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From: Junio C Hamano <junkio@cox.net>
Subject: Re: [PATCH 4/5] allow deepening of a shallow repository
Date: Fri, 17 Nov 2006 03:35:58 -0800
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	(Johannes Schindelin's message of "Fri, 17 Nov 2006 10:52:54 +0100
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To: Johannes Schindelin <Johannes.Schindelin@gmx.de>
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Johannes Schindelin <Johannes.Schindelin@gmx.de> writes:

>> Although I am reasonably sure we can eventually make it work, it
>> is very subtle and fragile -- somebody touching this code can
>> easily break it.
>
> I fully agree. Even the OA did not understand the code fully ;-)
>
> How about adding "int force_reparse" to the signature of 
> unregister_shallow()? (Just like we added "int cleanup" to 
> get_merge_bases() to avoid pilot errors.)

I do not think that's where its fragility lies.  It is that any
random place can later call parse_object() on the commit, after
you elaborately pre-parsed it with shallow so that it appears to
have fewer parents, with the expectation that nobody ever would
clear the parsed bit and cause it to be reparsed again.  With
that assumption, find_common() manipulated the shallow entry
after setting that scheme up.  A mechanism to prevent the commit
from getting re-parsed would have made it more robust.