A TChain whose trees have friend TChains stops updating

[lmoureaux]

Check duplicate issues.

• Checked for duplicates

Description

Consider the following tree loading structure:

TChain c2 ---> TTree ---(friend)---> TChain c1 ---> TTree

When iterating over c2, branches coming from c1 stop updating after the first file boundary is reached in c1. They start updating again once a file boundary is reached in c2.

I think this is distinct from #16804 because in this case the issues lie in TChain::LoadTree, while for #16804 TChain::LoadTree is the sole culprit.

As in #16804, the root cause of the issue is that the following code is not invoked often enough:

root/tree/tree/src/TChain.cxx

Lines 1407 to 1434 in 2cc4b70

	while ((frelement = (TChainElement*) fnext())) {
	void* addr = frelement->GetBaddress();
	if (addr) {
	TBranch* br = fTree->GetBranch(frelement->GetName());
	TBranch** pp = frelement->GetBranchPtr();
	if (pp) {
	// FIXME: What if br is zero here?
	*pp = br;
	}
	if (br) {
	if (!frelement->GetCheckedType()) {
	Int_t res = CheckBranchAddressType(br, TClass::GetClass(frelement->GetBaddressClassName()),
	(EDataType) frelement->GetBaddressType(), frelement->GetBaddressIsPtr());
	if ((res & kNeedEnableDecomposedObj) && !br->GetMakeClass()) {
	br->SetMakeClass(true);
	}
	frelement->SetDecomposedObj(br->GetMakeClass());
	frelement->SetCheckedType(true);
	}
	// FIXME: We may have to tell the branch it should
	// not be an owner of the object pointed at.
	br->SetAddress(addr);
	if (TestBit(kAutoDelete)) {
	br->SetAutoDelete(true);
	}
	}
	}
	}

I will call the operation above an "address refresh".

TChain has code triggering an address refresh when needed for friends of the current tree:

root/tree/tree/src/TChain.cxx

Lines 1384 to 1393 in 2cc4b70

	bool needUpdate = false;
	if (fTree->GetListOfFriends()) {
	for(auto fetree : ROOT::Detail::TRangeStaticCast<TFriendElement>(*fTree->GetListOfFriends())) {
	if (fetree->IsUpdated()) {
	needUpdate = true;
	fetree->ResetUpdated();
	}
	}
	}
	if (needUpdate) {

Unfortunately this doesn't work for two reasons:

1. The code cannot possibly be reached unless c2 itself has friends because of the following condition:
   

   root/tree/tree/src/TChain.cxx

   Line 1370 in 2cc4b70

   if (fFriends) {

   
   In our case c2 doesn't have friends, but the fTree does. This is easy to fix (or work around).
2. Even if fFriends is made non-null, fetree->IsUpdated() always returns false because TTree::LoadTree resets it:
   

   root/tree/tree/src/TTree.cxx

   Line 6512 in 18b4f31

   fe->ResetUpdated();

   
   Fixing this is much more annoying. For my application, I ended up subscribing to the Notify signal and calling TFriendElement::MarkUpdated again. This is only a partial solution because when c2 loads a new file, it doesn't reset friend elements that were marked updated, which would happen somewhere along this line:
   

   root/tree/tree/src/TChain.cxx

   Line 1666 in 2cc4b70

   while ((fe = (TFriendElement*) next())) {

Reproducer

Rename to recursiveBugDemo.C and run the following macro: recursiveBugDemo.txt

It works similarly to #16804 and should only print true.

ROOT version

Seen in:

• ROOT 6.30/04 (anaconda)
• ROOT 6.28/04 (LCG 104)

I couldn't find any code change in master in the relevant areas.

Installation method

anaconda and LCG 104

Operating system

Linux (CentOS 9, Ubuntu 22.04, Ubuntu 24.04)

Additional context

No response