1 files changed, 92 insertions, 0 deletions
diff --git a/roomserver/storage/sqlite3/previous_events_table.go b/roomserver/storage/sqlite3/previous_events_table.go
new file mode 100644
index 00000000..9ed64a38
--- /dev/null
+++ b/roomserver/storage/sqlite3/previous_events_table.go
@@ -0,0 +1,92 @@
+// Copyright 2017-2018 New Vector Ltd
+// Copyright 2019-2020 The Matrix.org Foundation C.I.C.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package sqlite3
+
+import (
+	"context"
+	"database/sql"
+
+	"github.com/matrix-org/dendrite/common"
+	"github.com/matrix-org/dendrite/roomserver/types"
+)
+
+const previousEventSchema = `
+  CREATE TABLE IF NOT EXISTS roomserver_previous_events (
+    previous_event_id TEXT NOT NULL,
+    previous_reference_sha256 BLOB NOT NULL,
+    event_nids TEXT NOT NULL,
+    UNIQUE (previous_event_id, previous_reference_sha256)
+  );
+`
+
+// Insert an entry into the previous_events table.
+// If there is already an entry indicating that an event references that previous event then
+// add the event NID to the list to indicate that this event references that previous event as well.
+// This should only be modified while holding a "FOR UPDATE" lock on the row in the rooms table for this room.
+// The lock is necessary to avoid data races when checking whether an event is already referenced by another event.
+const insertPreviousEventSQL = `
+	INSERT OR REPLACE INTO roomserver_previous_events
+	  (previous_event_id, previous_reference_sha256, event_nids)
+	  VALUES ($1, $2, $3)
+`
+
+// Check if the event is referenced by another event in the table.
+// This should only be done while holding a "FOR UPDATE" lock on the row in the rooms table for this room.
+const selectPreviousEventExistsSQL = `
+	SELECT 1 FROM roomserver_previous_events
+	  WHERE previous_event_id = $1 AND previous_reference_sha256 = $2
+`
+
+type previousEventStatements struct {
+	insertPreviousEventStmt       *sql.Stmt
+	selectPreviousEventExistsStmt *sql.Stmt
+}
+
+func (s *previousEventStatements) prepare(db *sql.DB) (err error) {
+	_, err = db.Exec(previousEventSchema)
+	if err != nil {
+		return
+	}
+
+	return statementList{
+		{&s.insertPreviousEventStmt, insertPreviousEventSQL},
+		{&s.selectPreviousEventExistsStmt, selectPreviousEventExistsSQL},
+	}.prepare(db)
+}
+
+func (s *previousEventStatements) insertPreviousEvent(
+	ctx context.Context,
+	txn *sql.Tx,
+	previousEventID string,
+	previousEventReferenceSHA256 []byte,
+	eventNID types.EventNID,
+) error {
+	stmt := common.TxStmt(txn, s.insertPreviousEventStmt)
+	_, err := stmt.ExecContext(
+		ctx, previousEventID, previousEventReferenceSHA256, int64(eventNID),
+	)
+	return err
+}
+
+// Check if the event reference exists
+// Returns sql.ErrNoRows if the event reference doesn't exist.
+func (s *previousEventStatements) selectPreviousEventExists(
+	ctx context.Context, txn *sql.Tx, eventID string, eventReferenceSHA256 []byte,
+) error {
+	var ok int64
+	stmt := common.TxStmt(txn, s.selectPreviousEventExistsStmt)
+	return stmt.QueryRowContext(ctx, eventID, eventReferenceSHA256).Scan(&ok)
+}