HTTP status codes reference

This page lists every response status that htmlform.Handler produces, the body it writes, and the cause. The errors of the function are mapped like for httpfun.Handler, see the HTTP status codes reference of go-function.

htmlform.Handler

Status	Body	Cause
200 OK	the HTML form	GET or HEAD
200 OK	written by the `ResultsWriter`	POST, the function returned without error
400 Bad Request	`invalid multipart form`	the POST body is not a valid `multipart/form-data` form, including URL-encoded forms
400 Bad Request	`invalid value for argument "a"`	the value of field `a` couldn't be parsed
400 Bad Request	`invalid argument "a": <validator error>`	the validator set with `SetArgValidator("a", ...)` returned an error
400 Bad Request	`argument "a" must be an uploaded file`	a non-empty text value was sent for a file argument
400 Bad Request	`argument "a" takes at most N files`	more files than an array argument `[N]fs.File` holds
403 Forbidden	`Forbidden`	a browser sent the POST from another origin, see `SetCrossOriginProtection`
200 OK	empty	POST, the function returned without error and the `ResultsWriter` is nil
405 Method Not Allowed	`Method Not Allowed`	any method other than GET, HEAD, and POST, with the header `Allow: GET, HEAD, POST`
413 Request Entity Too Large	`Request Entity Too Large`	the body exceeded the limit set by `SetMaxRequestBodySize`, default 100 MiB
any, 404, 500	as for `httpfun.Handler`	the function returned an error, mapped like the `httpfun.Handler` rows of the go-function reference
500 Internal Server Error	`Internal Server Error`	the function panicked, always recovered
500 Internal Server Error	`Internal Server Error`	a temporary file for an upload couldn't be written, or the form template failed

Notes:

The 400 errors for parsing, validation, and files, and the errors of the function, go to the ResultsWriter first, as its resultErr argument. A custom writer can render them, for example by showing the form again with the message. The built-in writers return them, and httpfun.HandleError writes the response.
The 400, 403, 405, and 413 responses for the form body itself (invalid multipart form, cross-origin, method, size) are written directly and skip the ResultsWriter and HandleError.
The function is not called for the 400, 403, 405, and 413 responses in the table. It can still cause a 4xx response itself by returning an error like httperr.NotFound or one wrapping os.ErrNotExist.
File arguments that uploads can't fill, like fs.MemFile or a file type in a struct field, are rejected by NewHandler before any request.

HTTP status codes reference of go-function: httpfun.Handler and how an error becomes a response
Security model of go-function: why responses hide argument values and internal errors