express-pouchdb allocates infinite memory when given a defunct PouchDB

[ben0x539]

var http = require('http');
var PouchDB = require('pouchdb');

var app = require('express-pouchdb')(PouchDB.defaults({
  prefix: './non-existent-path/',
}))

app.listen(3000, function() {
  http.get('http://localhost:3000/db/hello'); // delayed explosion
});

(node:13549) UnhandledPromiseRejectionWarning: OpenError: IO error: ./non-existent-path/_replicator/LOCK: No such file or directory

Despite the warning, we continue on with a non-functional pouchdb object backing those routes. When trying to handle a request, this bit of code gets stuck forever while allocating more and more promises:

pouchdb-server/packages/node_modules/express-pouchdb/lib/routes/authentication.js

Lines 55 to 65 in ef24d7d

	function getUsersDB() {
	// calls itself until usersDBPromise is a available
	if (!usersDBPromise) {
	return new Promise(function (resolve) {
	setImmediate(function () {
	resolve(getUsersDB());
	});
	});
	}
	return usersDBPromise;
	}

After busylooping while growing to a few gigabytes in size, node gets fed up and terminates.

This could probably be mitigated by only using one promise there and only repeating the setImmediate (or even setTimeout) call, but ultimately we should probably just be erroring out earlier and not let it come that far.

It's a very low impact crash that's easily avoided, but jumping into an unfamiliar codebase and not realizing the significance of the above warning, it took me a while to catch on. :)