Add IDBCursor.close()

[inexorabletash]

Cursors require maintaining a fair bit of state. Implementations may also do optimizations such as pre-fetching records. This state can only be completely released when either the cursor hits the end of its range or both the request and the cursor have been GC'd (since there's an implicit bidirectional dependency), or when the transaction ends.

It might be nice to expose a close() method on cursors to allow web apps to signal that a cursor can be tidied up prior transaction closing.

[inexorabletash]

TPAC 2019 Web Apps Indexed DB triage notes:

We are in favor, especially as we're already tweaking the IDBCursor interface in V3.

[inexorabletash]

This would be easiest to define and test if we allow calling close() at the same times that we allow calling continue(), namely when the owning transaction is active and the cursor's got value flag is true, and throw otherwise.

Otherwise, it's racy: there could be an outstanding continue() (or advance() etc) call running, with the associated request in a pending state.

@aliams @pwnall @asutherland - does that restriction sound acceptable?

[pwnall]

SGTM.

[aliams]

SGTM!

[asutherland]

TMSG. (To me, sounds good!)

[SteveBeckerMSFT]

TPAC 2024: We revisited this topic, which still seems like a good addition. Additional data and use cases would help browsers prioritize an implementation.