Bug: sum path parameter cannot decode other variants than first

[shagohead]

What version of ogen are you using?

v1.20.1

Can this issue be reproduced with the latest version?

Yes

What did you do?

Define sum path parameter with that schema:

oneOf:
  - type: integer
  - type: string

Try to decode in second, non integer variant.

What did you expect to see?

String variant value.

What did you see instead?

io.EOF

Explanation

Generated code will try to decode each variant sequentially, calling DecodeValue each time.
For path parameters decoder will be a pointer of uri.PathDecoder.
Its DecodeValue method calls cursor.readAll, which have that code:

func (c *cursor) readAll() (string, error) {
	if c.pos == len(c.src) {
		return "", io.EOF
	}

	defer func() { c.pos = len(c.src) }()
	return c.src[c.pos:], nil
}

On the first call, parameter value will be consumed and pos moves after it.
Subsequent calls results in returning io.EOF.

[shagohead]

I want to write PR and currently i see a two different ways to solve that issue with changing of:

• API of uri.Decoder
• behavior of PathDecoder

First brokes callers of Decoders or requires extending Decoder API with method like 'Capture'.

Second brokes path parameter decoder for non sum types and requires an explicit way of behaviour switching.