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[2.0 / next] Outside latest reads of a derivation expose errors and values at different times

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Assessment

Difficulty
4/5
Estimated time
3-5 days
Newbie friendliness
22/100
Issue type
Bug
Clarity
Mostly clear
Activity status
Active
Tech stack
javascript, typescript
Domain
frontend

Research direction

Start with the publication rules in packages/signals/docs/RULES-DERIVED.md, specifically D-22 and D-41, then reproduce in the playground steps listed in the issue. Compare how latest(derived) publishes a fulfilled value versus a rejected one while a sibling is pending, since the two paths currently diverge. Done means both outcomes become visible at the same point, with the held-frame behaviour matching the async sibling case. Design choices here are contested, so confirm the intended rule with maintainers before changing code.

Written by the indexing model from the issue text.

Description

Describe the bug

With derived = createMemo(() => latest(source)), outside reads of latest(derived) expose successful and failed outcomes at different points during a held update.

A successful result becomes readable before the frame reveals. A rejection only becomes observable through the derivation after reveal, even though direct latest(source) already throws it. An async sibling keeps the frame held in both cases.

Your Example Website or App

https://s.olid.uk/id/Bp_ILv8vREiTHbnn0x6Ppg

Steps to Reproduce the Bug or Issue

Wait for Derived effect: 1 and Committed round: 1. The Read latest button samples both accessors from an event handler, outside a reactive computation.

Rejected result:

  1. Click Start update, then Reject source.
  2. Click Read latest. The output shows latest(source): boom but latest(derived): 1. The read helper catches thrown errors and displays their messages. The committed round remains 1 because the sibling is still pending.
  3. Click Release sibling, then Read latest again. Both reads now throw boom, and the committed round is 2.

Successful result:

  1. Reload the playground.
  2. Click Start update, then Resolve source to 2 (control).
  3. Click Read latest before releasing the sibling. Both reads already return 2, while the committed round remains 1.
Expected behavior

Outside reads should follow a consistent publication rule for successful and failed outcomes.

The point at which latest(derived) exposes a new outcome should not depend on whether the source fulfills or rejects.

Screenshots or Videos

No response

Platform
  • OS: Linux
  • Browser: Chromium 154.0.8037.57
  • Solid: playground pinned to the next build at d5e32a9af41c7bfe0bc090f7438ee4297d540b17, which includes #4085.
Additional context

Related to #4085. Both the rejection and fulfillment paths were verified in the hosted playground.

The related publication rules are D-22 and D-41.

Dominant language
TypeScript
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  4. Open a pull request that references the issue number.

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