Optimise CheckServerAllowedToSeeEvent (#1602)

* Try to limit how many state events we have to unmarshal

* Comments

roomserver/internal/helpers/helpers.go

@@ -5,6 +5,7 @@ import (
 	"database/sql"
 	"errors"
 	"fmt"
+	"strings"
 
 	"github.com/matrix-org/dendrite/roomserver/api"
 	"github.com/matrix-org/dendrite/roomserver/auth"
@@ -222,12 +223,45 @@ func CheckServerAllowedToSeeEvent(
 		if errors.Is(err, sql.ErrNoRows) {
 			return false, nil
 		}
-		return false, err
+		return false, fmt.Errorf("roomState.LoadStateAtEvent: %w", err)
 	}
 
-	// TODO: We probably want to make it so that we don't have to pull
-	// out all the state if possible.
-	stateAtEvent, err := LoadStateEvents(ctx, db, stateEntries)
+	// Extract all of the event state key NIDs from the room state.
+	var stateKeyNIDs []types.EventStateKeyNID
+	for _, entry := range stateEntries {
+		stateKeyNIDs = append(stateKeyNIDs, entry.EventStateKeyNID)
+	}
+
+	// Then request those state key NIDs from the database.
+	stateKeys, err := db.EventStateKeys(ctx, stateKeyNIDs)
+	if err != nil {
+		return false, fmt.Errorf("db.EventStateKeys: %w", err)
+	}
+
+	// If the event state key doesn't match the given servername
+	// then we'll filter it out. This does preserve state keys that
+	// are "" since these will contain history visibility etc.
+	for nid, key := range stateKeys {
+		if key != "" && !strings.HasSuffix(key, ":"+string(serverName)) {
+			delete(stateKeys, nid)
+		}
+	}
+
+	// Now filter through all of the state events for the room.
+	// If the state key NID appears in the list of valid state
+	// keys then we'll add it to the list of filtered entries.
+	var filteredEntries []types.StateEntry
+	for _, entry := range stateEntries {
+		if _, ok := stateKeys[entry.EventStateKeyNID]; ok {
+			filteredEntries = append(filteredEntries, entry)
+		}
+	}
+
+	if len(filteredEntries) == 0 {
+		return false, nil
+	}
+
+	stateAtEvent, err := LoadStateEvents(ctx, db, filteredEntries)
 	if err != nil {
 		return false, err
 	}