Re: [PATCH] Implement fast hash-collision detection

On Tue, Nov 29, 2011 at 05:17, Nguyen Thai Ngoc Duy [off-list ref] wrote:

On Tue, Nov 29, 2011 at 4:07 PM, Jeff King [off-list ref] wrote:

quoted

However, it's harder for trees. Each entry needs to have the new digest
added, but there simply isn't room in the format. So we would have to
change the tree format, breaking interoperability with older versions of
git. And all of your tree sha1's would change as soon as you wrote them
with the new format. That's only slightly better than just swapping
sha1 out for a better algorithm.

I think we could hide the new digest after NUL at the end of path
name. C Git at least does not seem to care whatever after NUL.

No, you can't. The next byte after the NUL at the end of a path name
is the SHA-1 of that entry. After those 20 bytes of SHA-1 data is
consumed, the "mode SP name\0" of the next entry is parsed.

There is not room in the tree format for any additional data. Period.
At best you can modify the mode value to be a new octal code that is
not recognized by current Git implementations. But that doesn't get
you a whole lot of data, and its pretty risky change because its
rather incompatible.