Conflicting changes cause undefined behavior in Transformer during fork synchronization

[mindaugasdirg]

In iModel forking scenarios both source and target iModel can be modified and this also means that some changes can be conflicting like updating element in one iModel and deleting it in another. Currently, imodel-transformer ignores all conflicts and tries to iterate iModel like conflicts do not exist, which can cause weird and unexpected behaviors.

I think there is a need to decide how transformer should behave in case of conflicts and add it to the package.

[mindaugasdirg]

@JulijaRamoskiene found one such case which causes infinite loop in the IModelTransformer.

 it("should successfully process changes when child element has changes in source and it's parent was deleted in target", async () => {
    const sourceIModelName: string =
      IModelTransformerTestUtils.generateUniqueName("Source");
    const targetIModelName: string =
      IModelTransformerTestUtils.generateUniqueName("Target");
    const sourceIModelId = await HubWrappers.recreateIModel({
      accessToken,
      iTwinId,
      iModelName: sourceIModelName,
      noLocks: true,
    });
    const targetIModelId = await HubWrappers.recreateIModel({
      accessToken,
      iTwinId,
      iModelName: targetIModelName,
      noLocks: true,
    });
    const sourceDb = await HubWrappers.downloadAndOpenBriefcase({
      accessToken,
      iTwinId,
      iModelId: sourceIModelId,
    });
    const targetDb = await HubWrappers.downloadAndOpenBriefcase({
      accessToken,
      iTwinId,
      iModelId: targetIModelId,
    });

    const changes1ParentSubjectId = Subject.insert(
      sourceDb,
      IModel.rootSubjectId,
      "Change 1: Parent"
    );
    const subjectChildId = Subject.insert(sourceDb, changes1ParentSubjectId, "Change 1: Child");
    sourceDb.saveChanges();
    await sourceDb.pushChanges({ description: "change 1" });

    // process change 1
    let transformer = new IModelTransformer(sourceDb, targetDb);
    await transformer.process();
    transformer.updateSynchronizationVersion({initializeReverseSyncVersion: true});
    targetDb.saveChanges();

    // Update source iModel
    const subjectToUpdate = sourceDb.elements.getElementProps(subjectChildId);
    subjectToUpdate.userLabel = "Change 1: Child Updated";
    sourceDb.elements.updateElement(subjectToUpdate);
    sourceDb.saveChanges();
    await sourceDb.pushChanges({ description: "change 2" });

    // Update target iModel
    const changes1ParentSubject = sourceDb.elements.getElement(
      changes1ParentSubjectId
    );
    const targetChanges1ParentSubject = targetDb.elements.getElement(
      changes1ParentSubject.federationGuid!
    );
    targetDb.elements.deleteElement([targetChanges1ParentSubject.id]);
    // process change 2
    transformer = new IModelTransformer(sourceDb, targetDb, {
      argsForProcessChanges: {},
    });
    await expect(transformer.process()).to.be.eventually.fulfilled;
    // targetDb.saveChanges();

    // const queryReader = targetDb.createQueryReader(
    //   `SELECT COUNT(*) FROM ${Subject.classFullName}`
    // );
    // await queryReader.step();
    // const subjectCount = queryReader.current.toArray()[0];
    // expect(subjectCount).to.equal(3); // RootSubject + 2 created subjects
  });