[GitHub] [arrow-rs] Ted-Jiang commented on a diff in pull request #3115: Add COW conversion for Buffer and PrimitiveArray and unary_mut

[GitBox <gi...@apache.org>]

Ted-Jiang commented on code in PR #3115:
URL: https://github.com/apache/arrow-rs/pull/3115#discussion_r1023591486


##########
arrow-buffer/src/buffer/immutable.rs:
##########
@@ -227,6 +227,28 @@ impl Buffer {
     pub fn count_set_bits_offset(&self, offset: usize, len: usize) -> usize {
         UnalignedBitChunk::new(self.as_slice(), offset, len).count_ones()
     }
+
+    /// Returns `MutableBuffer` for mutating the buffer if this buffer is not shared.
+    pub fn into_mutable(self, len: usize) -> Result<MutableBuffer, Self> {
+        let offset_ptr = self.as_ptr();
+        let offset = self.offset;
+        let length = self.length;
+        Arc::try_unwrap(self.data)
+            .map(|bytes| {
+                // The pointer of underlying buffer should not be offset.

Review Comment:
   > For the limitation, I assume that a non-zero offset means the Bytes is shared/sliced from others. So it is disallowed to be mutable here.
   
   Just question:  i think `Arc::try_unwrap` has check the exactly one strong reference.
   And all `sliced`  use the `clone()`, So this is impossible panic right?  🤔  



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