encoding/json, encoding/xml: option to treat unknown fields as an error

[lukescott]

Currently unknown fields are silently ignored when decoding into a struct. I would like
to have an option to treat unknown fields as an error.

This would be extremely helpful for a REST API to help third-party developers identify
typos. In this particular case I don't need an "inline" tag as I don't need to
do anything with the extraneous fields. I just need to tell the recipient that they
included something the server doesn't understand.

[rsc]

Comment 1:

Labels changed: added release-none.

Status changed to Accepted.

[gopherbot]

Comment 2 by Kyle.A.Wolfe:

I would also like to see this, as well as a path to the field(s) that were not
unmarshalled Something like this maybe?
type UnmarshalResult struct {
    Path          string //"foo>bar"
    Unmarshalled  bool
}
func xml.UnmashalWithResult(xmlBytes []byte, v interface{}) []*UnmarshalResult
Adding this method rather than modifying the existing Unmarshal method should keep
backwards compatibility as well as provide us with this most useful feature

[gopherbot]

Comment 3 by Kyle.A.Wolfe:

Additionally, using `xml:ALL` option is not viable for large and living xml documents.
Placing a catchall on the root of the element will only work if the entire child element
was missed: See foo>c>c2 not showing up as a missed field:
http://play.golang.org/p/g3KPa3g4Dm

[gopherbot]

Comment 4 by Kyle.A.Wolfe:

I created a fork and added this functionality (not fully tested)

[gopherbot]

Comment 5 by Kyle.A.Wolfe:

I have forked this package and added this functionality while keep backwards
compatibility (not fully tested) https://github.com/KyleWolfe/xmlkeys

[griesemer]

Comment 6:

Labels changed: added repo-main.

[adg]

One way to do this would be to add a Strict method to the Encoder and Decoder types. While we're at it we could also make indentation on Encoder possible with an Indent method.

[h4ck3rm1k3]

I have a simple patch for this for json #10134

[cespare]

We should do this. Today, I'm aware of two workarounds:

• Separately unmarshal the object into a map in order to obtain and inspect the key set.
• Use pointers for all field values. For example, the github.com/google/go-github API wrapper uses pointer fields as a rule for this reason. (But even then, you can't distinguish between "k":null and a missing key.)

Besides some way to change the decoder "mode" (as @adg suggested), here's another way it could work: add a ,required struct field tag which causes Unmarshal to give an error if no key matches that field.

[adg]

Closing this in favor of #15314, which is identical and more active.