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CCDB: also read std::array members written by libstdc++ on libc++ builds #16127

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ktf merged 1 commit into from
Nov 15, 2026
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CCDB: also read std::array members written by libstdc++ on libc++ builds #16127

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79 changes: 59 additions & 20 deletions Framework/Core/src/DataRefUtils.cxx
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Original file line number Diff line number Diff line change
Expand Up @@ -84,30 +84,69 @@ void* extractFromTFile(TFile& file, TClass const* cl, const char* what)

#if defined(_LIBCPP_VERSION)
// The only member of std::array<T,N> is T _M_elems[N] in libstdc++, T __elems_[N] in libc++, so
// objects written on Linux carry a StreamerInfo with a checksum the libc++ class does not have.
// When opening the file, ROOT drops that StreamerInfo as an alias of the in-memory one
// (TStreamerInfo::BuildCheck), and the read fails with "Could not find the StreamerInfo with a
// checksum", leaving the array zero-filled. Register the on-file layout as a version of its own,
// objects written on Linux refer to a StreamerInfo checksum the libc++ class does not have, and
// the read fails with "Could not find the StreamerInfo with a checksum", leaving the array
// zero-filled (or -1 where the constructor put that). This happens for an std::array stored as
// the object itself (TRD's chamber status) and as a member (ITS GeometryTGeo::mLayerToWrapper).
// For each std::array the file describes, register its libstdc++ layout as a version of its own,
// under the in-memory member name, so that the payload, T[N] for both, is read as usual.
void DataRefUtils::registerLibstdcxxStdArray(TFile& file, TClass const* cl)
namespace
{
auto* infos = std::strncmp(cl->GetName(), "array<", 6) == 0 ? file.GetStreamerInfoCache() : nullptr;
auto* onfile = infos ? dynamic_cast<TStreamerInfo*>(infos->FindObject(cl->GetName())) : nullptr;
if (!onfile || onfile->GetNelement() != 1 || std::strcmp(onfile->GetElement(0)->GetName(), "_M_elems") != 0) {
return;
// TStreamerInfo::GetCheckSum for a class whose only member is "name" of type T[N]
UInt_t stdArrayCheckSum(TClass const* cl, TStreamerElement const* el, char const* name)
{
UInt_t id = 0;
auto add = [&id](char const* s) { for (; *s; ++s) { id = id * 3 + *s; } };
add(cl->GetName());
add(name);
add(el->GetTypeName());
for (int i = 0; i < el->GetArrayDim(); ++i) {
id = id * 3 + el->GetMaxIndex(i);
}
// Exists before BuildCheck, so that the on-file layout gets a version of its own
auto* current = static_cast<TStreamerInfo*>(const_cast<TClass*>(cl)->GetStreamerInfo());
auto* info = static_cast<TStreamerInfo*>(cl->FindStreamerInfo(onfile->GetCheckSum()));
if (!info) {
info = static_cast<TStreamerInfo*>(onfile->Clone());
info->BuildCheck(&file);
return id;
}
} // namespace

void DataRefUtils::registerLibstdcxxStdArray(TFile& file, TClass const* /*cl*/)
{
auto* infos = file.GetStreamerInfoCache();
if (!infos) {
return;
}
// Also when ROOT registered it itself: it then reads nothing, without any error
if (std::strcmp(info->GetElement(0)->GetName(), current->GetElement(0)->GetName()) != 0) {
info->GetElement(0)->SetName(current->GetElement(0)->GetName());
info->Clear("build");
info->BuildOld();
for (auto* obj : *infos) {
auto* onfile = dynamic_cast<TStreamerInfo*>(obj);
if (!onfile || std::strncmp(onfile->GetName(), "array<", 6) != 0 || onfile->GetNelement() != 1) {
continue;
}
auto* cl = TClass::GetClass(onfile->GetName());
if (!cl) {
continue;
}
// Exists before BuildCheck, so that the libstdc++ layout gets a version of its own
auto* current = static_cast<TStreamerInfo*>(cl->GetStreamerInfo());
if (!current || current->GetNelement() != 1) {
continue;
}
// ROOT may already have replaced the file's layout by the in-memory one (members of other
// classes): then rebuild the libstdc++ one, which differs only by the member name.
auto checksum = std::strcmp(onfile->GetElement(0)->GetName(), "_M_elems") == 0 ? onfile->GetCheckSum() : stdArrayCheckSum(cl, current->GetElement(0), "_M_elems");
auto* info = static_cast<TStreamerInfo*>(cl->FindStreamerInfo(checksum));
if (!info) {
info = static_cast<TStreamerInfo*>((std::strcmp(onfile->GetElement(0)->GetName(), "_M_elems") == 0 ? onfile : current)->Clone());
info->GetElement(0)->SetName("_M_elems");
info->SetCheckSum(checksum);
info->BuildCheck(&file);
info = static_cast<TStreamerInfo*>(cl->FindStreamerInfo(checksum));
if (!info) {
continue;
}
}
// Also when ROOT registered it itself: it then reads nothing, without any error
if (std::strcmp(info->GetElement(0)->GetName(), current->GetElement(0)->GetName()) != 0) {
info->GetElement(0)->SetName(current->GetElement(0)->GetName());
info->Clear("build");
info->BuildOld();
}
}
}
#endif
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