Unhandled promise rejection in future.js of @jupyterlab/services

[alexeckert]

Using v 2.0.3, when calling kernel.dispose() an unhandled future rejection might occur in node_modules/@jupyterlab/services/lib/kernel/future.js:155.

I've looked at the code, and it seems that there is some code which just tries to catch the rejected promise: See this._done.promise.catch(() => { /* no-op */ }); here https://github.com/jupyterlab/jupyterlab/blob/master/packages/services/src/kernel/future.ts#L162

However when I look at the transpiled future.js, this part is missing, which leads to unhandled promise rejections.

    KernelFutureHandler.prototype.dispose = function () {
        this._stdin = Private.noOp;
        this._iopub = Private.noOp;
        this._reply = Private.noOp;
        if (!this._testFlag(Private.KernelFutureFlag.IsDone)) {
            this._done.reject(new Error('Canceled'));
        }
        _super.prototype.dispose.call(this);
    };

[afshin]

This should be resolved with the merging of #4697 so the resolution will appear in our next release.

[jtpio]

I've seen something similar recently after using ClientSession to start Python kernels (on 0.34.10).

It also happens on master (tested on commit 98b4e6a).

There seems to be different ways to reproduce this by just starting consoles. Sometimes it happens right after a fresh start:

Another way would be:

1. Open a Python file
2. Create Console for Editor
3. Wait for the console to be ready
4. Refresh the whole webpage (Empty Cache and Hard Reload)
5. Wait for a few seconds

Stacktrace for reference

future.ts:174 Uncaught (in promise) Error: Canceled
    at KernelFutureHandler.dispose (future.ts:174)
    at _futures.forEach.future (default.ts:858)
    at Map.forEach (<anonymous>)
    at DefaultKernel._clearState (default.ts:857)
    at DefaultKernel.dispose (default.ts:231)
    at DefaultSession.dispose (default.ts:218)
    at editorTracker.forEach.file (index.ts:201)
    at algorithm_1.each.widget (instancetracker.ts:268)
    at Object.each (iter.js:60)
    at InstanceTracker.forEach (instancetracker.ts:267)