ContentFieldProperty is a CustomJsValue descriptor whose CustomValue maps
the stored JsonValue to a JsValue on first read. Existence and
enumerability questions never need that value, but they used to pay for
it: `in`, hasOwnProperty, propertyIsEnumerable, Object.keys/values/entries,
Object.assign, object spread and JSON.stringify all reached the object
through GetOwnProperty, which materializes the descriptor whose value the
caller then reads or discards.
Jint lets a host answer those questions directly through ProbeOwnProperty.
The override deliberately mirrors GetOwnProperty line for line, minus the
descriptor: same initialization, same toJSON exclusion, same lookup, and
the enumerable flag read off the descriptor rather than off its value. The
engine trusts the probe without re-verifying it on the hot path, so a wrong
Missing would silently drop the key from every enumeration above - which is
why the two are kept adjacent in the file, pinned by tests covering `in`,
hasOwnProperty, propertyIsEnumerable, Object.keys, spread and
JSON.stringify plus a delete and an add, and checked on every test run by
the host-contract verification enabled earlier in this branch.
ContentDataObject deliberately does not get the same override: its
GetOwnProperty auto-creates a field for any name probed, so a probe that
agreed with it at the same instant would have to do the same, and that
quirk is pre-existing tenant-visible behaviour this change has no business
altering.
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_016uV6H9cTntzsoKiaJRBn4f
Marko Lahma
4a7ccbf75a