From: Junio C Hamano <gitster@pobox.com>
To: Jonathan Tan <jonathantanmy@google.com>
Cc: git@vger.kernel.org, derrickstolee@github.com
Subject: Re: Changed path filter hash differs from murmur3 if char is signed
Date: Thu, 11 May 2023 15:51:16 -0700 [thread overview]
Message-ID: <xmqqjzxetrbv.fsf@gitster.g> (raw)
In-Reply-To: <20230511224101.972442-1-jonathantanmy@google.com> (Jonathan Tan's message of "Thu, 11 May 2023 15:40:59 -0700")
Jonathan Tan <jonathantanmy@google.com> writes:
> So...how do we proceed? I can see at least 2 ways:
>
> - Decide that we're going to stick with the details of the existing
> implementation and declare that "data" is always interpreted as signed.
> In that case, I would put "signed" wherever necessary, rename the
> function to something that is not "murmur3", and change the names of
> byte1 etc. to indicate that they are not constrained to be less than or
> equal to 0xff.
>
> - Bump the version number to 2 and correct the implementation to
> match murmur3 (so, "data" is unsigned). Then we would have to think of
> a transition plan. One possible one might be to always reject version
> 1 bloom filters, which I'm personally OK with, but it may seem too
> heavy a cost to some since in the perhaps typical case where a repo has
> filenames restricted to 0x7f and below, the existing bloom filters are
> still correct.
If path filter hashing were merely advisory, in the sense that if a
matching data is found, great, the processing goes faster, but if
not, we would get correct results albeit not so quickly, a third
option would be to just update the implementation without updating
the version number. But we may not be so lucky---you must have seen
a wrong result returned quickly, which is not what we want to see.
But if I recall correctly we made the file format in such a way that
bumping the version number is cheap in that transition can appear
seamless. An updated implementation can just be told to _ignore_
old and possibly incorrect Bloom filters until it gets told to
recompute, at which time it can write a correct one with a new
version number. So I would prefer your "Bump the version number and
ignore the old and possibly wrong data".
Thanks.
next prev parent reply other threads:[~2023-05-11 22:51 UTC|newest]
Thread overview: 7+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-05-11 22:40 Changed path filter hash differs from murmur3 if char is signed Jonathan Tan
2023-05-11 22:51 ` Junio C Hamano [this message]
2023-05-11 23:10 ` Taylor Blau
2023-05-12 17:33 ` Jonathan Tan
2023-05-12 19:42 ` Junio C Hamano
2023-05-12 20:54 ` Jonathan Tan
2023-05-12 21:27 ` Taylor Blau
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