[PATCH] fix: platform accordance while calculating murmur3

From: Chen Xuewei <316403398@qq.com>

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.

Signed-off-by: Chen Xuewei <316403398@qq.com>
---
    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(-)