From 84e328ebd5faa5ea9f721506d2d0d1820d9dc683 Mon Sep 17 00:00:00 2001
From: Raz Luvaton <16746759+rluvaton@users.noreply.github.com>
Date: Wed, 4 Mar 2026 22:21:02 +0200
Subject: [PATCH 1/2] fix: first next_back() on new RowsIter panics

---
 arrow-row/src/lib.rs | 38 +++++++++++++++++++++++++++++++++++++-
 1 file changed, 37 insertions(+), 1 deletion(-)

diff --git a/arrow-row/src/lib.rs b/arrow-row/src/lib.rs
index 9679c89b4807..1df79d3af44b 100644
--- a/arrow-row/src/lib.rs
+++ b/arrow-row/src/lib.rs
@@ -1415,7 +1415,7 @@ impl DoubleEndedIterator for RowsIter<'_> {
             return None;
         }
         // Safety: We have checked that `start` is less than `end`
-        let row = unsafe { self.rows.row_unchecked(self.end) };
+        let row = unsafe { self.rows.row_unchecked(self.end - 1) };
         self.end -= 1;
         Some(row)
     }
@@ -5651,4 +5651,40 @@ mod tests {
                 .contains("not yet implemented")
         );
     }
+
+    #[test]
+    fn empty_row_iter_next_back() {
+        let rows = RowConverter::new(vec![SortField::new(DataType::UInt8)])
+            .unwrap()
+            .empty_rows(0, 0);
+        let mut rows_iter = rows.iter();
+        assert_eq!(rows_iter.next_back(), None);
+        assert_eq!(rows_iter.next_back(), None);
+        assert_eq!(rows_iter.next_back(), None);
+    }
+
+    #[test]
+    fn row_iter_next_back() {
+        let row_converter = RowConverter::new(vec![SortField::new(DataType::UInt8)]).unwrap();
+        let mut rng = StdRng::seed_from_u64(42);
+        let array = generate_primitive_array::<UInt8Type>(&mut rng, 100, 0.8);
+        let rows = row_converter.convert_columns(&[Arc::new(array)]).unwrap();
+
+        let mut rows_iter = rows.iter();
+        let mut bytes: Vec<u8> = vec![];
+
+        while let Some(row) = rows_iter.next_back() {
+            bytes.extend(row.data.iter().rev());
+        }
+
+        bytes.reverse();
+
+        assert_eq!(
+            bytes,
+            &rows.buffer.as_slice()[..*rows.offsets.last().unwrap()]
+        );
+
+        assert_eq!(rows_iter.next_back(), None);
+        assert_eq!(rows_iter.next(), None);
+    }
 }

From 2977d07b661c47ca2935be1239e9fd381ed7b3a6 Mon Sep 17 00:00:00 2001
From: Raz Luvaton <16746759+rluvaton@users.noreply.github.com>
Date: Thu, 5 Mar 2026 11:28:24 +0200
Subject: [PATCH 2/2] do -1 first, and update safety comment

---
 arrow-row/src/lib.rs | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/arrow-row/src/lib.rs b/arrow-row/src/lib.rs
index 1df79d3af44b..078c4574775d 100644
--- a/arrow-row/src/lib.rs
+++ b/arrow-row/src/lib.rs
@@ -1414,9 +1414,12 @@ impl DoubleEndedIterator for RowsIter<'_> {
         if self.end == self.start {
             return None;
         }
-        // Safety: We have checked that `start` is less than `end`
-        let row = unsafe { self.rows.row_unchecked(self.end - 1) };
+
         self.end -= 1;
+
+        // Safety: By construction we create `end >= start`, so if `end` is not equal to `start` it cannot be less than `start`
+        //          therefore `end - 1` is within range
+        let row = unsafe { self.rows.row_unchecked(self.end) };
         Some(row)
     }
 }