[ktf]

This allows reading on macOS some of the TRD CCDB objects which are stored as std::array and get serialised differently on macOS wrt Linux. I think we already complained about this in ROOT ages ago. Given we are now entering a new era, maybe they will find a solution for it with some helps from friends.

The only member of std::array<T,N> is T _M_elems[N] in libstdc++ and
T _elems[N] in libc++, so the StreamerInfo of an array written on Linux
has a checksum (0x9189320d for array<int,540>) that the macOS class does
not have (0x919bc501). When the file is opened, TStreamerInfo::BuildCheck
(ROOT 6.40.04, "perfect alias" block, since 2014) drops the on-file
StreamerInfo as an alias of the in-memory one, and the read then fails:

Error in TBufferFile::ReadVersion: Could not find the StreamerInfo with
a checksum of 0x9189320d for the class "array<int,540>" in name.
Error in TBufferFile::CheckByteCount: object of class array<int,540>
read too few bytes: 6 instead of 2166

leaving the array zero-filled. TRD/Calib/DCSDPsFedChamberStatus is such an
object: every chamber looks off, the TRD digitizer writes 0 digits, and on
macOS the full system test has no TRD and no CTF. If the in-memory
StreamerInfo already exists when the file is opened, ROOT registers the
on-file one but cannot map _M_elems, and reads zeros without any error.

Before reading an array<...> object, DataRefUtils::registerLibstdcxxStdArray
registers the on-file layout as a class version of its own, with its member
renamed to the in-memory name, so ROOT finds it by checksum and reads the
T[N] payload, identical in both libraries, as usual. It is called from both
copies of extractFromTFile: CcdbApi (CcdbApi/BasicCCDBManager, DPL CCDB
fetcher) and DataRefUtils (DPL condition inputs).

Everything is inside #if defined(_LIBCPP_VERSION): libstdc++ builds (EPN,
Grid, CI) compile exactly the same code as before.

Verified on macOS 27.2, ROOT 6.40.04-alice:

• BasicCCDBManager read of DCSDPsFedChamberStatus at 1550600800000:
  540 zeros before, 521x3 + 19x0 after, the same as decoding the payload
  by hand; at 1720000000000: 431x3, 89x4, 2x5, 18x0.
• array<string,540> (TRD/Calib/DCSDPsFedCFGtag): 540 empty strings before,
  the real tags after.
• TRD-only o2-sim-digitizer-workflow on the 10-event PbPb FST sample:
  0 digits before, 126921 after.
  Plain-ROOT reproducer for the ROOT team: ~/src/data/ccdb-stdarray-libcxx-repro.

[ktf]

See also root-project/root#17216

[ktf]

code only relevant when building with libc++. Works on mac. merging.