[tahoe-lafs-trac-stream] [Tahoe-LAFS] #2823: "tahoe cp FILE ALIAS:" does unnecessary directory reads

[Tahoe-LAFS]

#2823: "tahoe cp FILE ALIAS:" does unnecessary directory reads
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 Reporter:  warner                |          Owner:
     Type:  defect                |         Status:  new
 Priority:  normal                |      Milestone:  undecided
Component:  code-frontend-cli     |        Version:  1.11.0
 Keywords:  performance tahoe-cp  |  Launchpad Bug:
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 I noticed that running `tahoe cp FILE my:` (where `my:` is an alias
 pointing at a directory with about 20 subdirectories) performs 20
 directory reads before finally doing the expected upload and read+write
 assignment to the top-level directory. The CLI frontend is doing 20
 separate WAPI `GET /uri/CAP?t=json` for each subdirectory.

 It looks like targetting `ALIAS:` (rather than a fully-qualified
 `ALIAS:FILE`) is putting tahoe-cp in a mode where it finishes with a WAPI
 `POST /uri/ALIAS?t=set_children`. To do that, it wants to have the full
 contents of the target directory. However I think it should be able to get
 that information from the initial GET, rather than needing to fetch the
 contents of each subdirectory.

 I don't remember why we're using `set_children` anyways. But I do remember
 an extensive investigation into how tahoe-cp ought to behave when the
 target is a directory (e.g. `tahoe cp DIR ALIAS:` or `tahoe cp DIR
 ALIAS:SUBDIR`). There might be a way to simplify this by transforming
 `tahoe cp X ALIAS:` into `tahoe cp X ALIAS:X`. Or at least an argument
 that the two ought to behave the same way.

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Ticket URL: <https://tahoe-lafs.org/trac/tahoe-lafs/ticket/2823>
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