Bug 265096 – Provide ability to convert one type of NativePromise into another

• Component: Web Template Framework
• Status: RESOLVED DUPLICATE

[Jean-Yves Avenard [:jya]]

The problem was mentioned in https://github.com/WebKit/WebKit/pull/20652#discussion_r1397622606

Where we did the following:
```
Ref<MediaPromise> SourceBufferPrivate::removeCodedFrames(const MediaTime& start, const MediaTime& end, const MediaTime& currentTime)
{
    MediaPromise::Producer producer;
    Ref promise = producer.promise();

    m_currentSourceBufferOperation = m_currentSourceBufferOperation->whenSettled(RunLoop::current(), [weakThis = WeakPtr { *this }, this, start, end, currentTime](auto&& result) mutable {
        if (!weakThis || !result)
            return MediaPromise::createAndReject(!result ? result.error() : PlatformMediaError::BufferRemoved);
        return removeCodedFramesInternal(start, end, currentTime);
    })->whenSettled(RunLoop::current(), [producer = WTFMove(producer)](auto&& result) mutable {
        producer.settle(result);
        return MediaPromise::createAndSettle(WTFMove(result));
    });
    return promise;
}
```

 m_currentSourceBufferOperation is a MediaPromise which is exclusive; you can only perform a single then/whenSettled on it. Exclusive promise are useful as it allows for move semantics and avoid needless copies of the resolved/rejected value.

So in the code above, we can't return m_currentSourceBufferOperation as it would mean you run then/whenSettled on it twice, which would assert.

One solution above would be to
1- make MediaPromise (NativePromise<void, PlatformMediaError>) a non-exclusive promise or
2- make m_currentSourceBufferOperation be a NonExclusive promise and allows to convert a non exclusive promise into an exclusive one.

Doing 1- would require changing all the MediaPromise use points to no longer expect a r-valued result (most are using `(auto&& result)` as their prototype
2- allows for less disruptive code and allow for rather more elegant code. It would also be a bit more efficient as you no longer require to dispatch a task

So the above could be written:
```
    m_currentSourceBufferOperation = m_currentSourceBufferOperation->whenSettled(RunLoop::current(), [weakThis = WeakPtr { *this }, this, start, end, currentTime](auto&& result) mutable {
        if (!weakThis || !result)
            return OperationPromise::createAndReject(!result ? result.error() : PlatformMediaError::BufferRemoved);
        return static_cast<Ref<OperationPromise>>(removeCodedFramesInternal(start, end, currentTime).get());
    });
    return m_currentSourceBufferOperation.get();
```
where m_currentSourceBufferOperation.get is now a non-exclusive promise.

Adopting solution 2, also gets us close to achieving bug 264028

[Radar WebKit Bug Importer]

<rdar://problem/118610126>

[Jean-Yves Avenard [:jya]]

Pull request: https://github.com/WebKit/WebKit/pull/20713

[Jean-Yves Avenard [:jya]]

Pull request: https://github.com/WebKit/WebKit/pull/20721

[Jean-Yves Avenard [:jya]]

Combined changes with bug 264028

*** This bug has been marked as a duplicate of bug 264028 ***