JMAP RFC-8621 : Back-reference resolution

1. 1. Why

https://jmap.io/spec-core.html#references-to-previous-method-results

To allow clients to make more efficient use of the network and avoid round trips, an argument to one method can be taken from the result of a previous method call in the same request.

Example:

[[ "Foo/changes", {
    "accountId": "A1",
    "sinceState": "abcdef"
}, "t0" ],
[ "Foo/get", {
    "accountId": "A1",
    "#ids": {
        "resultOf": "t0",
        "name": "Foo/changes",
        "path": "/created"
    }
}, "t1" ]]

`back references` that mean server will reuse the result of a previous method call in the same request.
Process is:

• When processing a method call, the server MUST first check the arguments object for any names beginning with *#*.
• If found, the result reference should be resolved and the value used as the “real” argument.
• If any result reference fails to resolve, the whole method MUST be rejected with an *invalidResultReference* error.
• If an arguments object contains the same argument name in normal and referenced form (e.g., foo and #foo), the method MUST return an *invalidArguments* error.

A *ResultReference* object has the following properties:

• *resultOf*: String The method call id (see Section 3.1.1) of a previous method call in the current request.
• *name*: String The required name of a response to that method call.
• *path*: String A pointer into the arguments of the response selected via the name and resultOf properties. This is a JSON Pointer, except it also allows the use of * to map through an array (see the description below).

1. 1. How

https://github.com/linagora/james-project/pull/3683 demonstrated a proof of concept doing JSON substitution within the method arguments (relying on play-JSON JSON transformers) before the actual processing start

This makes back-reference resolution completly orthogonal to the Methods being written.

1. 1. Definition of Done

Write some simple memory integration tests demonstrating back-references are resolved. Also test back-reference resolution failure.