Re: [WISH] Store also tag dereferences in packed-refs

From: Jakub Narebski <jnareb@gmail.com>
Date: Nov 20, 2006, 09:40 UTC
Message-ID: <ejrt37$fg9$1@sea.gmane.org>
In-Reply-To: <7vr6vy7smi.fsf@assigned-by-dhcp.cox.net>

Junio C Hamano wrote:

Show 13 quoted lines

> Linus Torvalds <torvalds@osdl.org> writes:
> 
>> So I'd suggest adding - at the very top of the ref-pack file - a line line
>>
>>      # Ref-pack version 2
>>
>> which will be ignored by the current ref-pack reader (again, because it's 
>> not a valid ref line), but we can use it in the future to specify further 
>> extensions if we want to.
>>
>> Now somebody would just need to implement that ;)
> 
> For this particular one, there is no need for version 2.

Actually, I think it is both true and untrue. True, because we need some indicator that we trust packed-refs file to provide tag dereferences to distinguish between the case when there are no tag objects at all, so there are no tag dereferences in packed-refs, and the situation where we use packed-refs generated by older git, and there are no tag dereferences in packed-refs because git didn't saved it.

Untrue, because it is not enough. In the case[*1*] when packed-refs was created with tag dereferences, then some "heavyweight" tags were added by older version of git (adding references doesn't rewrite packed-refs if I understand correctly), then we use new git again and trust that there are no derefs...

[*1*] For example when git repository is on the network filesystem, but
programs are installed locally, and perhaps computers in the network are
heterogenic (perhaps even different architectures: PC vs. Sun and/or
different operating systems: Linux vs. FreeBSD vs. Solaris vs.
Windows+Cygwin) and have different versions of git installed (perhaps
one of them is "your" machine, where you have admin rights, and you have
newest git installed there). Or for example using git repository on USB
stick, again on different computers with different version of git installed.
 
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To summarize, we have the following proposals of the packed-refs format extension

The unusable Linux Torvalds proposal (unusable because of requiring newer packed-refs work with older git, for example in the case [*1*] or the case of git downgrade):

lt>    <sha1><space><name>[<space><sha1-of-deref>]*

Linus Torvalds "Now somebody would just need to implement that ;)" proposal:

lt> <sha1> refs/tags/tagname lt> ^<sha1-unpeeled> lt> ^<sha1-unpeeled-of-unpeeled> lt> ... lt> <sha1> refs/tags/othertag

Junio C Hamano proposal _with code_ (proposal with code usually wins). Less elegant IMVHO, but perhaps better.

jc> My current wip does: jc> jc> SHA-1 SP name LF jc> SHA-1 SP SP name^{} LF jc> jc> the latter of which is ignored by code in the wild and the new jc> code can take advantage of (and fall back the usual deref_tag jc> when it is not available).

-- 
Jakub Narebski
Warsaw, Poland
ShadeHawk on #git