Migration causes failure if the specific code already exists on the engine code is migrating to

[marusshi]

Code Engine keeps track of all codes it is running in a field called codes, a mapping of the code name to Code object. When code migrates from engine n₀ to n₁, n₁'s codes field is updated to the code recovered from the snapshot

        var code = Code.fromSnapshot(kwargs.snapshot);
        self.codes[code.name] = code;

This will cause problems in an example like this:

1. Code Engine n₀ is running Foo.js
2. Code Engine n₁ is running Foo.js
3. n₁ migrates its Foo.js instance to n₀
4. n₀'s original Code object for Foo.js is overwritten and its reference is lost

This will specifically cause problems for the Scheduler because it assumes n₀ still has reference to its original Foo.js. If the next schedule is computed where n₀ should migrate its original Foo.js, an error like this will be thrown:

TypeError: Cannot read property 'snapshot' of undefined at Object.migrate_code (ThingsJS\lib\core\CodeEngine.js:269:69)

[marusshi]

I found a workaround for the SchedulerService class right now by running the code with unique code names for all components but this probably isn't optimal because it creates a new Code object every time instead of appending the new instance to the existing Code object.

[jungkumseok]

Fixed in 7f88ebd