TBulkRead fails with "fExtraBasket should have been set to nullptr by GetFreshBasket"

[ktf]

Describe the bug

I am trying to use the new TBulkRead API. I have a set of branches that I read with (updated with full reproducer):

#include <TBufferFile.h>
#include <TFile.h>
#include <TTree.h>
#include <iostream>

void foo() {
  auto f = TFile::Open("http://hyperloop.cern.ch/train-workdir/testdata/LFN/alice/data/2015/LHC15o/000245064/pass5_lowIR/PWGZZ/Run3_Conversion/71_20200915-2255_child_1/0935/AO2D.root");
  auto t = (TTree*)f->Get("O2track");
  auto e = t->GetEntries();
  auto b = t->GetBranch("fAlpha");
  assert(b);
  int pos = 0;
  while (pos < e) {
    TBufferFile buf(TBuffer::EMode::kWrite, 32*1024);
    auto &r = b->GetBulkRead();
    auto s = r.GetBulkEntries(pos, buf);
    pos += s;
    std::cout << "Read " << s << " elements " << std::endl;
    b->Print();
  }
}

however when I get to read the last but one buffer, I get:

Read 1000 elements
*Br 3398 :fAlpha    : fAlpha/F                                               *
*Entries :  3399743 : Total  Size=   13939349 bytes  File Size  =   11798760 *
*Baskets :     3399 : Basket Size=       1000 bytes  Compression=   1.17     *
*............................................................................*
Fatal: fExtraBasket == nullptr && "fExtraBasket should have been set to nullptr by GetFreshBasket" violated at line 1474 of `/Users/ktf/src/sw/SOURCES/ROOT/v6-20-02-alice7/v6-20-02-alice7/tree/tree/src/TBranch.cxx'
aborting

Expected behavior

Reading all the baskets, with the last one returning the appropriate number of entries.

To Reproduce

Setup

6.20.0

Additional context

TFile with a few TTree in it, all the branches have basic types or arrays of basic types.

[ktf]

For the record, for other files, with larger baskets, it happens in the middle of the tree reading.

[pzhristov]

@ktf There is an orphan p->Print() in the reproducer above.

[ktf]

@pzhristov fixed

[ktf]

If I understand correctly TBranch::GetBasketAndFirst() sometimes does not go through the GetFreshBasket statement, but through GetFreshCluster(). Line 1223-1226 of TBranch.cxx.

[pcanal]

It turns out that #6417 and this issue have the same original cause (not being able to handle basket that are not stored (on their own) in the file).

[pcanal]

If I understand correctly TBranch::GetBasketAndFirst() sometimes does not go through the GetFreshBasket statement, but through GetFreshCluster(). Line 1223-1226 of TBranch.cxx

Indeed and the rest of the code assumed it always went through GetFreshBasket.