gh-155389: Return bytes from _pyio.BytesIO.peek()

peek() returned a slice of the internal bytearray, where read() converts with
take_bytes(). It also did not coerce its size through __index__ and did not
hold the lock while slicing, both of which read() and the C implementation do.

Author: fedonman

Lib/_pyio.py

@@ -1003,9 +1003,18 @@ def tell(self):
     def peek(self, size=0):
         if self.closed:
             raise ValueError("peek on closed file")
-        if size < 1:
-            return self._buffer[self._pos:self._pos + io.DEFAULT_BUFFER_SIZE]
-        return self._buffer[self._pos:self._pos + size]
+        try:
+            size_index = size.__index__
+        except AttributeError:
+            raise TypeError(f"{size!r} is not an integer")
+        else:
+            size = size_index()
+
+        with self._lock:
+            if size < 1:
+                size = io.DEFAULT_BUFFER_SIZE
+            b = self._buffer[self._pos:self._pos + size]
+            return b.take_bytes()
 
     def truncate(self, pos=None):
         if self.closed:

Lib/test/test_io/test_memoryio.py

@@ -570,6 +570,10 @@ def test_peek(self):
         buf = self.buftype("1234567890")
         with self.ioclass(buf) as memio:
             self.assertEqual(memio.tell(), 0)
+            # bytearray(b'1') == b'1', so the type has to be asserted separately.
+            self.assertIsInstance(memio.peek(), bytes)
+            self.assertIsInstance(memio.peek(1), bytes)
+            self.assertEqual(memio.peek(IntLike(3)), buf[:3])
             self.assertEqual(memio.peek(1), buf[:1])
             self.assertEqual(memio.peek(1), buf[:1])
             self.assertEqual(memio.peek(), buf)