Re: Changed path filter hash differs from murmur3 if char is signed

From: Junio C Hamano <gitster@pobox.com>
Date: May 12, 2023, 19:42 UTC
Message-ID: <xmqqbkippca5.fsf@gitster.g>
In-Reply-To: <20230512173330.1072880-1-jonathantanmy@google.com>

Jonathan Tan <jonathantanmy@google.com> writes:

Show 5 quoted lines

> Yes - if the bloom filter contained junk data (in our example, created
> using a different hash function on filenames that have characters that
> exceed 0x7f), the bloom filter would report "no, this commit does not
> contain a change in such-and-such path" and then we would skip the
> commit, even if the commit did have a change in that path.

Just to help my understanding (read: I am not suggesting this as one of the holes to exploit to help a smooth transition), does the above mean that, as long as the path we are asking about does not have a byte with the high-bit set, we would be OK, even if the Bloom filter were constructed with a bad function and there were other paths that had such a byte?

Show 7 quoted lines

> I don't have statistics on this, but if the majority of repos have
> only <=0x7f filenames (which seems reasonable to me), this might save
> sufficient work that we can proceed with bumping the version number and
> ignoring old data.
>
>> Better yet, we should be able to reuse existing Bloom filter data for
>> paths that have all characters <=0xff, and only recompute them where

"ff" -> "7f" I presume?

>> necessary. That makes much more sense than the previous paragraph.