This commit changes the `Hdlc::Decoder` implementation to report
non-zero length received frames shorter than FCS size as an error
`OT_ERROR_PARSE` (instead of ignoring such frames during decoding).
Since the recent changes in `Hdlc::Decoder` buffer model delegates
the management of the frame buffer to the user of `Decoder`,
clearing frame buffer after an error should be be performed by the
`Decoder` user from the `FrameHandler` callback. The change in this
commit addresses an issue where a small (single-byte) frame would
not have been cleared from the decoder buffer and included in the
next frame. This commit also updates the `test_hdlc` unit test to
cover small frame error.
This commit updates HDLC implementation to harmonize the buffer model
used by `Hdlc::Encoder` and `Hdlc::Decoder`.
The new model defines a common class `Hdlc::FrameWritePointer` which
provides a minimum set of APIs used by `Encoder/Decoder` for writing
frames, while not defining the underlying buffer space or how the
buffer is managed. `Encoder` or `Decoder` users are expected to use
sub-classes of `FrameWritePointer` to add the buffer space and
implement the frame buffer management scheme.
Two sub-classes `Hdlc::FrameBuffer` and `Hdlc::MultiFrameBuffer` are
also provided which respectively allow storing a single frame or
multiple frames (FIFO queue of frames) in a buffer of given size.
This commit also updates the `NcpUart` code to use the new buffer
model.
Finally, this commit adds an HDLC unit test `test_hdlc.cpp` which
covers the following:
- Verify behavior of `Hdlc::FrameBuffer` (single frame buffer),
- Verify behavior of `Hdlc::MultiFrameBuffer` (FIFO queue),
- Verify behavior of `Hdlc::Encoder` and `Hdlc::Decoder`,
- Test encoder/decoder with randomly generated frames (fuzz test).
Abtin Keshavarzian