[neilberkman]

A frame that deserializes correctly can still violate the protocol: a text frame that is not valid UTF-8, a continuation frame with no message in progress, a data frame in the middle of a fragmented message, a compressed frame when compression was not negotiated or that fails to inflate, or a fragmented message over the configured limit. Bandit.WebSocket.Connection returned these from handle_frame/3 as {:error, reason, connection} with the bare string as the reason, so the handler process stopped abnormally. Each one produced a GenServer crash report containing the whole handler state, the WebSock state included, and websocket_options: [log_protocol_errors: false] did not apply to them. #610 fixed this for errors found while deserializing a frame; these are the ones found afterwards.

This change adds do_protocol_error/4 next to do_error/4 in Bandit.WebSocket.Connection. It calls do_error/4 unchanged, logs through Bandit.Logger.maybe_log_websocket_protocol_error/2, and returns {:error, {:shutdown, reason}, connection}. The ten call sites where the peer caused the error use it. Errors returned or raised by the WebSock callbacks, and the deflation error on an outgoing message, still go through do_error/4 and behave as before, crash report included.

The close code sent to the client, the reason passed to terminate/2, and the error value in the [:bandit, :websocket, :stop] event are unchanged. The connection process exit reason changes from the string to {:shutdown, string}. The error in [:thousand_island, :connection, :stop] is also unchanged by default; with silent_terminate_on_error: true it becomes {:shutdown, string}, as it already is for deserialization errors.

Nine tests in the "protocol error logging" block of test/bandit/websocket/protocol_test.exs: invalid UTF-8, unexpected continuation and unexpected compressed frame, each with log_protocol_errors set to :short, :verbose and false. Each asserts the close frame, the terminate/2 reason, a {:shutdown, reason} exit with no crash report, and one log line or none. The existing tests that exercise these errors now also assert that no crash report is logged for them, which covers all ten call sites; fourteen existing tests fail without the change.

Verified with mix format --check-formatted, mix compile --warnings-as-errors, mix credo --strict, mix dialyzer, and mix test on Elixir 1.20.4 / OTP 29 (1083 passed, 1 skipped, 8 excluded; the 3 failures are in HibernationTest and also fail on main under OTP 29, where a hibernating process reports {:gen_server, :loop_hibernate, 4}).

[mtrudel]

Looks good!