[mafredri]

Accept writes a plain text http.Error response when it rejects a handshake request. A server that needs a structured error body, such as RFC 9457 JSON (#545), cannot replace that response. A second write after Accept returns only appends to the plain text body: the status and text/plain content type stay Accept's, and net/http logs a superfluous WriteHeader call.

AcceptOptions.ErrorHandler func(w http.ResponseWriter, r *http.Request, status int, err error) writes that response instead. It receives the status Accept chose and the error Accept returns, before wrapping. w.Header() already holds the headers Accept set, such as Sec-WebSocket-Version: 13 for an unsupported version. It is called for every rejection before the 101 response: request validation, origin, and a ResponseWriter without http.Hijacker. It is not called when hijacking fails, because Accept has already written 101 Switching Protocols. A handler that writes nothing leaves net/http's default 200 OK with an empty body.

A nil ErrorHandler keeps the previous response. The Accept doc now says not to write to w after Accept returns an error.

Tests cover the handler for all 11 rejections. A separate test pins the default status, headers and body for all 11 and passes on master. For four rejections, the raw responses match master byte for byte, excluding Date.

Fixes #545

Decision log

• Handler field instead of a typed error plus an opt-out flag: with the flag, callers must write after a rejection but not after a hijack failure, and a wrong branch gives 200 OK or a double write.
• The handler receives r, so one handler shared across requests can use it, for example r.Context(). Allow customize how HTTP errors are rendered when websocket.Accept fails to upgrade #545 discussed capturing the request in a closure instead, which does not work for shared options.
• Documentation alone was rejected because callers still could not change the body (Allow customize how HTTP errors are rendered when websocket.Accept fails to upgrade #545, Support disabling error writing via AcceptOptions #152).
• No exported per-reason errors; status and error text are enough to render a response, and sentinels can be added later.
• For an invalid OriginPatterns entry the handler receives "Forbidden", as Accept returns today; the pattern error is still only logged.
• Without http.Hijacker, the handler receives the descriptive error, while the nil default body stays "Not Implemented".
• No recover around the handler: a panic behaves like any other handler panic in net/http.
• The hijack failure after the 101 is unchanged; whether to hijack before writing 101 is a separate decision.
• The field is also added to the js/wasm AcceptOptions stub so shared code compiles.

🤖 This PR was created with the help of Coder Agents.