gh-129005: Avoid copy in _pyio.FileIO.readinto

Lib/_pyio.py

@@ -1695,10 +1695,20 @@ def readall(self):
     def readinto(self, b):
         """Same as RawIOBase.readinto()."""
         m = memoryview(b).cast('B')
-        data = self.read(len(m))
-        n = len(data)
-        m[:n] = data
-        return n
+        self._checkClosed()
+        self._checkReadable()
+        try:
+            if hasattr(os, 'readv'):
+                return os.readv(self._fd, (m, ))
+
+            data = os.read(self._fd, len(m))
+            n = len(data)
+            m[:n] = data
+            return n
+        except BlockingIOError:
+            return None
+
+
 
     def write(self, b):
         """Write bytes b to file, return number written.

Misc/NEWS.d/next/Library/2025-01-18-15-29-08.gh-issue-129005.mM5dmV.rst

@@ -0,0 +1 @@
+Change ``_pyio.FileIO.readinto`` to use :func:`os.readv` avoiding a copy that was present when using :func:`os.read` on platforms with :func:`os.readv`