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Add Range (#1037)

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* Add Range

* Use Range
This commit is contained in:
Kegsay 2020-05-15 09:41:12 +01:00 committed by GitHub
parent 419ff150d4
commit 2b5052eccf
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GPG Key ID: 4AEE18F83AFDEB23
11 changed files with 108 additions and 74 deletions
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2
syncapi/storage/interface.go
View File

@ -64,7 +64,7 @@ type Database interface {
// Returns a map following the format data[roomID] = []dataTypes
// If no data is retrieved, returns an empty map
// If there was an issue with the retrieval, returns an error
GetAccountDataInRange(ctx context.Context, userID string, oldPos, newPos types.StreamPosition, accountDataFilterPart *gomatrixserverlib.EventFilter) (map[string][]string, error)
GetAccountDataInRange(ctx context.Context, userID string, r types.Range, accountDataFilterPart *gomatrixserverlib.EventFilter) (map[string][]string, error)
// UpsertAccountData keeps track of new or updated account data, by saving the type
// of the new/updated data, and the user ID and room ID the data is related to (empty)
// room ID means the data isn't specific to any room)

11
syncapi/storage/postgres/account_data_table.go
View File

@ -100,19 +100,12 @@ func (s *accountDataStatements) InsertAccountData(
func (s *accountDataStatements) SelectAccountDataInRange(
ctx context.Context,
userID string,
oldPos, newPos types.StreamPosition,
r types.Range,
accountDataEventFilter *gomatrixserverlib.EventFilter,
) (data map[string][]string, err error) {
data = make(map[string][]string)
// If both positions are the same, it means that the data was saved after the
// latest room event. In that case, we need to decrement the old position as
// it would prevent the SQL request from returning anything.
if oldPos == newPos {
oldPos--
}
rows, err := s.selectAccountDataInRangeStmt.QueryContext(ctx, userID, oldPos, newPos,
rows, err := s.selectAccountDataInRangeStmt.QueryContext(ctx, userID, r.Low(), r.High(),
pq.StringArray(filterConvertTypeWildcardToSQL(accountDataEventFilter.Types)),
pq.StringArray(filterConvertTypeWildcardToSQL(accountDataEventFilter.NotTypes)),
accountDataEventFilter.Limit,

4
syncapi/storage/postgres/invites_table.go
View File

@ -117,10 +117,10 @@ func (s *inviteEventsStatements) DeleteInviteEvent(
// selectInviteEventsInRange returns a map of room ID to invite event for the
// active invites for the target user ID in the supplied range.
func (s *inviteEventsStatements) SelectInviteEventsInRange(
ctx context.Context, txn *sql.Tx, targetUserID string, startPos, endPos types.StreamPosition,
ctx context.Context, txn *sql.Tx, targetUserID string, r types.Range,
) (map[string]gomatrixserverlib.HeaderedEvent, error) {
stmt := common.TxStmt(txn, s.selectInviteEventsInRangeStmt)
rows, err := stmt.QueryContext(ctx, targetUserID, startPos, endPos)
rows, err := stmt.QueryContext(ctx, targetUserID, r.Low(), r.High())
if err != nil {
return nil, err
}

16
syncapi/storage/postgres/output_room_events_table.go
View File

@ -155,13 +155,13 @@ func NewPostgresEventsTable(db *sql.DB) (tables.Events, error) {
// Results are bucketed based on the room ID. If the same state is overwritten multiple times between the
// two positions, only the most recent state is returned.
func (s *outputRoomEventsStatements) SelectStateInRange(
ctx context.Context, txn *sql.Tx, oldPos, newPos types.StreamPosition,
ctx context.Context, txn *sql.Tx, r types.Range,
stateFilter *gomatrixserverlib.StateFilter,
) (map[string]map[string]bool, map[string]types.StreamEvent, error) {
stmt := common.TxStmt(txn, s.selectStateInRangeStmt)
rows, err := stmt.QueryContext(
ctx, oldPos, newPos,
ctx, r.Low(), r.High(),
pq.StringArray(stateFilter.Senders),
pq.StringArray(stateFilter.NotSenders),
pq.StringArray(filterConvertTypeWildcardToSQL(stateFilter.Types)),
@ -198,8 +198,8 @@ func (s *outputRoomEventsStatements) SelectStateInRange(
// since it'll just mark the event as not being needed.
if len(addIDs) < len(delIDs) {
log.WithFields(log.Fields{
"since": oldPos,
"current": newPos,
"since": r.From,
"current": r.To,
"adds": addIDs,
"dels": delIDs,
}).Warn("StateBetween: ignoring deleted state")
@ -298,7 +298,7 @@ func (s *outputRoomEventsStatements) InsertEvent(
// from sync.
func (s *outputRoomEventsStatements) SelectRecentEvents(
ctx context.Context, txn *sql.Tx,
roomID string, fromPos, toPos types.StreamPosition, limit int,
roomID string, r types.Range, limit int,
chronologicalOrder bool, onlySyncEvents bool,
) ([]types.StreamEvent, error) {
var stmt *sql.Stmt
@ -307,7 +307,7 @@ func (s *outputRoomEventsStatements) SelectRecentEvents(
} else {
stmt = common.TxStmt(txn, s.selectRecentEventsStmt)
}
rows, err := stmt.QueryContext(ctx, roomID, fromPos, toPos, limit)
rows, err := stmt.QueryContext(ctx, roomID, r.Low(), r.High(), limit)
if err != nil {
return nil, err
}
@ -331,10 +331,10 @@ func (s *outputRoomEventsStatements) SelectRecentEvents(
// from a given position, up to a maximum of 'limit'.
func (s *outputRoomEventsStatements) SelectEarlyEvents(
ctx context.Context, txn *sql.Tx,
roomID string, fromPos, toPos types.StreamPosition, limit int,
roomID string, r types.Range, limit int,
) ([]types.StreamEvent, error) {
stmt := common.TxStmt(txn, s.selectEarlyEventsStmt)
rows, err := stmt.QueryContext(ctx, roomID, fromPos, toPos, limit)
rows, err := stmt.QueryContext(ctx, roomID, r.Low(), r.High(), limit)
if err != nil {
return nil, err
}

57
syncapi/storage/shared/syncserver.go
View File

@ -54,17 +54,22 @@ func (d *Database) GetEventsInStreamingRange(
roomID string, limit int,
backwardOrdering bool,
) (events []types.StreamEvent, err error) {
r := types.Range{
From: from.PDUPosition(),
To: to.PDUPosition(),
Backwards: backwardOrdering,
}
if backwardOrdering {
// When using backward ordering, we want the most recent events first.
if events, err = d.OutputEvents.SelectRecentEvents(
ctx, nil, roomID, to.PDUPosition(), from.PDUPosition(), limit, false, false,
ctx, nil, roomID, r, limit, false, false,
); err != nil {
return
}
} else {
// When using forward ordering, we want the least recent events first.
if events, err = d.OutputEvents.SelectEarlyEvents(
ctx, nil, roomID, from.PDUPosition(), to.PDUPosition(), limit,
ctx, nil, roomID, r, limit,
); err != nil {
return
}
@ -167,10 +172,10 @@ func (d *Database) RetireInviteEvent(
// If no data is retrieved, returns an empty map
// If there was an issue with the retrieval, returns an error
func (d *Database) GetAccountDataInRange(
ctx context.Context, userID string, oldPos, newPos types.StreamPosition,
ctx context.Context, userID string, r types.Range,
accountDataFilterPart *gomatrixserverlib.EventFilter,
) (map[string][]string, error) {
return d.AccountData.SelectAccountDataInRange(ctx, userID, oldPos, newPos, accountDataFilterPart)
return d.AccountData.SelectAccountDataInRange(ctx, userID, r, accountDataFilterPart)
}
// UpsertAccountData keeps track of new or updated account data, by saving the type
@ -417,7 +422,7 @@ func (d *Database) syncPositionTx(
func (d *Database) addPDUDeltaToResponse(
ctx context.Context,
device authtypes.Device,
fromPos, toPos types.StreamPosition,
r types.Range,
numRecentEventsPerRoom int,
wantFullState bool,
res *types.Response,
@ -443,11 +448,11 @@ func (d *Database) addPDUDeltaToResponse(
var deltas []stateDelta
if !wantFullState {
deltas, joinedRoomIDs, err = d.getStateDeltas(
ctx, &device, txn, fromPos, toPos, device.UserID, &stateFilter,
ctx, &device, txn, r, device.UserID, &stateFilter,
)
} else {
deltas, joinedRoomIDs, err = d.getStateDeltasForFullStateSync(
ctx, &device, txn, fromPos, toPos, device.UserID, &stateFilter,
ctx, &device, txn, r, device.UserID, &stateFilter,
)
}
if err != nil {
@ -455,14 +460,14 @@ func (d *Database) addPDUDeltaToResponse(
}
for _, delta := range deltas {
err = d.addRoomDeltaToResponse(ctx, &device, txn, fromPos, toPos, delta, numRecentEventsPerRoom, res)
err = d.addRoomDeltaToResponse(ctx, &device, txn, r, delta, numRecentEventsPerRoom, res)
if err != nil {
return nil, err
}
}
// TODO: This should be done in getStateDeltas
if err = d.addInvitesToResponse(ctx, txn, device.UserID, fromPos, toPos, res); err != nil {
if err = d.addInvitesToResponse(ctx, txn, device.UserID, r, res); err != nil {
return nil, err
}
@ -534,8 +539,12 @@ func (d *Database) IncrementalSync(
var joinedRoomIDs []string
var err error
if fromPos.PDUPosition() != toPos.PDUPosition() || wantFullState {
r := types.Range{
From: fromPos.PDUPosition(),
To: toPos.PDUPosition(),
}
joinedRoomIDs, err = d.addPDUDeltaToResponse(
ctx, device, fromPos.PDUPosition(), toPos.PDUPosition(), numRecentEventsPerRoom, wantFullState, res,
ctx, device, r, numRecentEventsPerRoom, wantFullState, res,
)
} else {
joinedRoomIDs, err = d.CurrentRoomState.SelectRoomIDsWithMembership(
@ -589,6 +598,10 @@ func (d *Database) getResponseWithPDUsForCompleteSync(
if err != nil {
return
}
r := types.Range{
From: 0,
To: toPos.PDUPosition(),
}
res = types.NewResponse(toPos)
@ -611,8 +624,7 @@ func (d *Database) getResponseWithPDUsForCompleteSync(
// See: https://github.com/matrix-org/synapse/blob/v0.19.3/synapse/handlers/sync.py#L316
var recentStreamEvents []types.StreamEvent
recentStreamEvents, err = d.OutputEvents.SelectRecentEvents(
ctx, txn, roomID, types.StreamPosition(0), toPos.PDUPosition(),
numRecentEventsPerRoom, true, true,
ctx, txn, roomID, r, numRecentEventsPerRoom, true, true,
)
if err != nil {
return
@ -644,7 +656,7 @@ func (d *Database) getResponseWithPDUsForCompleteSync(
res.Rooms.Join[roomID] = *jr
}
if err = d.addInvitesToResponse(ctx, txn, userID, 0, toPos.PDUPosition(), res); err != nil {
if err = d.addInvitesToResponse(ctx, txn, userID, r, res); err != nil {
return
}
@ -686,11 +698,11 @@ var txReadOnlySnapshot = sql.TxOptions{
func (d *Database) addInvitesToResponse(
ctx context.Context, txn *sql.Tx,
userID string,
fromPos, toPos types.StreamPosition,
r types.Range,
res *types.Response,
) error {
invites, err := d.Invites.SelectInviteEventsInRange(
ctx, txn, userID, fromPos, toPos,
ctx, txn, userID, r,
)
if err != nil {
return err
@ -726,12 +738,11 @@ func (d *Database) addRoomDeltaToResponse(
ctx context.Context,
device *authtypes.Device,
txn *sql.Tx,
fromPos, toPos types.StreamPosition,
r types.Range,
delta stateDelta,
numRecentEventsPerRoom int,
res *types.Response,
) error {
endPos := toPos
if delta.membershipPos > 0 && delta.membership == gomatrixserverlib.Leave {
// make sure we don't leak recent events after the leave event.
// TODO: History visibility makes this somewhat complex to handle correctly. For example:
@ -739,10 +750,10 @@ func (d *Database) addRoomDeltaToResponse(
// TODO: This will fail on join -> leave -> sensitive msg -> join -> leave
// in a single /sync request
// This is all "okay" assuming history_visibility == "shared" which it is by default.
endPos = delta.membershipPos
r.To = delta.membershipPos
}
recentStreamEvents, err := d.OutputEvents.SelectRecentEvents(
ctx, txn, delta.roomID, types.StreamPosition(fromPos), types.StreamPosition(endPos),
ctx, txn, delta.roomID, r,
numRecentEventsPerRoom, true, true,
)
if err != nil {
@ -872,7 +883,7 @@ func (d *Database) fetchMissingStateEvents(
// A list of joined room IDs is also returned in case the caller needs it.
func (d *Database) getStateDeltas(
ctx context.Context, device *authtypes.Device, txn *sql.Tx,
fromPos, toPos types.StreamPosition, userID string,
r types.Range, userID string,
stateFilter *gomatrixserverlib.StateFilter,
) ([]stateDelta, []string, error) {
// Implement membership change algorithm: https://github.com/matrix-org/synapse/blob/v0.19.3/synapse/handlers/sync.py#L821
@ -886,7 +897,7 @@ func (d *Database) getStateDeltas(
var deltas []stateDelta
// get all the state events ever between these two positions
stateNeeded, eventMap, err := d.OutputEvents.SelectStateInRange(ctx, txn, fromPos, toPos, stateFilter)
stateNeeded, eventMap, err := d.OutputEvents.SelectStateInRange(ctx, txn, r, stateFilter)
if err != nil {
return nil, nil, err
}
@ -947,7 +958,7 @@ func (d *Database) getStateDeltas(
// updates for other rooms.
func (d *Database) getStateDeltasForFullStateSync(
ctx context.Context, device *authtypes.Device, txn *sql.Tx,
fromPos, toPos types.StreamPosition, userID string,
r types.Range, userID string,
stateFilter *gomatrixserverlib.StateFilter,
) ([]stateDelta, []string, error) {
joinedRoomIDs, err := d.CurrentRoomState.SelectRoomIDsWithMembership(ctx, txn, userID, gomatrixserverlib.Join)
@ -972,7 +983,7 @@ func (d *Database) getStateDeltasForFullStateSync(
}
// Get all the state events ever between these two positions
stateNeeded, eventMap, err := d.OutputEvents.SelectStateInRange(ctx, txn, fromPos, toPos, stateFilter)
stateNeeded, eventMap, err := d.OutputEvents.SelectStateInRange(ctx, txn, r, stateFilter)
if err != nil {
return nil, nil, err
}

11
syncapi/storage/sqlite3/account_data_table.go
View File

@ -91,19 +91,12 @@ func (s *accountDataStatements) InsertAccountData(
func (s *accountDataStatements) SelectAccountDataInRange(
ctx context.Context,
userID string,
oldPos, newPos types.StreamPosition,
r types.Range,
accountDataFilterPart *gomatrixserverlib.EventFilter,
) (data map[string][]string, err error) {
data = make(map[string][]string)
// If both positions are the same, it means that the data was saved after the
// latest room event. In that case, we need to decrement the old position as
// it would prevent the SQL request from returning anything.
if oldPos == newPos {
oldPos--
}
rows, err := s.selectAccountDataInRangeStmt.QueryContext(ctx, userID, oldPos, newPos)
rows, err := s.selectAccountDataInRangeStmt.QueryContext(ctx, userID, r.Low(), r.High())
if err != nil {
return
}

4
syncapi/storage/sqlite3/invites_table.go
View File

@ -121,10 +121,10 @@ func (s *inviteEventsStatements) DeleteInviteEvent(
// selectInviteEventsInRange returns a map of room ID to invite event for the
// active invites for the target user ID in the supplied range.
func (s *inviteEventsStatements) SelectInviteEventsInRange(
ctx context.Context, txn *sql.Tx, targetUserID string, startPos, endPos types.StreamPosition,
ctx context.Context, txn *sql.Tx, targetUserID string, r types.Range,
) (map[string]gomatrixserverlib.HeaderedEvent, error) {
stmt := common.TxStmt(txn, s.selectInviteEventsInRangeStmt)
rows, err := stmt.QueryContext(ctx, targetUserID, startPos, endPos)
rows, err := stmt.QueryContext(ctx, targetUserID, r.Low(), r.High())
if err != nil {
return nil, err
}

16
syncapi/storage/sqlite3/output_room_events_table.go
View File

@ -146,13 +146,13 @@ func NewSqliteEventsTable(db *sql.DB, streamID *streamIDStatements) (tables.Even
// Results are bucketed based on the room ID. If the same state is overwritten multiple times between the
// two positions, only the most recent state is returned.
func (s *outputRoomEventsStatements) SelectStateInRange(
ctx context.Context, txn *sql.Tx, oldPos, newPos types.StreamPosition,
ctx context.Context, txn *sql.Tx, r types.Range,
stateFilterPart *gomatrixserverlib.StateFilter,
) (map[string]map[string]bool, map[string]types.StreamEvent, error) {
stmt := common.TxStmt(txn, s.selectStateInRangeStmt)
rows, err := stmt.QueryContext(
ctx, oldPos, newPos,
ctx, r.Low(), r.High(),
/*pq.StringArray(stateFilterPart.Senders),
pq.StringArray(stateFilterPart.NotSenders),
pq.StringArray(filterConvertTypeWildcardToSQL(stateFilterPart.Types)),
@ -195,8 +195,8 @@ func (s *outputRoomEventsStatements) SelectStateInRange(
// since it'll just mark the event as not being needed.
if len(addIDs) < len(delIDs) {
log.WithFields(log.Fields{
"since": oldPos,
"current": newPos,
"since": r.From,
"current": r.To,
"adds": addIDsJSON,
"dels": delIDsJSON,
}).Warn("StateBetween: ignoring deleted state")
@ -308,7 +308,7 @@ func (s *outputRoomEventsStatements) InsertEvent(
func (s *outputRoomEventsStatements) SelectRecentEvents(
ctx context.Context, txn *sql.Tx,
roomID string, fromPos, toPos types.StreamPosition, limit int,
roomID string, r types.Range, limit int,
chronologicalOrder bool, onlySyncEvents bool,
) ([]types.StreamEvent, error) {
var stmt *sql.Stmt
@ -318,7 +318,7 @@ func (s *outputRoomEventsStatements) SelectRecentEvents(
stmt = common.TxStmt(txn, s.selectRecentEventsStmt)
}
rows, err := stmt.QueryContext(ctx, roomID, fromPos, toPos, limit)
rows, err := stmt.QueryContext(ctx, roomID, r.Low(), r.High(), limit)
if err != nil {
return nil, err
}
@ -340,10 +340,10 @@ func (s *outputRoomEventsStatements) SelectRecentEvents(
func (s *outputRoomEventsStatements) SelectEarlyEvents(
ctx context.Context, txn *sql.Tx,
roomID string, fromPos, toPos types.StreamPosition, limit int,
roomID string, r types.Range, limit int,
) ([]types.StreamEvent, error) {
stmt := common.TxStmt(txn, s.selectEarlyEventsStmt)
rows, err := stmt.QueryContext(ctx, roomID, fromPos, toPos, limit)
rows, err := stmt.QueryContext(ctx, roomID, r.Low(), r.High(), limit)
if err != nil {
return nil, err
}

18
syncapi/storage/tables/interface.go
View File

@ -11,29 +11,29 @@ import (
type AccountData interface {
InsertAccountData(ctx context.Context, txn *sql.Tx, userID, roomID, dataType string) (pos types.StreamPosition, err error)
// SelectAccountDataInRange returns a map of room ID to a list of `dataType`. The range is exclusive of `lowPos` and inclusive of `hiPos`.
SelectAccountDataInRange(ctx context.Context, userID string, lowPos, hiPos types.StreamPosition, accountDataEventFilter *gomatrixserverlib.EventFilter) (data map[string][]string, err error)
// SelectAccountDataInRange returns a map of room ID to a list of `dataType`.
SelectAccountDataInRange(ctx context.Context, userID string, r types.Range, accountDataEventFilter *gomatrixserverlib.EventFilter) (data map[string][]string, err error)
SelectMaxAccountDataID(ctx context.Context, txn *sql.Tx) (id int64, err error)
}
type Invites interface {
InsertInviteEvent(ctx context.Context, txn *sql.Tx, inviteEvent gomatrixserverlib.HeaderedEvent) (streamPos types.StreamPosition, err error)
DeleteInviteEvent(ctx context.Context, inviteEventID string) error
// SelectInviteEventsInRange returns a map of room ID to invite events. The range is exclusive of `startPos` and inclusive of `endPos`.
SelectInviteEventsInRange(ctx context.Context, txn *sql.Tx, targetUserID string, startPos, endPos types.StreamPosition) (map[string]gomatrixserverlib.HeaderedEvent, error)
// SelectInviteEventsInRange returns a map of room ID to invite events.
SelectInviteEventsInRange(ctx context.Context, txn *sql.Tx, targetUserID string, r types.Range) (map[string]gomatrixserverlib.HeaderedEvent, error)
SelectMaxInviteID(ctx context.Context, txn *sql.Tx) (id int64, err error)
}
type Events interface {
SelectStateInRange(ctx context.Context, txn *sql.Tx, oldPos, newPos types.StreamPosition, stateFilter *gomatrixserverlib.StateFilter) (map[string]map[string]bool, map[string]types.StreamEvent, error)
SelectStateInRange(ctx context.Context, txn *sql.Tx, r types.Range, stateFilter *gomatrixserverlib.StateFilter) (map[string]map[string]bool, map[string]types.StreamEvent, error)
SelectMaxEventID(ctx context.Context, txn *sql.Tx) (id int64, err error)
InsertEvent(ctx context.Context, txn *sql.Tx, event *gomatrixserverlib.HeaderedEvent, addState, removeState []string, transactionID *api.TransactionID, excludeFromSync bool) (streamPos types.StreamPosition, err error)
// SelectRecentEvents returns events between the two stream positions: exclusive of `fromPos` and inclusive of `toPos`.
// SelectRecentEvents returns events between the two stream positions: exclusive of low and inclusive of high.
// If onlySyncEvents has a value of true, only returns the events that aren't marked as to exclude from sync.
// Returns up to `limit` events.
SelectRecentEvents(ctx context.Context, txn *sql.Tx, roomID string, fromPos, toPos types.StreamPosition, limit int, chronologicalOrder bool, onlySyncEvents bool) ([]types.StreamEvent, error)
// SelectEarlyEvents returns the earliest events in the given room, exclusive of `fromPos` and inclusive of `toPos`.
SelectEarlyEvents(ctx context.Context, txn *sql.Tx, roomID string, fromPos, toPos types.StreamPosition, limit int) ([]types.StreamEvent, error)
SelectRecentEvents(ctx context.Context, txn *sql.Tx, roomID string, r types.Range, limit int, chronologicalOrder bool, onlySyncEvents bool) ([]types.StreamEvent, error)
// SelectEarlyEvents returns the earliest events in the given room.
SelectEarlyEvents(ctx context.Context, txn *sql.Tx, roomID string, r types.Range, limit int) ([]types.StreamEvent, error)
SelectEvents(ctx context.Context, txn *sql.Tx, eventIDs []string) ([]types.StreamEvent, error)
}

15
syncapi/sync/requestpool.go
View File

@ -184,11 +184,20 @@ func (rp *RequestPool) appendAccountData(
return data, nil
}
r := types.Range{
From: req.since.PDUPosition(),
To: currentPos,
}
// If both positions are the same, it means that the data was saved after the
// latest room event. In that case, we need to decrement the old position as
// results are exclusive of Low.
if r.Low() == r.High() {
r.From--
}
// Sync is not initial, get all account data since the latest sync
dataTypes, err := rp.db.GetAccountDataInRange(
req.ctx, userID,
types.StreamPosition(req.since.PDUPosition()), types.StreamPosition(currentPos),
accountDataFilter,
req.ctx, userID, r, accountDataFilter,
)
if err != nil {
return nil, err

28
syncapi/types/types.go
View File

@ -47,6 +47,34 @@ type StreamEvent struct {
ExcludeFromSync bool
}
// Range represents a range between two stream positions.
type Range struct {
// From is the position the client has already received.
From StreamPosition
// To is the position the client is going towards.
To StreamPosition
// True if the client is going backwards
Backwards bool
}
// Low returns the low number of the range.
// This represents the position the client already has and hence is exclusive.
func (r *Range) Low() StreamPosition {
if !r.Backwards {
return r.From
}
return r.To
}
// High returns the high number of the range
// This represents the position the client is going towards and hence is inclusive.
func (r *Range) High() StreamPosition {
if !r.Backwards {
return r.To
}
return r.From
}
// SyncTokenType represents the type of a sync token.
// It can be either "s" (representing a position in the whole stream of events)
// or "t" (representing a position in a room's topology/depth).