QCBOREncode_FinishGetSize not usable if the original buffer is too small

[BrianSipos]

I would like to use a simple pattern to encode variable-length PDUs with an optimistic initial size estimate to avoid double-encoding under good conditions.

The pattern is:

1. Allocate a small fixed-size buffer
2. Encode to that buffer as normal
3. Use QCBOREncode_FinishGetSize() to determine if the buffer was large enough or not
4. If it was not, allocate a properly sized (larger) buffer
5. Encode to the dynamically sized buffer
6. Use QCBOREncode_Finish() to finalize

My problem is currently that QCBOREncode_FinishGetSize() only sets the encoded size if the error state is QCBOR_SUCCESS but not if it is QCBOR_ERR_BUFFER_TOO_SMALL.

QCBOR/src/qcbor_encode.c

Lines 1081 to 1083 in 9f09f0c

	if(nReturn == QCBOR_SUCCESS) {
	*puEncodedLen = Enc.len;
	}

Is there a reason for that condition to be present in FinishGetSize at all?
Let the caller check the error code to see if the size is meaningful in its context.

[laurencelundblade]

Hi Brian, I'll look at it some more, but encoding stops immediately when an error occurs so it doesn't calculate the whole length. There are also errors that are not due to the length. Maybe there can be an exception for QCBOR_ERR_BUFFER_TOO_SMALL, but there will have to be a lot of checking and testing to be sure it's OK and it may increase code size.

[BrianSipos]

Thank you. Yes, this would be to treat QCBOR_ERR_BUFFER_TOO_SMALL the same condition as using the zero-length buffer to obtain the full size (which currently exists).

[laurencelundblade]

Still working on this. It's a significant change to UsefulBuf error semantics so I'm hesitant.

[laurencelundblade]

I've made the change to UsefulOutBuf to have error semantics in #341. It was needed for error codes for streaming.

I haven't reflected it up into the QCBOR API. Still have to look at that.