DPL Analysis: make sure VLAs use only one basket

Otherwise it seems to create issues with ROOT bulk reading if the baskets
are split across multiple clusters.

In particular, each row needs 4 bytes for the size of the elements in the row
and the _size branch needs to also have its size correctly set.

(cherry picked from commit 1561ebb)

Author: ktf

Framework/Core/include/Framework/TableTreeHelpers.h

@@ -83,8 +83,9 @@ class ColumnToBranch
   ColumnToBranch(ColumnToBranch const& other) = delete;
   ColumnToBranch(ColumnToBranch&& other) = delete;
   void at(const int64_t* pos);
-  int fieldSize() const { return mFieldSize; }
-  char const* branchName() const { return mBranchName.c_str(); }
+  [[nodiscard]] int fieldSize() const { return mFieldSize; }
+  [[nodiscard]] int columnEntries() const { return mColumn->length(); }
+  [[nodiscard]] char const* branchName() const { return mBranchName.c_str(); }
 
  private:
   void accessChunk();

Framework/Core/src/TableTreeHelpers.cxx

@@ -454,9 +454,23 @@ std::shared_ptr<TTree> TableToTree::process()
   }
 
   for (auto& reader : mColumnReaders) {
-    int idealBasketSize = 1024 + reader->fieldSize() * mRows; // minimal additional size needed, otherwise we get 2 baskets
+    int idealBasketSize = 1024 + reader->fieldSize() * reader->columnEntries(); // minimal additional size needed, otherwise we get 2 baskets
     int basketSize = std::max(32000, idealBasketSize);        // keep a minimum value
+    // std::cout << "Setting baskets size for " << reader->branchName() << " to " << basketSize << " =  1024 + "
+    //           << reader->fieldSize() << " * " << reader->columnEntries() << ". mRows was " << mRows << std::endl;
     mTree->SetBasketSize(reader->branchName(), basketSize);
+    // If it starts with fIndexArray, also set the size branch basket size
+    if (strncmp(reader->branchName(), "fIndexArray", strlen("fIndexArray")) == 0) {
+      std::string sizeBranch = reader->branchName();
+      sizeBranch += "_size";
+      //  std::cout << "Setting baskets size for " << sizeBranch << " to " << basketSize << " =  1024 + "
+      //            << reader->fieldSize() << " * " << reader->columnEntries() << ". mRows was " << mRows << std::endl;
+      // One int per array to keep track of the size
+      int idealBasketSize = 4 * mRows + 1024 + reader->fieldSize() * reader->columnEntries(); // minimal additional size needed, otherwise we get 2 baskets
+      int basketSize = std::max(32000, idealBasketSize);                                      // keep a minimum value
+      mTree->SetBasketSize(sizeBranch.c_str(), basketSize);
+      mTree->SetBasketSize(reader->branchName(), basketSize);
+    }
   }
 
   while (row < mRows) {