gh-155074: Fix remaining-space calculation in _pyio.BufferedReader.readinto()

Lib/_pyio.py

@@ -1198,7 +1198,8 @@ def _readinto(self, buf, read1):
             while written < len(buf):
 
                 # First try to read from internal buffer
-                avail = min(len(self._read_buf) - self._read_pos, len(buf))
+                avail = min(len(self._read_buf) - self._read_pos,
+                            len(buf) - written)
                 if avail:
                     buf[written:written+avail] = \
                         self._read_buf[self._read_pos:self._read_pos+avail]

Lib/test/test_io/test_bufferedio.py

@@ -313,6 +313,14 @@ def test_readinto(self):
         self.assertEqual(bufio.readinto(b), 1)
         self.assertEqual(b, b"cb")
 
+    def test_readinto_with_remaining_buffered_data(self):
+        bufio = self.tp(self.BytesIO(b"abcd"), buffer_size=2)
+        self.assertEqual(bufio.read(1), b"a")
+        b = bytearray(2)
+        self.assertEqual(bufio.readinto(b), 2)
+        self.assertEqual(b, b"bc")
+        self.assertEqual(bufio.read(), b"d")
+
     def test_readinto1(self):
         buffer_size = 10
         rawio = self.MockRawIO((b"abc", b"de", b"fgh", b"jkl"))

Misc/NEWS.d/next/Library/2026-08-02-19-08-17.gh-issue-155074.Vj8qL2.rst

@@ -0,0 +1,2 @@
+Fix the pure Python implementation of ``BufferedReader.readinto()`` raising
+:exc:`ValueError` after partially filling the destination buffer.