Refactor cacheable cursor create time

Author: ftkg

server/core_channel.go

@@ -138,7 +138,6 @@ WHERE stream_mode = $1 AND stream_subject = $2::UUID AND stream_descriptor = $3:
 		userIDOne := stream.Subject.String()
 		userIDTwo := stream.Subcontext.String()
 		messages := make([]*api.ChannelMessage, 0, limit)
-		createTimeNano := make(map[string]int64)
 		var nextCursor, prevCursor *channelMessageListCursor
 
 		var dbID string
@@ -148,19 +147,23 @@ WHERE stream_mode = $1 AND stream_subject = $2::UUID AND stream_descriptor = $3:
 		var dbContent string
 		var dbCreateTime pgtype.Timestamptz
 		var dbUpdateTime pgtype.Timestamptz
+		var dbFirstCreateTimeNano int64
+		var dbLastCreateTimeNano int64
 		for rows.Next() {
 			if len(messages) >= limit {
+				dbLastCreateTimeNano = dbCreateTime.Time.UnixNano()
 				nextCursor = &channelMessageListCursor{
 					StreamMode:       stream.Mode,
 					StreamSubject:    stream.Subject.String(),
 					StreamSubcontext: stream.Subcontext.String(),
 					StreamLabel:      stream.Label,
-					CreateTime:       dbCreateTime.Time.UnixNano(),
+					CreateTime:       dbLastCreateTimeNano,
 					Id:               dbID,
 					Forward:          forward,
 					IsNext:           true,
 				}
 				break
+
 			}
 
 			err = rows.Scan(&dbID, &dbCode, &dbSenderID, &dbUsername, &dbContent, &dbCreateTime, &dbUpdateTime)
@@ -181,7 +184,6 @@ WHERE stream_mode = $1 AND stream_subject = $2::UUID AND stream_descriptor = $3:
 				UpdateTime: &timestamppb.Timestamp{Seconds: dbUpdateTime.Time.Unix()},
 				Persistent: &wrapperspb.BoolValue{Value: true},
 			}
-			createTimeNano[dbID] = dbCreateTime.Time.UnixNano()
 			switch stream.Mode {
 			case StreamModeChannel:
 				message.RoomName = stream.Label
@@ -194,14 +196,17 @@ WHERE stream_mode = $1 AND stream_subject = $2::UUID AND stream_descriptor = $3:
 
 			messages = append(messages, message)
 
+			if dbFirstCreateTimeNano == 0 {
+				dbFirstCreateTimeNano = dbCreateTime.Time.UnixNano()
+			}
 			// There can only be a previous page if this is a paginated listing.
 			if incomingCursor != nil && prevCursor == nil {
 				prevCursor = &channelMessageListCursor{
 					StreamMode:       stream.Mode,
 					StreamSubject:    stream.Subject.String(),
 					StreamSubcontext: stream.Subcontext.String(),
 					StreamLabel:      stream.Label,
-					CreateTime:       dbCreateTime.Time.UnixNano(),
+					CreateTime:       dbFirstCreateTimeNano,
 					Id:               dbID,
 					Forward:          forward,
 					IsNext:           false,
@@ -210,6 +215,7 @@ WHERE stream_mode = $1 AND stream_subject = $2::UUID AND stream_descriptor = $3:
 		}
 		_ = rows.Close()
 
+		flipped := false
 		if incomingCursor != nil && !incomingCursor.IsNext {
 			// If this was a previous page listing, flip the results to their normal order and swap the cursors.
 			nextCursor, prevCursor = prevCursor, nextCursor
@@ -223,17 +229,22 @@ WHERE stream_mode = $1 AND stream_subject = $2::UUID AND stream_descriptor = $3:
 			for i, j := 0, len(messages)-1; i < j; i, j = i+1, j-1 {
 				messages[i], messages[j] = messages[j], messages[i]
 			}
+			flipped = true
 		}
 
 		var cacheableCursor *channelMessageListCursor
 		if l := len(messages); l > 0 {
+			createTime := dbLastCreateTimeNano
+			if flipped {
+				createTime = dbFirstCreateTimeNano
+			}
 			// There is at least 1 message returned by the listing, so use it as the foundation of a new cacheable cursor.
 			cacheableCursor = &channelMessageListCursor{
 				StreamMode:       stream.Mode,
 				StreamSubject:    stream.Subject.String(),
 				StreamSubcontext: stream.Subcontext.String(),
 				StreamLabel:      stream.Label,
-				CreateTime:       createTimeNano[messages[l-1].MessageId],
+				CreateTime:       createTime,
 				Id:               messages[l-1].MessageId,
 				Forward:          true,
 				IsNext:           true,
@@ -348,7 +359,6 @@ WHERE stream_mode = $1 AND stream_subject = $2::UUID AND stream_descriptor = $3:
 	if start > 0 {
 		// Check if there might be a prev cursor
 		setPrevCursor = true
-		firstRecords = firstRecords[:len(firstRecords)-1]
 	}
 
 	records = records[start:end]