Async effects/promises not immediately flushing

[dangodai]

Describe the bug

I have run into a scenario where async code of any kind seems to not immediately be run/flushed. In other words $effect.pending will be stuck > 0 and things like async deriveds will not re-run even when the value they depend on changes. They will eventually be run if something else triggers a flush of pending promises.

I am not certain I have found the minimal reproduction, but after many hours I have narrowed it down to swapping out recursive dynamic components.

Reproduction

Reproduction: https://svelte.dev/playground/eee80b71655641e3b7e0eb5afcb93469?version=5.39.5

This reproduction shows a basic tree structure rendered by the Node component. The displayed Message component contains async work.

Click the "Swap children" button which will replace the existing Message with a pair of Messages nested one level deeper.

Observe that the $effect.pending() displayed is stuck > 0. Observe that the async derived that doubles the input number does not re-run after the initial calculation.

Logs

System Info

System:
    OS: Linux 6.15 Arch Linux
    CPU: (12) x64 AMD Ryzen 5 2600 Six-Core Processor
    Memory: 4.58 GB / 15.54 GB
    Container: Yes
    Shell: 5.3.3 - /bin/bash
  Binaries:
    Node: 18.16.0 - ~/.nvm/versions/node/v18.16.0/bin/node
    Yarn: 1.22.22 - /usr/bin/yarn
    npm: 9.5.1 - ~/.nvm/versions/node/v18.16.0/bin/npm
    pnpm: 10.13.1 - ~/.local/share/pnpm/pnpm
  Browsers:
    Chromium: 139.0.7258.66

Severity

blocking an upgrade

[noobmaster19]

facing a similar issue

[dangodai]

Release 5.39.7 fixed part of the issue, but not all of it.

The initial async work still seems to be stuck (at least $effect.pending() is stuck > 0). Updating values the async derived depends on now gets them unstuck, instead of having to depend on something external to clear them.

[PatrickG]

Seems like #16897 fixes it completely. REPL
Can we close this @dangodai?

[dangodai]

@PatrickG in practical terms this is fixed for my real application where I encountered the problem.

I upgraded my original reproduction to 5.39.9 (the release of that PR) and noticed that $effect.pending() will be stuck at 2 after pressing "Swap children" until the source value of the async derived changes. This seems to be happening when no async work is actually being done (i.e. just awaiting a primitive), whereas actual async work like your setTimeout version seems to behave correctly.

Here is an updated REPL: https://svelte.dev/playground/eee80b71655641e3b7e0eb5afcb93469?version=5.39.9

This doesn't affect me since I don't tend to await non-async work in the real world, but that's still valid Javascript and you may want to track that issue whether here or separately. I think sometimes people await arguments of optional promises (e.g. arg: T | Promise<T>), so I could see this coming up for someone.