Setting admins with couchConfig at database creation crashes

[QuentinRoy]

It is often a good practice to always setup an admin of a database, i.e. at db creation, c.f. #239.
Unfortunately, the following crashes the first time the server is launches (even if one doesn't wait for the callback):

// Create the app.
const app = express();
const expressPouchDB = ExpressPouchDB(PouchDB);
// Set up the admin.
await new Promise((resolve, reject) => {
  expressPouchDB.couchConfig.set(
    'admins',
    config.adminName,
    config.adminPassword,
    err => {
      if (err) reject(err);
      else resolve()
    }
  )
});
// Go.
app.use(expressPouchDB);
app.listen(config.port);

with:

TypeError: refreshUsersDBImpl is not a function
    at CouchConfig.refreshUsersDB (/Users/qroy/Workspace/Code/project/node_modules/express-pouchdb/lib/routes/authentication.js:74:12)
    at CouchConfig.emit (events.js:182:13)
    at /Users/qroy/Workspace/Code/project/node_modules/express-pouchdb/lib/config-infrastructure.js:136:10
    at FSReqWrap.oncomplete (fs.js:145:20)

Subsequent launches would work however. It seems the issue is due to the _users database not being ready yet when the admin is set. Unfortunately, some process still try to update it without waiting.
A somewhat ridiculous workaround is to do this:

// Create the app.
const app = express();
const expressPouchDB = ExpressPouchDB(PouchDB);
// Wait for the _users database to be ready 😔
// (I've tried `await DB('_users').info()` instead without success).
await DB('_users')
  .get('whatever')
  .catch(() => {});
// Set up the admin.
await new Promise((resolve, reject) => {
  expressPouchDB.couchConfig.set(
    'admins',
    config.adminName,
    config.adminPassword,
    err => {
      if (err) reject(err);
      else resolve()
    }
  )
});
// Go.
app.use(expressPouchDB);
app.listen(config.port);

[marten-de-vries]

Thanks for reporting. Sadly, I don't have time to fix this myself currently, but this is what's going on:

The problem is that refreshUsersDBImpl is only assigned after pouchdb-auth is set up: https://github.com/pouchdb/pouchdb-server/blob/master/packages/node_modules/express-pouchdb/lib/routes/authentication.js#L41

The easiest workaround would probably be to let https://github.com/pouchdb/pouchdb-server/blob/master/packages/node_modules/express-pouchdb/lib/routes/authentication.js#L74 retry calling refreshUsersDBImpl on the next tick if it currently fails. The hack of awaiting the _users db only works by coincidence, I think. What's important is that some time simply passes.

Calling on the next tick could be done using process.nextTick, or using promises.

PRs welcome!