On Fri, 13 Sep 2013, Nicolas Pitre wrote:
> On Fri, 13 Sep 2013, Duy Nguyen wrote:
> > On Fri, Sep 13, 2013 at 8:27 PM, Nicolas Pitre <n...@fluxnic.net> wrote:
> > > However I can see that, as you say, the same base object is repeatedly
> > > referenced. This means that we need to parse it over and over again
> > > just to find the right offset where the needed entries start. We
> > > probably end up skipping over the first entries in a tree object
> > > multiple times. And that would be true even when the core code learns
> > > to walk pv4 trees directly.
> > >
> > > So here's the beginning of a tree offset cache to mitigate this problem.
> > > It is incomplete as the cache function is not implemented, etc. But
> > > that should give you the idea.
> > Thanks. I'll have a closer look and maybe complete your patch.
> I've committed an almost final patch and pushed it out. It is disabled
> right now due to some bug I've not found yet. But you can have a look.
Found the bug.
The cache is currently updated by the caller. The caller may ask for a
copy of 2 entries from a base object, but that base object may itself
copy those objects from somewhere else in a larger chunk.
Let's consider this example:
0 (0) copy 2 entries from tree B starting at entry 0
1 (2) copy 1 entry from tree B starting at entry 3
0 (0) copy 6 entries from tree C starting at entry 0
1 (6) entry "foo.txt"
2 (7) entry "bar.txt"
Right now, the code calls decode_entries() to decode 2 entries from tree
B but those entries are part of a copy from tree C. When that call
returns, the cache is updated as if tree B entry #2 would start at
offset 1 but this is wrong because offset 0 in tree B covers 6 entries
and therefore offset 1 is for entry #6.
So this needs a rethink.
I won't be able to work on this for a few days.
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