Re: [gentoo-dev] [RFC] DIGESTS metadata variable for cache validation

[Zac Medico <zmedico@gentoo.org>]

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Tiziano Müller wrote:
> Am Montag, den 02.02.2009, 12:34 -0800 schrieb Zac Medico:
>> For the digest format, I suggest that we use the leftmost 10
>> hexadecimal digits of the SHA-1 digest. The rationale for limiting
>> it to 10 digits (out of 40) is to save space. Due to the avalanche
>> effect [2], 10 digits should be sufficient to ensure that problems
>> resulting from hash collisions are extremely unlikely.
> I'd recommend to prefix the digest with a "{TYPE}" (like for hashed
> passwords) to be able to change the digest algorithm as needed
> (especially in regards to the current SHA successor competition).
> This allows a future package manager which might use SHA-3 for hashing
> (once it's released) to still check old digests. Furthermore it would
> allow for easier transition and only needs a definition of allowed
> hashes instead of a specific one.

I like that idea. That way it's not necessary to bump the EAPI in
order to change the hash function. So, a typical DIGESTS value might
look like this:

SHA1 02021be38b a28b191904 3992945426 6ec21b29a3

>> The primary reason to use a digest for cache validation instead of a
>> timestamp is that it allows the cache validation mechanism to work
>> even if the tree is distributed with a protocol that does not
>> preserve timestamps, such as git or subversion. This would make it
> Well, usually you don't keep intermediate or generated files in a VCS,
> so why the metadata?

People who distribute overlays commonly ask if it's possible to
distribute metadata cache with the overlay. Using a format that
doesn't rely on timestamps will allow them to distribute metadata
cache using their existing infrastructure, which is typically git or
subversion. In addition to overlays, it would also be useful for
forks of the entire gentoo tree, such as the funtoo tree [1].

[1] http://github.com/funtoo/portage/tree/master
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Thanks,
Zac
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