ShortHeaderPacket.build / LongHeaderPacket.build crashed with
`IllegalArgumentException: packet too short for HP sample` whenever the
plaintext payload was small enough that pnLen + payload < 4 — most
visibly on the 1-RTT path when buildApplicationPacket emitted a single
1-byte PING (PTO probe with no ACKs queued and no streams to drain) and
the packet number still fit in 1 byte. The crash tore down the writer
loop, which surfaced upstream as moq-lite "subscribe stream FIN before
reply" because in-flight bidi streams got FIN'd by the peer.
RFC 9001 §5.4.2 mandates the sender pad the plaintext so the encrypted
output (plaintext + 16-byte AEAD tag) has at least 20 bytes after the
packet-number offset for the 16-byte HP sample. The fix pads the
plaintext with trailing 0x00 bytes — those decode as PADDING frames per
RFC 9000 §19.1 and decodeFrames already absorbs them. For long-header
packets the padded size feeds back into the Length varint.
Claude
edd6eb5c10