failing test: loading empty doc from network

[geoffreylitt]

In Jacquard, we have a use case where we sometimes try to load an empty doc from network. It's a metadata doc which sometimes has some stuff in it, but other times it starts out just being empty.

It turns out that if you do this, automerge-repo will never resolve the .doc() promise, so our app hangs. (This PR has a failing test for that)

The underlying cause is this code in DocHandle, which uses heads to decide whether to do a state transition:

 #checkForChanges(before: A.Doc<T>, after: A.Doc<T>) {
    const beforeHeads = A.getHeads(before)
    const afterHeads = A.getHeads(after)
    const docChanged = !headsAreSame(afterHeads, beforeHeads)
    if (docChanged) {
      this.emit("heads-changed", { handle: this, doc: after })

The "empty" handle pre-load has heads [], and the doc loaded from network also has heads [], so the state transition doesn't happen.

@joshuahhh and I got this far and we weren't sure what made sense to do next, thoughts @alexjg ?

For now we were able to workaround in our app by just making a change in the empty docs to avoid this case.

[alexjg]

Interesting, in the latest alpha of automerge-repo I think we may fix this. We made the decision to always create a document with an empty change in it to distinguish a new document from one we are requesting. This is effectively what you are doing as well.

[geoffreylitt]

Great, sounds like that'll fix it! I'll just close this issue and then we can confirm fixed once we upgrade to that.