pythongh-129005: _pyio.BufferedIO Remove copy on readall

Author: cmaloney

Lib/_pyio.py

@@ -1062,6 +1062,9 @@ def _read_unlocked(self, n=None):
                 if chunk is None:
                     return buf[pos:] or None
                 else:
+                    # Avoid slice + copy if there is no data in buf.
+                    if not buf:
+                        return chunk
                     return buf[pos:] + chunk
             chunks = [buf[pos:]]  # Strip the consumed bytes.
             current_size = 0

Misc/NEWS.d/next/Library/2025-01-28-21-34-10.gh-issue-129005.WNaBND.rst

@@ -0,0 +1 @@
+Optimize ``_pyio.FileIO.read`` by removing an unnecessary copy