Comment 3 for bug 1934992

Hi Wayne! Thanks for commenting.

> It's only the 128-bit hash that depends on 0.8.0.
> The 0.7 version works fine with rsync, giving it
> the 64-bit and 32-bit hashes.

Yes. Except it seems that if you switch the libxxhash0 from 0.8 to 0.7, you get different behaviour.

rsync doesn't check what kind of xxh128 is produced, so we end up with a mismatch.

Steps to reproduce:

  wget -q https://launchpad.net/ubuntu/+archive/primary/+files/rsync_3.2.3-2ubuntu1_amd64.deb

  wget -q https://launchpad.net/ubuntu/+archive/primary/+files/libxxhash0_0.7.3-1_amd64.deb

  wget -q https://launchpad.net/ubuntu/+archive/primary/+files/libxxhash0_0.8.0-1ubuntu1.20.10.1_amd64.deb

focal-node-1:

  sudo dpkg -i libxxhash0_0.7.3-1_amd64.deb \
    rsync_3.2.3-2ubuntu1_amd64.deb
  touch empty-file.txt
  echo A > non-empty-file.txt

focal-node-2:

  sudo dpkg -i rsync_3.2.3-2ubuntu1_amd64.deb \
    libxxhash0_0.8.0-1ubuntu1.20.10.1_amd64.deb
  rsync -v --debug=nstr \
    focal-node-1:*empty-file.txt \
    .

Result:

  Client negotiated checksum: xxh128
  empty-file.txt
  WARNING: empty-file.txt failed verification
    -- update discarded (will try again).
  non-empty-file.txt
  WARNING: non-empty-file.txt failed verification
    -- update discarded (will try again).
  empty-file.txt
  ERROR: empty-file.txt failed verification
    -- update discarded.
  non-empty-file.txt
  ERROR: non-empty-file.txt failed verification
    -- update discarded.

  sent 104 bytes received 255 bytes 239.33 bytes/sec
  total size is 2 speedup is 0.01
  rsync error: some files/attrs were not transferred
    (see previous errors) (code 23) at main.c(1816)
    [generator=3.2.3]

focal-node-2:

  $ ls *empty*
  ls: cannot access '*empty*': No such file or directory

I don't mind if I don't get xxh128 and get some poorer hash. But I _do_ mind if I get a hash that produces different results.

If I install libxxhash0 0.7.3 on both: things work.

If I install libxxhash0 0.8.x on both: things work.

But when there is a mismatch, things break. And uselessly too. I ended up syncing lots of GBs multiple times because our job kept retrying.

I hope that clarifies the situation.

Walter

P.S. Alternative solutions could be:
- not exporting xxh128 functions from libxxhash0 0.7.3 (but it might be a bit late for that);
- checking that xxh128 produces sane values in rsync before choosing that option.