[treereader] Correctly treat empty trees with range loop iteration

instead of allowing a spurious iteration with zero entries.

Fixes #16249

Author: dpiparo

tree/treeplayer/inc/TTreeReader.h

@@ -78,10 +78,28 @@ class TTreeReader: public TObject {
          fEntry(entry), fReader(&reader) {}
 
       /// Compare two iterators for equality.
-      bool operator==(const Iterator_t& lhs) const {
+      bool operator==(const Iterator_t &lhs) const {
          // From C++14: value initialized (past-end) it compare equal.
-         if (!IsValid() && !lhs.IsValid()) return true;
-         return fEntry == lhs.fEntry && fReader == lhs.fReader;
+         if (!IsValid() && !lhs.IsValid())
+            return true;
+         // The iterators refer to different readers
+         if (fReader != lhs.fReader)
+            return false;
+         // #16249: range based loop and the tree has zero entries
+         // as well as analogous cases.
+         // Getting the number of events can have a cost, for example in
+         // case of chains of remote files accessible with high latency.
+         // However, it is reasonable to assume that if iterators are
+         // being compared is because an iteration is taking place,
+         // therefore such cost has to be paid anyway, it's just
+         // anticipated.
+         if (fReader->GetTree()->GetEntriesFast() == 0 && fEntry == 0 && !lhs.IsValid()) {
+            return true;
+         }
+         if (lhs.fReader->GetTree()->GetEntriesFast() == 0 && lhs.fEntry == 0 && !IsValid()) {
+            return true;
+         }
+         return fEntry == lhs.fEntry;
       }
 
       /// Compare two iterators for inequality.
@@ -241,7 +259,7 @@ class TTreeReader: public TObject {
       return Iterator_t(*this, 0);
    }
    /// Return an iterator beyond the last TTree entry.
-   Iterator_t end() const { return Iterator_t(); }
+   Iterator_t end() { return Iterator_t(*this, -1); }
 
 protected:
    using NamedProxies_t = std::unordered_map<std::string, std::unique_ptr<ROOT::Internal::TNamedBranchProxy>>;

tree/treeplayer/test/basic.cxx

@@ -509,3 +509,22 @@ TEST(TTreeReaderBasic, DisappearingBranch)
    gSystem->Unlink("DisappearingBranch0.root");
    gSystem->Unlink("DisappearingBranch1.root");
 }
+
+// Issue #16249
+TEST(TTreeReaderBasic, ZeroEntriesTree) {
+    TTree t("t","t");
+    TTreeReader tr(&t);
+    auto b = tr.begin();
+    auto e = tr.end();
+    auto n_iterations = 0;
+    for ([[maybe_unused]] auto &d : tr) {
+      ++n_iterations;
+    }
+    EXPECT_EQ(n_iterations, 0);
+    EXPECT_TRUE(b == e);
+    EXPECT_TRUE(b == b);
+    EXPECT_TRUE(e == e);
+    auto b_copy(b);
+    EXPECT_TRUE(b++ == e);
+    EXPECT_TRUE(++b_copy == e);
+}