Correct MSP-message-sending message truncation issue - #33949
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peterbarker merged 2 commits intoAug 12, 2026
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The guard here contradicted the comment above it, which allows a send if either the transmit buffer is empty or the frame fits in it. Testing !tx_pending() instead of tx_pending() inverted that: a frame was dropped when the buffer was empty but too small, and sent unconditionally whenever a transmit was already pending. Writes do not block, so sending unconditionally with insufficient space wrote as much of the frame as would fit and discarded the rest, putting a truncated frame on the wire. The write return values were ignored, so the caller was told the whole frame had been sent. The empty-buffer case exists upstream only because a blocking write could push out a frame bigger than the buffer. ArduPilot has no blocking writes, so require the frame to fit and drop it otherwise.
Check that a frame is sent whole when there is room for it, on both an idle and a busy port, and dropped rather than truncated when there is not. Uses a uart which truncates writes to the space available, as the ChibiOS and Linux backends do.
rishabsingh3003
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Summary
Fix inverted check when sending MSP traffic
Classification & Testing (check all that apply and add your own)
Description
Original betaflight line: https://github.com/betaflight/betaflight/blob/master/src/main/msp/msp_serial.c#L311 - you can see we inverted the pending check when porting ("empty" vs "pending" and both lines have a
!)tx_pending doesn't make a huge amount of sense when our writes are non-blocking anyway. So just make sure the thing fits.