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-----BEGIN PGP SIGNED MESSAGE----- |
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Hash: SHA1 |
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Hi, |
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|
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I'd like to add a new metadata cache value called DIGESTS which will |
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contain a space separated list of digests which can be |
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used to validate the metadata cache. Like INHERITED and |
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DEFINED_PHASES [1], it will be automatically generated. The first |
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digest in the list will correspond to the ebuild. If there are any |
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inherited eclasses, the digests of those eclasses will follow in a |
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space separated list, in the same order that they occur in the |
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INHERITED variable. The value of the DIGESTS variable will be on |
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line 18 of the metadata cache (just after DEFINED_PHASES). |
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|
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For the digest format, I suggest that we use the leftmost 10 |
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hexadecimal digits of the SHA-1 digest. The rationale for limiting |
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it to 10 digits (out of 40) is to save space. Due to the avalanche |
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effect [2], 10 digits should be sufficient to ensure that problems |
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resulting from hash collisions are extremely unlikely. |
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|
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The primary reason to use a digest for cache validation instead of a |
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timestamp is that it allows the cache validation mechanism to work |
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even if the tree is distributed with a protocol that does not |
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preserve timestamps, such as git or subversion. This would make it |
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possible to distribute metadata cache directly from git and |
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subversion repositories (among others). Since a digest is inherently |
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more expensive to obtain than a timestamp, package managers may use |
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the Manifest entries as a digest cache, in order to avoid the need |
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to compute digests of ebuilds during dependency calculations. |
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|
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Does the suggested approach seem reasonable? Would anybody like to |
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suggest any changes? |
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|
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[1] |
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http://archives.gentoo.org/gentoo-dev/msg_8c34d8efbc0d31ab28c517403dc83f62.xml |
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[2] http://en.wikipedia.org/wiki/Avalanche_effect |
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- -- |
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Thanks, |
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Zac |
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|
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|
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=JSDL |
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