TFile-s that I create (using an hadd-like script that uses TTree::MergeTrees for one of the input trees) using v6.20.08 cannot be read by v6.18.04. When opening the file I get:
Error R__unzip_header: error in header. Values: 5a53
Info in <TFile::GetStreamerInfoList>: cannot find the StreamerInfo record in file inputs/v20lbob.pr
Is this a known / expected limitation of backward compatibility?
Is there any reasonable workaround that does not force me to upgrade ROOT in sync on all relevant computers?
505 is ZSTD (level 5) which is not supported by v6_18, however it is not the default.
So we need to track down how the files ended up using ZSTD (hadd might just be re-using the compression level in the original file ; hadd can also be used to switch the compression setting).