[tahoe-dev] Tahoe Access Control

[Brian Warner]

On 6/3/11 11:45 AM, Zooko O'Whielacronx wrote:
> On Fri, Jun 3, 2011 at 4:28 AM, Greg Troxel <gdt at ir.bbn.com> wrote:
>>
>> But, in wiki:Capabilities, it says that a directory is just a mutable
>> file with special interpretation.
> 
> This is perhaps a misleading statement. The "special interpretation"
> includes an added layer of decryption. Probably that statement should
> just be struck from wiki:Capabilities.

Hm, I kinda disagree. The "special interpretation" means parsing the
file's contents to build a mapping from child names to child caps, then
providing a different set of methods (dirnodes have
list/get/add/replace/remove, files have read/write). (ok, plus the extra
decryption, but that's less significant than the table-parsing).

Maybe we should clarify it to explain that directories are *backed* by
files. I remember using that "just a mutable file" phrase to teach the
reader that they didn't need to be afraid of dirnodes; that they were
re-using existing technology (which the reader would already understand,
having read about mutable files in the previous chapters), and only
emphasizing the new parts (the layer of interpretation that makes
dirnodes special).


>> but I expected to get the directory contents.
> 
> Change the URI from URI:DIR2etc. to URI:SSKetc.

Oh, I just thought of a precedent for this *not* working as you
expected. Once upon a time in the UNIX world, when directories were
fairly new, you could 'cat' or 'vi' a directory, and you'd get a
weird-looking file with some filenames and some other binary gunk. In
theory you could probably edit that "file" to modify the directory, but
nobody sane would do that directly. Only tools like "mv" did stuff like
that, and eventually that functionality was moved down into the kernel
and the filesystem code.

Later, when the authors wised up and realized that allowing this sort of
pseudo-casting wasn't helping anyone but filesystem hackers, they made
it an error to use open() on a directory. Tahoe's webapi behaves like
the latter/wiser UNIX interface.

cheers,
 -Brian