[tahoe-lafs-trac-stream] [tahoe-lafs] #1877: SFTP: use new eventual utilities from deferredutil

[tahoe-lafs]

#1877: SFTP: use new eventual utilities from deferredutil
-------------------------------------------------+-------------------------
 Reporter:  davidsarah                           |          Owner:
     Type:  defect                               |  davidsarah
 Priority:  minor                                |         Status:  new
Component:  code-frontend                        |      Milestone:  1.11.0
 Keywords:  sftp deferred eventual cloud-        |        Version:  1.9.2
  backend leasedb easy cleanup                   |  Launchpad Bug:
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 The cloud backend + leasedb merge adds several deferred utilities:
 [https://github.com/davidsarah/tahoe-
 lafs/commit/7671b3e33f0c91e764fb869175ff].

 {{{eventually_callback}}} and {{{eventually_errback}}} were modelled on
 the versions already in sftpd.py, with an error-reporting improvement.
 eventual_chain is an abstraction of the pattern
 {{{
 source.addCallbacks(eventually_callback(target),
 eventually_errback(target))
 }}}
 which is used twice in sftpd.py. So we should change sftpd to use these
 versions, once the cloud backend + leasedb merge has landed.

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Ticket URL: <https://tahoe-lafs.org/trac/tahoe-lafs/ticket/1877>
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