1 files changed, 36 insertions, 8 deletions

diff --git a/syncapi/sync/request.go b/syncapi/sync/request.go
index 35a15f6f..a5d2f60f 100644
--- a/syncapi/sync/request.go
+++ b/syncapi/sync/request.go
@@ -16,8 +16,10 @@ package sync
 
 import (
 	"context"
+	"errors"
 	"net/http"
 	"strconv"
+	"strings"
 	"time"
 
 	"github.com/matrix-org/dendrite/clientapi/auth/authtypes"
@@ -36,7 +38,7 @@ type syncRequest struct {
 	device        authtypes.Device
 	limit         int
 	timeout       time.Duration
-	since         *types.StreamPosition // nil means that no since token was supplied
+	since         *types.SyncPosition // nil means that no since token was supplied
 	wantFullState bool
 	log           *log.Entry
 }
@@ -73,15 +75,41 @@ func getTimeout(timeoutMS string) time.Duration {
 }
 
 // getSyncStreamPosition tries to parse a 'since' token taken from the API to a
-// stream position. If the string is empty then (nil, nil) is returned.
-func getSyncStreamPosition(since string) (*types.StreamPosition, error) {
+// types.SyncPosition. If the string is empty then (nil, nil) is returned.
+// There are two forms of tokens: The full length form containing all PDU and EDU
+// positions separated by "_", and the short form containing only the PDU
+// position. Short form can be used for, e.g., `prev_batch` tokens.
+func getSyncStreamPosition(since string) (*types.SyncPosition, error) {
 	if since == "" {
 		return nil, nil
 	}
-	i, err := strconv.Atoi(since)
-	if err != nil {
-		return nil, err
+
+	posStrings := strings.Split(since, "_")
+	if len(posStrings) != 2 && len(posStrings) != 1 {
+		// A token can either be full length or short (PDU-only).
+		return nil, errors.New("malformed batch token")
+	}
+
+	positions := make([]int64, len(posStrings))
+	for i, posString := range posStrings {
+		pos, err := strconv.ParseInt(posString, 10, 64)
+		if err != nil {
+			return nil, err
+		}
+		positions[i] = pos
+	}
+
+	if len(positions) == 2 {
+		// Full length token; construct SyncPosition with every entry in
+		// `positions`. These entries must have the same order with the fields
+		// in struct SyncPosition, so we disable the govet check below.
+		return &types.SyncPosition{ //nolint:govet
+			positions[0], positions[1],
+		}, nil
+	} else {
+		// Token with PDU position only
+		return &types.SyncPosition{
+			PDUPosition: positions[0],
+		}, nil
 	}
-	token := types.StreamPosition(i)
-	return &token, nil
 }