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

[fedonman]

_pyio.BytesIO.peek() returned a slice of the internal bytearray, so it gave a bytearray where the C version and the docs say bytes. It also did not accept an __index__-able size, and did not take the lock while slicing.

It now does what read() does: coerce the size, take the lock, convert with take_bytes().

test_peek only used assertEqual, and bytearray(b'1') == b'1', so it passed either way. It now checks the type and an __index__ size, and fails without the change.

BytesIO.peek() is new in 3.16, so there is no NEWS entry.

• Issue: _pyio.BytesIO.peek() returns bytearray instead of bytes #155389

[cmaloney]

cc: @marcelm@vstinner

[cmaloney]

I don't think the size piece here needs to be in the lock

[fedonman]

Done, thanks. Only the slicing is under the lock now.

[cmaloney]

that the size is ignored previously is actually intentional, that's the behavior in the C as well:

cpython/Modules/_io/bufferedio.c

Lines 951 to 982 in 998b890

	/*[clinic input]
	@critical_section
	_io._Buffered.peek
	size: Py_ssize_t = 0
	/
	[clinic start generated code]*/
	staticPyObject*
	_io__Buffered_peek_impl(buffered*self, Py_ssize_tsize)
	/*[clinic end generated code: output=ba7a097ca230102b input=56733376f926d982]*/
	{
	PyObject*res=NULL;
	CHECK_INITIALIZED(self)
	CHECK_CLOSED(self, "peek of closed file")
	if (!ENTER_BUFFERED(self))
	returnNULL;
	if (self->writable) {
	res=buffered_flush_and_rewind_unlocked(self);
	if (res==NULL)
	goto end;
	Py_CLEAR(res);
	}
	res=_bufferedreader_peek_unlocked(self);
	end:
	LEAVE_BUFFERED(self)
	returnres;
	}

[fedonman]

I think that one is _io._Buffered.peek (BufferedReader), which does ignore size. BytesIO.peek is in bytesio.c and uses it: size < 1 becomes DEFAULT_BUFFER_SIZE, then it clamps to what is left in the buffer.

io.BytesIO.peek(2) -> b'ab'
BufferedReader.peek(2) -> b'bcdef'

So the Python fallback should honour it too, which is what the __index__ coercion is for.

[aisk]

It's better to add a test to check if TypeError is raised for peek(1.5).