feat(backend): main handling of readonly sharing

* Saving works as before
* Don't broadcast drawevents from readonly whiteboard (prevents malicious use)

scripts/services/ReadOnlyBackendService.js

@@ -0,0 +1,71 @@
+const { v4: uuidv4 } = require("uuid");
+
+class ReadOnlyBackendService {
+    /**
+     * Mapping from an editable whiteboard id to the matching read-only whiteboard id
+     * @type {Map<string, string>}
+     * @private
+     */
+    #idToReadOnlyId = new Map();
+
+    /**
+     * Mapping from a read-only whiteboard id to the matching editable whiteboard id
+     *
+     * @type {Map<string, string>}
+     * @private
+     */
+    #readOnlyIdToId = new Map();
+
+    /**
+     * Make sure a whiteboardId is ignited in the service
+     *
+     * If it's not found in the service, we assume that it's an editable whiteboard
+     *
+     * @param {string} whiteboardId
+     */
+    init(whiteboardId) {
+        const idToReadOnlyId = this.#idToReadOnlyId;
+        const readOnlyIdToId = this.#readOnlyIdToId;
+
+        if (!idToReadOnlyId.has(whiteboardId) && !readOnlyIdToId.has(whiteboardId)) {
+            const readOnlyId = uuidv4();
+            idToReadOnlyId.set(whiteboardId, readOnlyId);
+            readOnlyIdToId.set(readOnlyId, whiteboardId);
+        }
+    }
+
+    /**
+     * Get the read-only id corresponding to a whiteboard id
+     *
+     * @param {string} whiteboardId
+     * @return {string}
+     */
+    getReadOnlyId(whiteboardId) {
+        // make sure it's inited
+        this.init(whiteboardId);
+        return this.#idToReadOnlyId.get(whiteboardId);
+    }
+
+    /**
+     * Get the id corresponding to readonly id
+     *
+     * @param {string} readOnlyId
+     * @return {string}
+     */
+    getIdFromReadOnlyId(readOnlyId) {
+        return this.#readOnlyIdToId.get(readOnlyId);
+    }
+
+    /**
+     * Tell is whiteboard id corresponds to a read-only whiteboard
+     *
+     * @param whiteboardId
+     * @return {boolean}
+     */
+    isReadOnly(whiteboardId) {
+        this.init(whiteboardId);
+        return this.#readOnlyIdToId.has(whiteboardId);
+    }
+}
+
+module.exports = new ReadOnlyBackendService();