Allow `onLoadDocument` hook to return a state delta instead of a Y.Doc

[HalfVoxel]

The problem I am facing
Hocuspocus will immediately convert any Y.Doc returned by the onLoadDocument hook into an update.
This is the relevant code from hocuspocus:

await this.hooks('onLoadDocument', hookPayload, (loadedDocument: Doc | undefined) => {
      // if a hook returns a Y-Doc, encode the document state as update
      // and apply it to the newly created document
      // Note: instanceof doesn't work, because Doc !== Doc for some reason I don't understand
      if (
        loadedDocument?.constructor.name === 'Document'
        || loadedDocument?.constructor.name === 'Doc'
      ) {
        applyUpdate(document, encodeStateAsUpdate(loadedDocument))
      }
    })

Since we load documents in the update format anyway, it's just redundant work for us to convert our updates to Y.Docs just to be able to return them from the onLoadDocument hook.

The solution I would like
Change the onLoadDocument hook to have the return type Y.Doc | { update: Uint8Array, encodingVersion: number } instead of just Y.Doc.

CC: @ViktorQvarfordt

[HalfVoxel]

Turns out we can avoid this by only modifying hookPayload.document, and returning nothing from our hook. Right now we were doing both! We both applied our updates to hookPayload.document and then we returned hookPayload.document, which made hocuspocus apply the updates again. It's kinda odd that there are two ways of acomplishing the same thing. Maybe this should be clarified in the docs, or potentially hocuspocus should assert that the return value is not document.

[chrsan]

I was just confused about this myself when looking at the database extension and the server logic mentioned by @HalfVoxel above. The database extension applies the update on the payload document and then returns that same document which made hocuspocus apply them again.