Ensure that the sandbox has started before taking a filesystem snapshot

[thecodingwizard]

Previously, if you tried to take a filesystem snapshot of a sandbox that hasn't been started yet, it would fail with an error:

import modal

app = modal.App.lookup("my-app", create_if_missing=True)


with modal.enable_output():
    sb = modal.Sandbox.create(app=app, gpu="H100")
    sb.terminate()

    img = sb.snapshot_filesystem()
    print(img)

This PR fixes this issue by blocking until the sandbox has started (or at least until it has been picked up by a worker) before executing the snapshot request.

[pawalt]

I thought you were planning on moving this logic into the RPC?

[thecodingwizard]

Yeah, but this RPC has a timeout property, and I didn't want that timeout to be the time for both sandbox startup and snapshotting. Otherwise if it takes a very long time for the sandbox to start up, then the snapshot only has a small amount of time to complete.