Add conditional code for platforms without os.readv

Author: cmaloney

Lib/_pyio.py

@@ -1695,12 +1695,18 @@ def readall(self):
     def readinto(self, b):
         """Same as RawIOBase.readinto()."""
         m = memoryview(b).cast('B')
-        self._checkClosed()
-        self._checkReadable()
-        try:
-            return os.readv(self._fd, (m, ))
-        except BlockingIOError:
-            return None
+        if hasattr(os, 'readv'):
+            self._checkClosed()
+            self._checkReadable()
+            try:
+                return os.readv(self._fd, (m, ))
+            except BlockingIOError:
+                return None
+        else:
+            data = self.read(len(m))
+            n = len(data)
+            m[:n] = data
+            return n
 
     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

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