Cleanup also current file in TChain::GetEntries

[vepadulano]

Followup to #19817

In the method we need to check if the current TTree has a TTreeCache associated with the current file. The previous logic was getting the pointer to the current file but calling LoadTree afterwards, which deleted the current file and switched it for a new instance. In turn, this would cause bad reads as highlighted by a run of the associated unit test with the address sanitizer:

==18501==ERROR: AddressSanitizer: heap-use-after-free on address 0x61800004be60 at pc 0x000108735e2c bp 0x00016b501950 sp 0x00016b501948
READ of size 8 at 0x61800004be60 thread T0
    #0 0x000108735e28 in TFile::GetCacheRead(TObject const*) const TFile.cxx:1262
    https://github.com/root-project/root/issues/1 0x00010a6e0758 in TTree::GetReadCache(TFile*) const TTree.cxx:6403
    https://github.com/root-project/root/pull/2 0x00010a6e79c0 in TTree::GetReadCache(TFile*, bool) TTree.cxx:6416
    https://github.com/root-project/root/pull/3 0x00010acd2f58 in TTreeReader::Restart() TTreeReader.cxx:574
    https://github.com/root-project/root/pull/4 0x00010acd2634 in TTreeReader::SetEntriesRange(long long, long long) TTreeReader.cxx:550
    https://github.com/root-project/root/pull/5 0x0001048fd23c in main test_ttreereader_uninitialized_chain.cpp:28
    https://github.com/root-project/root/pull/6 0x000190a96b94 in start+0x17b8 (dyld:arm64e+0xfffffffffff3ab94)

0x61800004be60 is located 480 bytes inside of 824-byte region [0x61800004bc80,0x61800004bfb8)
freed by thread T0 here:
    #0 0x00010c757b0c in _ZdlPv+0x74 (libclang_rt.asan_osx_dynamic.dylib:arm64e+0x4bb0c)
    https://github.com/root-project/root/issues/1 0x000107556f6c in TStorage::ObjectDealloc(void*) TStorage.cxx:325
    https://github.com/root-project/root/pull/2 0x0001074a8a68 in TObject::operator delete(void*) TObject.cxx:1189
    https://github.com/root-project/root/pull/3 0x000108732650 in TFile::~TFile() TFile.cxx:569
    https://github.com/root-project/root/pull/4 0x00010a5c6e04 in TChain::LoadTree(long long) TChain.cxx:1449
    https://github.com/root-project/root/pull/5 0x00010acd2dfc in TTreeReader::Restart() TTreeReader.cxx:571
    https://github.com/root-project/root/pull/6 0x00010acd2634 in TTreeReader::SetEntriesRange(long long, long long) TTreeReader.cxx:550
    https://github.com/root-project/root/pull/7 0x0001048fd23c in main test_ttreereader_uninitialized_chain.cpp:28
    https://github.com/root-project/root/pull/8 0x000190a96b94 in start+0x17b8 (dyld:arm64e+0xfffffffffff3ab94)

previously allocated by thread T0 here:
    #0 0x00010c7576e4 in _Znwm+0x74 (libclang_rt.asan_osx_dynamic.dylib:arm64e+0x4b6e4)
    https://github.com/root-project/root/issues/1 0x000107556eec in TStorage::ObjectAlloc(unsigned long) TStorage.cxx:293
    https://github.com/root-project/root/pull/2 0x0001056ee26c in TObject::operator new(unsigned long) TObject.h:187
    https://github.com/root-project/root/pull/3 0x00010875b3c8 in TFile::Open(char const*, char const*, char const*, int, int) TFile.cxx:3956
    https://github.com/root-project/root/pull/4 0x00010a5c71a0 in TChain::LoadTree(long long) TChain.cxx:1481
    https://github.com/root-project/root/pull/5 0x00010a5c0f04 in TChain::GetEntries() const TChain.cxx:910
    https://github.com/root-project/root/pull/6 0x0001048fd224 in main test_ttreereader_uninitialized_chain.cpp:28
    https://github.com/root-project/root/pull/7 0x000190a96b94 in start+0x17b8 (dyld:arm64e+0xfffffffffff3ab94)

Change the logic to instead load the tree beforehand, so the pointer to the file will be the correct one.

Fixes CI failures such as https://github.com/root-project/root/actions/runs/17610706130/job/50031825233?pr=19860#step:6:4762

[ferdymercury]

maybe this if-statement is no longer needed if you have the assert above.

[vepadulano]

Indeed. Although these changes are causing further side effects in a few tests. We may have to revert everything and go back to the drawing board

[ferdymercury]

What about:

   if (auto curFile = fTree->GetCurrentFile()) {
      if (!fTree->GetTree()) {
         fTree->LoadTree(0);
         curFile = fTree->GetCurrentFile();
      }
      if (fTree->GetTree()) {

(In case the side-effects appear when curFile is null)

[github-actions]

Test Results

    19 files      19 suites   3d 10h 12m 28s ⏱️
 3 663 tests  3 660 ✅ 0 💤  3 ❌
68 220 runs  68 194 ✅ 5 💤 21 ❌

For more details on these failures, see this check.

Results for commit afb31f5.

♻️ This comment has been updated with latest results.

[vepadulano]

I have updated this PR with a second version of the approach. It now also deals with the root cause which is the side-effect caused by TChain::GetEntries, but in a way that keeps all the side-effects of the method. It becomes another alternative to #19873 . Note that this PR will still fail the same tests as before due to the introduction of yet another side-effect. This is meant for discussion with @pcanal