExtensionToDeviceSequence"
+	v2.addAccount(t, alice, aliceToken)
+	toDeviceMsgs := []json.RawMessage{
+		json.RawMessage(`{"sender":"alice","type":"something","content":{"foo":"1"}}`),
+		json.RawMessage(`{"sender":"alice","type":"something","content":{"foo":"2"}}`),
+		json.RawMessage(`{"sender":"alice","type":"something","content":{"foo":"3"}}`),
+		json.RawMessage(`{"sender":"alice","type":"something","content":{"foo":"4"}}`),
+	}
+	v2.queueResponse(alice, sync2.SyncResponse{
+		ToDevice: sync2.EventsResponse{
+			Events: toDeviceMsgs,
+		},
+	})
+
+	hiSince := 999999
+	res := v3.mustDoV3Request(t, aliceToken, sync3.Request{
+		Lists: map[string]sync3.RequestList{"a": {
+			Ranges: sync3.SliceRanges{
+				[2]int64{0, 10}, // doesn't matter
+			},
+		}},
+		Extensions: extensions.Request{
+			ToDevice: &extensions.ToDeviceRequest{
+				Core:  extensions.Core{Enabled: &boolTrue},
+				Since: fmt.Sprintf("%d", hiSince),
+			},
+		},
+	})
+	m.MatchResponse(t, res, m.MatchList("a", m.MatchV3Count(0)), m.MatchToDeviceMessages(toDeviceMsgs), func(res *sync3.Response) error {
+		// ensure that we return a lower numbered since token
+		got, err := strconv.Atoi(res.Extensions.ToDevice.NextBatch)
+		if err != nil {
+			return err
+		}
+		if got >= hiSince {
+			return fmt.Errorf("next_batch got %v wanted lower than %v", got, hiSince)
+		}
+		return nil
+	})
+}
+
 // tests that the account data extension works:
 // 1- check global account data is sent on first connection
 // 2- check global account data updates are proxied through