payload object is unexpectedly mutated during findAll / findRecord

[Gaurav0]

Discovered during writing #6754, see comments in the PR.

Description

If you call await store.findRecord('person', 1) and the mock server in your test returns this payload, which is a Plain Old JavaScript Object:

{
  "data": {
    "id": 1,
    "type": "person",
    "attributes: { ... }
  }
}   

then since IDENTIFIERS feature flag was turned on, that payload object was mutated to:

{
  "data": {
    "id": 1,
    "type": "person",
    "lid": "some uuid or other string from generateIdForRecord",
    "attributes: { ... }
  }
} 

Expected Result: Payload object is unchanged, but record returned by findRecord has an added lid.
Actual Result: Payload object is modified to contain an lid.

Note: Fixing this bug may cause a performance regression. Furthermore, users are unlikely to notice this issue outside of testing with a mock server.

Versions

Run the following command and paste the output below: yarn list ember-source && yarn list ember-cli && yarn list ember-data.

yarn list v1.19.0
warning Filtering by arguments is deprecated. Please use the pattern option instead.
└─ [email protected]
Done in 1.95s.
yarn list v1.19.0
warning Filtering by arguments is deprecated. Please use the pattern option instead.
└─ [email protected]
Done in 1.99s.
yarn list v1.19.0
warning Filtering by arguments is deprecated. Please use the pattern option instead.
└─ [email protected]
Done in 2.88s.

(This bug is likely reproducible in the 3.15 beta)

[runspired]

overall this has proved to not be an issue; however, it was a limitation that prevented payloads being fully opaque to the store. emberjs/rfcs#854 addresses this and emberjs/rfcs#860 updates request fulfillment. Collectively they ensure we can thread enough context to the cache that this mutation is no longer necessary.