On 09/05/2013 11:36 AM, Jeff King wrote:
[...]
I haven't looked carefully at the pack v4 patches yet, but I suspect
that yes, it's still a problem. The premise of pack v4 is that we can do
better by not storing the raw git object bytes, but rather storing
specialized representations of the various components. For example, by
using an integer to store the mode rather than the ascii representation.
But that representation does not represent the "oops, I have a 0-padded
mode" quirk. And we have to be able to recover the original object, byte
for byte, from the v4 representation (to verify sha1, or to generate a
loose object or v2 pack).
There are basically two solutions:
1. Add a single-bit flag for "I am 0-padded in the real data". We
could probably even squeeze it into the same integer.
2. Have a "classic" section of the pack that stores the raw object
bytes. For objects which do not match our expectations, store them
raw instead of in v4 format. They will not get the benefit of v4
optimizations, but if they are the minority of objects, that will
only end up with a slight slow-down.
As I said, I have not looked carefully at the v4 patches, so maybe they
handle this case already. But of the two solutions, I prefer (2). Doing
(1) can solve _this_ problem, but it complicates the format, and does
nothing for any future compatibility issues. Whereas (2) is easy to
implement, since it is basically just pack v2 (and implementations would
need a pack v2 reader anyway).
3. Keep those objects in v2 packs instead of the v4 pack. Transfers
would have to be v3 or multi-pack transfers would need to be supported.
4. Don't use v4 packs with projects that have "crufty" objects. Projects
with such objects may choose to pay the "cost" to "upgrade" to v4
compatibility.
There's nothing that requires the next pack format to support all of the
broken stuff that's happened over the years.
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