[tree] Fix bulk read inflating the reported branch size

tree/tree/src/TBranch.cxx

@@ -1409,8 +1409,11 @@ Int_t TBranch::GetBasketAndFirst(TBasket *&basket, Long64_t &first,
             // Disassociate basket from memory buffer for bulk IO
             // When the user provides a memory buffer (i.e., for bulk IO), we should
             // make sure to drop all references to that buffer in the TTree afterward.
+            // Use RemoveAt (rather than a plain slot assignment) so that the array's
+            // fLast bookkeeping is updated; otherwise the branch would be streamed with
+            // an extra (empty) basket slot, spuriously increasing its reported size.
             fCurrentBasket = nullptr;
-            fBaskets[fReadBasket] = nullptr;
+            fBaskets.RemoveAt(fReadBasket);
          } else {
             fCurrentBasket = basket;
          }
@@ -1515,7 +1518,10 @@ Int_t TBranch::GetBulkEntries(Long64_t entry, TBuffer &user_buf)
          user_buf.SetBuffer(buf->Buffer(), buf->BufferSize());
          buf->ResetBit(TBufferIO::kIsOwner);
          fCurrentBasket = nullptr;
-         fBaskets[fReadBasket] = nullptr;
+         // Use RemoveAt so the array's fLast bookkeeping is updated; a plain slot
+         // assignment would leave the branch streaming an extra (empty) basket slot,
+         // spuriously increasing its reported size (see issue #8961).
+         fBaskets.RemoveAt(fReadBasket);
       } else {
          // This is the only copy, we can't return it as is to the user, just make a copy.
          if (user_buf.BufferSize() < buf->BufferSize()) {
@@ -1633,7 +1639,10 @@ Int_t TBranch::GetEntriesSerialized(Long64_t entry, TBuffer &user_buf, TBuffer *
          user_buf.SetBuffer(buf->Buffer(), buf->BufferSize());
          buf->ResetBit(TBufferIO::kIsOwner);
          fCurrentBasket = nullptr;
-         fBaskets[fReadBasket] = nullptr;
+         // Use RemoveAt so the array's fLast bookkeeping is updated; a plain slot
+         // assignment would leave the branch streaming an extra (empty) basket slot,
+         // spuriously increasing its reported size (see issue #8961).
+         fBaskets.RemoveAt(fReadBasket);
       } else {
          // This is the only copy, we can't return it as is to the user, just make a copy.
          if (user_buf.BufferSize() < buf->BufferSize()) {

tree/tree/test/BulkApi.cxx

@@ -1,4 +1,5 @@
 #include <cstdio>
+#include <memory>
 
 #include "Bytes.h"
 #include "TBranch.h"
@@ -255,3 +256,43 @@ TEST_F(BulkApiTest, BulkInMem)
          break;
    }
 }
+
+// https://github.com/root-project/root/issues/8961
+// Reading a branch with the bulk API must not change its reported size: the bulk
+// path temporarily loads a basket into the fBaskets array and must remove it
+// cleanly afterwards, otherwise the branch streams an extra (empty) basket slot
+// and its total size grows by 4 bytes on every read.
+TEST_F(BulkApiTest, SizeInvariantAfterBulkRead)
+{
+   const std::string fileName = "BulkApiSizeInvariant.root";
+   {
+      // Heap-allocated to avoid the intermittent failures that stack-allocated
+      // TFile/TTree have caused in the past. The TTree is owned by the file.
+      std::unique_ptr<TFile> f(TFile::Open(fileName.c_str(), "recreate"));
+      f->SetCompressionLevel(0);
+      auto t = new TTree("T", "T");
+      int x = 0;
+      t->Branch("x", &x, 8000, 0); // small basket size -> many baskets
+      for (int i = 0; i < 100000; ++i) {
+         x = i;
+         t->Fill();
+      }
+      f->Write();
+   }
+
+   std::unique_ptr<TFile> f(TFile::Open(fileName.c_str()));
+   ASSERT_NE(nullptr, f);
+   auto t = f->Get<TTree>("T"); // owned by the file
+   ASSERT_NE(nullptr, t);
+   TBranch *b = t->GetBranch("x");
+   ASSERT_NE(nullptr, b);
+
+   const Long64_t sizeBefore = b->GetTotalSize();
+
+   TBufferFile buf(TBuffer::EMode::kWrite, 32 * 1024);
+   auto s = b->GetBulkRead().GetBulkEntries(0, buf);
+   ASSERT_GT(s, 0) << "Did not read any entry with the bulk API.";
+
+   const Long64_t sizeAfter = b->GetTotalSize();
+   EXPECT_EQ(sizeBefore, sizeAfter) << "The branch size must be invariant from the API used to read it.";
+}