encoding/xml: Add (*Encoder).Close() to check for unclosed elements #53346
Description
When using (*xml.Encoder).EncodeToken, the Encoder checks for mismatched elements. That is, it returns an error if e.g. a <a> element is closed with a </b> token or if there is an extra </c> token after all elements have already been closed.
However, if the user simply forgets to close all elements, the Encoder emits incorrect XML without reporting an error. See this code, for an example. So currently, the user must track the tokens they encode themselves, to make sure the emitted XML is valid.
Flush returns an error, but as it is possible to encode more tokens after Flush has been called, it can't complain about unclosed elements itself.
I thus propose to add a new method to Encoder:
// Close the Encoder, indicating that no more data will be written. It flushes any buffered XML to the underlying writer
// and returns an error if the written XML is invalid (e.g. by containing unclosed elements).
// After Close has been called, any Encode method return errors.
func (enc *Encoder) Close() errorNaming the method Close is an indication that it should be called and the returned error should be checked by the user.