binder: Dispatch transact() calls on an Executor when FLAG_ONEWAY would not be respected.

[jdcormie]

Fixes #8914

[markb74]

Thanks so much for this!

[ejona86]

@jdcormie, @markb74, this release() is suspicious. Should it be done within the executor instead?

[jdcormie]

The OneWayBinderProxy contract says that callers of transact() must unconditionally call close() on 'wrappedParcel'. If we called release() inside the Runnable, the non-thread-safe wrappedParcel might be accessed concurrently from multiple threads.

The idea here is to transfer ownership to the Runnable as a raw Parcel in an Exception safe way. There are two cases. Either:

1. execute() returns successfully, the Runnable promises to eventually recycle() 'parcel' in transactAndRecycleParcel(), and we release ownership of 'wrappedParcel' on the very next line.
   OR
2. execute() throws RejectedExecutionException. In this case, the Runnable never runs and wrappedParcel.release() is never called so the caller of OneWayBinderProxy#transact() will recycle the Parcel when it calls ParcelHolder#close() in its finally.

LMK if you see a problem or have a better suggestion.

[ejona86]

Ah, release() is not like a close(). It steals the ownership instead of releasing the object. (The holder releases ownership because it is transferring to the caller; the caller is not releasing ownership overall which is what I'm used to with a release method, where release is similar to unref/close.)

I think this is fine then. Thank you!