[tahoe-dev] Hack Fest Day One report

[David-Sarah Hopwood]

On 31/03/12 21:15, Zooko Wilcox-O'Hearn wrote:
> • On the topic of issue 1., above, I suggested an alternative way to
> prevent cycling endlessly through the local filesystem. The primary
> proposal -- already partially implemented by amiller -- is to have a
> limit on how many symlinks you'll cross over before you stop. If you
> stop, you should probably print out a complete listing of how you got
> to where you are, and which steps were traversing symlinks, so that
> the user can figure out if this was really a cycle and if so how they
> want to break the cycle.
> 
> My alternative proposal is that for each directory that you visit,
> learn its device id and inode number and use those as a unique
> identifier for this directory in the context of this run of "tahoe
> backup", and if you encounter a directory for the second time then use
> the same within-tahoe-lafs-directory that you created for it the first
> time.
> 
> That's way better! It is almost elegant. But as Brian then pointed
> out, it can't work. Because all directories (except for the root dir)
> that are produced by a "tahoe backup" are immutable. You can't have a
> cycle of immutable directories, because it is impossible to get a link
> to an immutable directory before providing the initial and final
> contents of it.

What can't work is constructing a cycle of Tahoe links matching the
cycle of symlinks. But if all you want to do is detect cycles in order
to, say, output a warning and skip the link that completes the cycle,
there's no problem.

> • This led me to off-handedly comment that such a thing would be
> possible with Petrification a.k.a. revocation of write authority
> (#954) which inspired David-Sarah to announce that they were going to
> implement #954 during Hack Fest. That would be very ambitious, but
> David-Sarah often says that they will do ambitious things and then
> does them. I remember that the "Drop-Upload" feature was implemented
> in a single afternoon by David-Sarah at The Second Tahoe-LAFS Summit.

In this particular case I said I might do this ambitious thing, and
then realized there are a bunch of complicated design issues that I
wasn't going to have time to work through.

-- 
David-Sarah Hopwood ⚥

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