We now have two (background) Firefox Account consumers, Sync and RL. Due to time pressure, we haven't implemented a nice API for them both to update the account state; instead, they both try to own the state, resulting in races. The effect is most noticeable when an account is created or re-connected: it's frequent that both consumers will try to fetch keys using the same keyFetchToken; one will win the race and proceed to start syncing; the other will fail to fetch keys and drive the account to Separated (in order to fetch keys). Result: a terrible user experience. As a step towards a better API, we need to protect the Account state with a critical section. Eventually, we transition Sync to requesting token server tokens from the FxAccountAuthenticator using Android's APIs, which makes this much cleaner (since there's then a single consumer to handle locking).

rnewman: how do you feel about this? It's working well locally. I intended to include telemetry for how long we wait and how long we hold the lock, but I then observed (and explained in the second commit comment) that we should very rarely wait.

Backed out for bustage. https://hg.mozilla.org/integration/fx-team/rev/cb309087f978 https://treeherder.mozilla.org/logviewer.html#?job_id=2452077&repo=fx-team