[tahoe-lafs-trac-stream] [Tahoe-LAFS] #2780: use OneShotObserverList on magic-folder branch

[Tahoe-LAFS]

#2780: use OneShotObserverList on magic-folder branch
--------------------+---------------------------
 Reporter:  warner  |          Owner:  daira
     Type:  task    |         Status:  new
 Priority:  normal  |      Milestone:  undecided
Component:  code    |        Version:  1.11.0
 Keywords:          |  Launchpad Bug:
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 From today's devchat: refactor !StorageFarmBroker on the magic-folder
 branch to use `util.observer.OneShotObserverList` instead of passing in a
 Deferred.

 Add a method named `when_first_connected()`, which returns a Deferred that
 fires when the broker first achieves a minimum threshold of connected
 storage servers. If the method is called after that first transition, it
 will return `defer.succeed()`.

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Ticket URL: <https://tahoe-lafs.org/trac/tahoe-lafs/ticket/2780>
Tahoe-LAFS <https://Tahoe-LAFS.org>
secure decentralized storage