Plugin cyclical check ignores optional plugins

[vidartf]

There was recently an issue on the jupyterlab repo, where a cyclical plugin dependency was reported. The error ended up being reported as a Maximum call stack size exceeded. It turns out that the reason why the phosphor cycle check did not catch it was because one of the parts of the loop was an optional dependency.

Relevant code at time of submission:

phosphor/packages/application/src/index.ts

Lines 692 to 725 in 0d84258

	function ensureNoCycle(data: IPluginData, pluginMap: PluginMap, serviceMap: ServiceMap): void {
	// Bail early if there cannot be a cycle.
	if (!data.provides || data.requires.length === 0) {
	return;
	}
	// Setup a stack to trace service resolution.
	let trace = [data.id];
	// Throw an exception if a cycle is present.
	if (data.requires.some(visit)) {
	throw new Error(`Cycle detected: ${trace.join(' -> ')}.`);
	}
	function visit(token: Token<any>): boolean {
	if (token === data.provides) {
	return true;
	}
	let id = serviceMap.get(token);
	if (!id) {
	return false;
	}
	let other = pluginMap[id];
	if (other.requires.length === 0) {
	return false;
	}
	trace.push(id);
	if (other.requires.some(visit)) {
	return true;
	}
	trace.pop();
	return false;
	}
	}