TBulkRead does not work when TFile is missing

[ktf]

Standalone TTree does not allow for TBulkRead

I created a standalone TTree, without a backing TFile, for test purposes. While using TTreeReader works just fine, using TBulkRead does not work.

Expected behavior

Given the API to request a TBulkRead does not involve a TFile, I would have expected it to work without.

Setup

6.20.0

[pcanal]

This is a reproducer:

#include "TTree.h"
#include "TBufferFile.h"

void bulkmem() {
   TTree t("t","t");
   int i = 3;
   t.Branch("fAlpha",&i);
   for(i = 0; i < 100000; ++i)
      t.Fill();

   t.Print("");

   TBufferFile buf(TBuffer::EMode::kWrite, 32*1024);
   auto &r = t.GetBranch("fAlpha")->GetBulkRead();

   int iteration = 0;
   Long64_t events = t.GetEntries();
   while (events) {
      auto s = r.GetBulkEntries(0, buf);
      events -= s;
      fprintf(stderr, "#%d %lld events left %d\n", iteration, events, (int)s);
      ++iteration;
      if (iteration > 2)
         break;
   }
}

where the 2nd iteration (and subsequent) have GetBulkEntries returning 0;

Inadvertently this shows that the TBasket are being dropped (and hence data is lost since it is not backed by a file) after the first use (i.e. the loop is not advancing). This is a fatal problem but might not have been the original problem.

[pcanal]

With

#include "TTree.h"
#include "TBufferFile.h"

void bulkmem() {
   TTree t("t","t");
   int i = 3;
   t.Branch("fAlpha",&i);
   for(i = 0; i < 100000; ++i)
      t.Fill();

   t.Print("");

   TBufferFile buf(TBuffer::EMode::kWrite, 32*1024);
   auto &r = t.GetBranch("fAlpha")->GetBulkRead();

   int iteration = 0;
   Long64_t nentries = t.GetEntries();
   Long64_t event_idx = 0;
   while (nentries) {
      auto s = r.GetBulkEntries(event_idx, buf);
      nentries -= s;
      event_idx += s;
      fprintf(stderr, "#%d %lld events left %d\n", iteration, nentries, (int)s);
      ++iteration;
      if (iteration > 2)
         break;
   }
}

I get

Fatal: fExtraBasket == nullptr && "fExtraBasket should have been set to nullptr by GetFreshBasket" violated at line 1475 of `/Users/pcanal/root_working/code/rootcling/tree/tree/src/TBranch.cxx'

[pcanal]

It turns out that #6416 and this issue have the same original cause (not being able to handle basket that are not stored (on their own) in the file).