feat(json-crdt): 🎸 when forking, detect if sid was already used

Author: streamich

src/json-crdt/model/Model.ts

@@ -34,6 +34,24 @@ export class Model<N extends JsonNode = JsonNode<any>> implements Printable {
    */
   public static readonly sid = randomSessionId;
 
+  /**
+   * Use this method to generate a random session ID for an existing document.
+   * It checks for the uniqueness of the session ID given the current peers in
+   * the document. This reduces the chance of collision substantially.
+   *
+   * @returns A random session ID that is not used by any peer in the current
+   *     document.
+   */
+  public rndSid(): number {
+    const clock = this.clock;
+    const sid = clock.sid;
+    const peers = clock.peers;
+    while (true) {
+      const candidate = randomSessionId();
+      if (sid !== candidate && !peers.has(candidate)) return candidate;
+    }
+  }
+
   /**
    * Create a CRDT model which uses logical clock. Logical clock assigns a
    * logical timestamp to every node and operation. Logical timestamp consists
@@ -46,7 +64,7 @@ export class Model<N extends JsonNode = JsonNode<any>> implements Printable {
    * @deprecated Use `Model.create()` instead.
    */
   public static readonly withLogicalClock = (clockOrSessionId?: clock.ClockVector | number): Model => {
-    return Model.create(undefined, clockOrSessionId);
+    return Model.create(void 0, clockOrSessionId);
   };
 
   /**
@@ -62,7 +80,7 @@ export class Model<N extends JsonNode = JsonNode<any>> implements Printable {
    * @deprecated Use `Model.create()` instead: `Model.create(undefined, SESSION.SERVER)`.
    */
   public static readonly withServerClock = (time: number = 1): Model => {
-    return Model.create(undefined, new clock.ServerClockVector(SESSION.SERVER, time));
+    return Model.create(void 0, new clock.ServerClockVector(SESSION.SERVER, time));
   };
 
   /**
@@ -193,7 +211,7 @@ export class Model<N extends JsonNode = JsonNode<any>> implements Printable {
     const first = patches[0];
     const sid = first.getId()!.sid;
     if (!sid) throw new Error('NO_SID');
-    const model = Model.withLogicalClock(sid);
+    const model = Model.create(void 0, sid);
     model.applyBatch(patches);
     return model;
   }
@@ -436,7 +454,7 @@ export class Model<N extends JsonNode = JsonNode<any>> implements Printable {
    * @param sessionId Session ID to use for the new model.
    * @returns A copy of this model with a new session ID.
    */
-  public fork(sessionId: number = Model.sid()): Model<N> {
+  public fork(sessionId: number = this.rndSid()): Model<N> {
     const copy = Model.fromBinary(this.toBinary()) as unknown as Model<N>;
     if (copy.clock.sid !== sessionId && copy.clock instanceof clock.ClockVector)
       copy.clock = copy.clock.fork(sessionId);

src/json-crdt/model/__tests__/Model.cloning.spec.ts

@@ -165,6 +165,23 @@ describe('fork()', () => {
     expect(doc2.view()).toEqual([true, 1, 2, 'lol']);
     expect(doc1.clock.sid !== doc2.clock.sid).toBe(true);
   });
+
+  test('does not reuse existing session IDs when forking', () => {
+    const rnd = Math.random;
+    let i = 0;
+    Math.random = () => {
+      i++;
+      return i < 20 ? 0.5 : i < 24 ? 0.1 : i < 30 ? 0.5 : rnd();
+    };
+    const model = Model.create();
+    model.api.root(123);
+    const model2 = model.fork();
+    const model3 = model2.fork();
+    expect(model.clock.sid).not.toBe(model2.clock.sid);
+    expect(model3.clock.sid).not.toBe(model2.clock.sid);
+    expect(model3.clock.sid).not.toBe(model.clock.sid);
+    Math.random = rnd;
+  });
 });
 
 describe('reset()', () => {