fix: platform accordance while calculating murmur3

From: Chen Xuewei <[email protected]>

It is known that whether the highest bit is extended when char cast to uint32, depends on CPU architecture, which will lead different hash value. This is a fix to accord all architecture behaviour.

---
fix: platform accordance while calculating murmur3

Short Description
=================

fix: platform accordance while calculating murmur3

It is known that whether the highest bit is extended when char cast to uint32, depends on CPU architecture, which will lead different hash value. This is a fix to accord all architecture behaviour.

Problem backgroud:
==================

when using git log --max-count=1 <commit> -- <path> in an mixed cpu cluster environment both arm and x86 in a cluster as a service, where the <path> character is chinese or some other character that the highest bit of char is 1. all machines share the same repo disk. It happened that sometimes you can get the searched file among commit, sometimes you cannot.

Conditions
==========

1. file path include chinese characters or other characters that the highest bit is 1.
2. mixed cpu architecture as a git cluster service

Reason
======

when you have over 2 machines (both arm and x86 are included at least one) as a git server cluster. once you open the commit-graph's bloom_filter feature. The bloom filter stores the file path as hash values using the murmur3 function. suppose the arm take it this time, then the char's highest bit is not extended. for example, on arm, char(11100110) to uint32(00000000 00000000 00000000 11100110) on x86, char(11100110) to uint32(11111111 11111111 11111111 11100110) then according to the murmur3 function that git currently use, the calculated hash value will be different. If the value was calculated through the same cpu architure machine, then it is ok. however, sometimes the hash value is calculated through a different cpu architure machine, then you cannot get the searched file. for example, bloom_filter's hash set is calculated through arm, and query through x86. So the hash value is incorrect, then missed the searched file.

Solution
========

No matter what the highest 24 bits will be when char cast to uint32, the murmur3 function only cares about the char part , which is only the lowest 8 bits, so we can use & 0xFF(11111111) to the casted uint32 value to choose only the lowest 8 bits.

Others
======

after fixed the bug, the historical bloom_filter data stored in commit-graph need to be updated. because the path's hash value is already calculated through a bad way. so we need to update it. this need to be done in repository

Published-As: https://github.com/gitgitgadget/git/releases/tag/pr-git-1636%2Fcdegree%2Fmaster-v1
Fetch-It-Via: git fetch https://github.com/gitgitgadget/git pr-git-1636/cdegree/master-v1
Pull-Request: https://github.com/git/git/pull/1636

 bloom.c | 10 +++++-----
 1 file changed, 5 insertions(+), 5 deletions(-)

index 1474aa19fa5..bc40edac795 100644
@@ -116,11 +116,11 @@ uint32_t murmur3_seeded(uint32_t seed, const char *data, size_t len)
 
 	uint32_t k;
 	for (i = 0; i < len4; i++) {
-		uint32_t byte1 = (uint32_t)data[4*i];
-		uint32_t byte2 = ((uint32_t)data[4*i + 1]) << 8;
-		uint32_t byte3 = ((uint32_t)data[4*i + 2]) << 16;
-		uint32_t byte4 = ((uint32_t)data[4*i + 3]) << 24;
-		k = byte1 | byte2 | byte3 | byte4;
+		uint32_t byte1 = ((uint32_t)data[4*i]) & 0xFF;
+		uint32_t byte2 = ((uint32_t)data[4*i + 1]) & 0xFF;
+		uint32_t byte3 = ((uint32_t)data[4*i + 2]) & 0xFF;
+		uint32_t byte4 = ((uint32_t)data[4*i + 3]) & 0xFF;
+		k = byte1 | (byte2 << 8) | (byte3 << 16) | (byte4 << 24);
 		k *= c1;
 		k = rotate_left(k, r1);
 		k *= c2;

base-commit: a26002b62827b89a19b1084bd75d9371d565d03c

gitgitgadget

Hi Chen,

Thanks for your patch. A similar fix is being pursued in [1], part of which includes [2], which I believe is functionally equivalent to your patch here.

We would not want to impose that burden on all users upon upgrading to the latest Git version. In [1] we are perusing an approach where:

- The Bloom data is stored with a version identifier, meaning that we can still use the existing/non-murmur3 Bloom filters after upgrading.

- When the user decides to upgrade from v1 -> v2 Bloom filters, we reuse the existing Bloom filter data when possible, namely when all paths within a tree have no non-ASCII characters.

If you have thoughts on the approach in [1], they would be most welcome.

Thanks,
Taylor

[1]: https://lore.kernel.org/git/[email protected]/
[2]: https://lore.kernel.org/git/f6ab427ead86bc82284b2c721f3c177947ece3c9.1697653929.git.me@ttaylorr.com/

Jonathan and Taylor, isn't this what you two were working together on? How would we want to proceed?

Chen, using the right implementation of the hash function to be used after the next rebuild of the Bloom data has so far been treated as only one part of the solution (the others include "how to deal with the existing data? do we have a way to tell our binary to safely ignore the Bloom data using a wrong hash?" and "how to make sure old binaries do not get confused by the Bloom data using the right/new hash?").

Jonathan and Taylor's (stalled) effort is here

    https://lore.kernel.org/git/[email protected]

Thanks.

They are indeed similar. I think that Jonathan and my series would supersede this effort.

But I would appreciate if Chen took a look at the approach in that series to make sure that we're all on the same page and that Jonathan and I aren't missing anything.

Thanks,
Taylor