Re: [PATCH] git tag --contains : avoid stack overflow

[Jeff King <peff@peff.net>]

On Sun, Nov 11, 2012 at 05:46:32PM +0100, René Scharfe wrote:

> >However, I couldn't reproduce it on Linux : where the windows
> >implementations crashes at a ~32000 depth (*not* exactly 32768, mind
> >you), on linux it happily went through 100000 commits. I didn't take
> >time to look much further, but maybe on my 64 bit Linux VM, the process
> >can afford to reserve a much bigger address range for the stack of each
> >thread than the 1Mb given to 32 bit processes on windows.
> >Jean-Jacques.
> 
> I can reproduce it on Linux (Debian testing amd64) with ulimit -s
> 1000 to reduce the stack size from its default value of 8MB.
> 
> After reverting ffc4b8012d9a4f92ef238ff72c0d15e9e1b402ed (tag: speed
> up --contains calculation) the test passes even with the smaller
> stack, but it makes "git tag --contains" take thrice the time as
> before.

Right, I am not too surprised.  That function replaced the original
algorithm with a much faster depth-first recursive one. I haven't looked
closely yet at Jean-Jacques' iterative adaptation, but that direction
seems like a good fix for now.

Ultimately, I have some ideas for doing this in a breadth-first way,
which would make it more naturally iterative. It would involve having N
bits of storage per commit to check N tags, but it would mean that we
could get accurate answers in the face of clock skew (like the
merge-base calculation, it would merely get slower in the face of skew).

But since I haven't worked on that at all, fixing the depth-first
algorithm to be iterative makes sense to me.

-Peff

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