pythongh-129005: _pyio.BufferedIO remove copy on readall

Slicing buf and appending chunk would always result in a copy. Commonly
in a readall there is no already read data in buf, and the amount of
data read may be large, so the copy is expensive.

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/2025-01-29-00-00-01.gh-issue-129005.aV_3O8.rst

@@ -0,0 +1,2 @@
+Remove an unnecessary copy when ``_pyio.BufferedReader.read`` is called to
+read all data from a file and has no data already in buffer.