gh-140650: fix SystemError in io.BufferedWriter.close when closed errors

[inventshah]

Restores the 3.14 behavior of emitting a TypeError. Preservation of the sign helps in the error checking here:

cpython/Modules/_io/bufferedio.c

Lines 556 to 563 in 3dab11f

	/* gh-138720: Use IS_CLOSED to match flush CHECK_CLOSED. */
	r = IS_CLOSED(self);
	if (r < 0)
	goto end;
	if (r > 0) {
	res = Py_NewRef(Py_None);
	goto end;
	}

db68bfc changed from buffered_closed to IS_CLOSED.

• Issue: SystemError in io.BufferedWriter #140650

[serhiy-storchaka]

This does not look right. If IS_CLOSED() returns a positive value and sets an error, there is a bug in IS_CLOSED().

[cmaloney]

@serhiy-storchaka : Overall problem is code assumes IS_CLOSED can't result in an exception being set but if a user-provided implementation does unusual things (ex. closed = NotImplemented) then that breaks.

Another option here is "Exception" can be turned into a True result for "IS_CLOSED" + clear the exception but that seems like hiding an implemention issue rather than helping resolve it to me.

[serhiy-storchaka]

IS_CLOSED() should return -1 on error.

[cmaloney]

Also update CHECK_CLOSED to handle IS_CLOSED returning non-zero + setting an exception. In particular, if there's an exception CHECK_CLOSED should always return.

[inventshah]

Should I still be using PyErr_Occurred() or based on @serhiy-storchaka's feedback, should we make IS_CLOSED() return -1 on exception and check the sign in both CHECK_CLOSED() and _io__Buffered_close_impl()?

[serhiy-storchaka]

PyErr_Occurred() is only used when we cannot distinguish the value signalling an error from a valid value, like in PyLong_AsLong(). IS_CLOSED() returns -1 on error, 0 -- false, 1 -- true. Or so it should be.