Hi,
During some tests (on 5.3.0 and in 4.X) I noticed that for some very small files (4 bytes, all zero valued bytes), the checksums fail in TPC (with root://).
The checksum of the file is 00040001.

Looking at the logs, in non TPC mode:
xrdcp -f -d3 testfile_4B -C adler32:00040001 root://ceph-gw1.gridpp.rl.ac.uk:1094//dteam:test/jwalder/checksum_test
...
[2021-07-16 10:54:16.395552 +0100][Dump ][Utility ] Checksum for /dteam:test/jwalder/checksum_test checksum: adler32:40001
[2021-07-16 10:54:16.395586 +0100][Info ][Utility ] Checksum verification: succeeded.
...

It can be seen that the preceding "000" is not printed (although the stored checksum is correct).
In the non-TPC mode the verification is successful.

In TPC mode:
xrdcp -f -d3 --tpc delegate only -C adler32:00040001 root://golias100.farm.particle.cz:1094//dpm/farm.particle.cz/home/dteam/test/jwalder_ckstest root://ceph-gw8.gridpp.rl.ac.uk:1094//dteam:test/jwalder/checksum_test

[2021-07-16 10:58:47.215790 +0100][Dump ][Utility ] Checksum for /dteam:test/jwalder/checksum_test checksum: adler32:40001
Run: [ERROR] CheckSum error

This time there is a mismatch - I'm assuming because of the dropped 000 ?
Again, the stored checksum is still correct.

In the last case, if the checksum is requested, but the source not provided, then it works again:
xrdcp -f -d3 --tpc delegate only -C adler32 root://golias100.farm.particle.cz:1094//dpm/farm.particle.cz/home/dteam/test/jwalder_ckstest root://ceph-gw1.gridpp.rl.ac.uk:1094//dteam:test/jwalder/checksum_test

[2021-07-16 11:09:50.834772 +0100][Dump ][Utility ] Checksum for /dteam:test/jwalder/checksum_test checksum: adler32:40001
[2021-07-16 11:09:50.834798 +0100][Info ][Utility ] Checksum verification: succeeded.

I'm not sure where in the code to start looking though, but have (hopefully) attached the test file.
testfile_4B.txt

I'm sure this is a rather pathological case, but hopefully useful to report.
Thanks,
James

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