Karl Fogel wrote:
>> being unchanged. However, with rep-sharing, we get:
>> v |
>> +-----+ +-----+ |
>> | | delta | | delta +-+
>> | | ----> | | ----> |o|
>> | | | | +-+
>> +-----+ +-----+
>> A_1 A_2 A_3
>> Where both A_1 and A_2 remain unchanged, and A_3, instead of being a new
>> representation, just refers to the previously written identical
>> representation, A_1. This turns out to be a problem, because A_3 is
>> really just A_1, which is a delta against A_2, which is a delta against
>> A_1, which is a delta against A_2, ad nauseam.
>> See the problem?
> Well, no, wait. Under a sharing scheme, if when you add a string you
> discover it has the same (fulltext) checksum as some existing string,
> then no new deltification happens -- instead, the new string is just a
> reference to the existing string. No cycle danger here.
> In other words, both A1 and A3 would be deltas against A2.
> I didn't read the rest of the mail closely, because it seemed to be
> solving a problem whose existence I'm denying here :-).
Deltification happens in a separate phase of the commit, Karl. By the time
we perform deltification, we've completely forgotten that we recycled a
representation/string somewhere along the way and should avoid deltifying
against that object.
Trust me, this problem *does* exist. Compile the fs-rep-sharing code with
BDB support right now, and run the test suite. But, uh, save your Emacs
buffer state before doing so.
C. Michael Pilato <cmpilato_at_collab.net>
CollabNet <> www.collab.net <> Distributed Development On Demand
Received on 2008-03-27 21:23:40 CET