Re: fix git-pack-redundant crashing sometimes

From: Junio C Hamano <junkio@cox.net>
Date: Nov 16, 2005, 20:13 UTC
Message-ID: <7vlkzo73m1.fsf@assigned-by-dhcp.cox.net>
In-Reply-To: <Pine.LNX.4.64.0511151552400.11232@g5.osdl.org>

Linus Torvalds <torvalds@osdl.org> writes:

Show 8 quoted lines

> On Tue, 15 Nov 2005, Alex Riesen wrote:
>> 
>> Sorry, it doesn't. The code loops here:
>> 
>> 401             /* find the permutations which contain all missing objects */
>> 402             perm_all = perm = get_all_permutations(non_unique);
>
> Looks like the whole thing is exponential.

This reminds me of one thing I have been meaning to remove.

The $GIT_OBJECT_DIRECTORY/info/pack file produced by update-server-info records not just the packs (P lines), but its dependencies (D lines) and top objects (T lines). If there are more than one packs, D lines are meant to help the downloader to figure out what other packs are needed to complete.

"meant to" is the keyword here. The original heuristics was very bad (still correct but way suboptimal) and it has never been updated. I do not use that information myself in http fetch, and I do not think any Porcelain uses it.

Top objects are tags and commits in the pack that are not reachable from any other object in the same pack. This can be useful if you obtain just a packfile -- you can run server-info to produce info/pack file, and resurrect the branch heads and tags by using them --- you would not know what they are called, though. I have been thinking it may be worthwhile to also record bottom objects (B lines, naturally) --- tags and commits in the pack that refer to tags or commits not present in the same pack. This would help commit walkers to decide which pack is more efficient to slurp when faced with multiple choices. If you see a pack all whose bottom objects you have in your repository, fetching that would complete the commit walking, so the choice would become the matter of choosing such a pack among multiple that has smallest number of objects you do not have locally -- you would have the index file for each pack at that point.