fix(signals): a reader mounted mid-flight reads a derived store like its memo (#4491)

.changeset/derived-store-midflight-mount.md

@@ -0,0 +1,5 @@
+---
+"@solidjs/signals": patch
+---
+
+A reader of a derived store that mounts while the derive's input is in flight is held until it answers, as a reader of the async memo is, instead of showing the previous answer first: `createStore(d => { d.v = m1() })` over a memo that reads `latest()`, read mid-flight, no longer shows the old question's value before the new one (#4491).

packages/signals/docs/RULES-DERIVED.md

@@ -252,7 +252,7 @@ Effect-phase code that runs after the frame is decided reads the frame as it sta
 
 **From:** F-9, F-1. **Scenarios:** S-36, S-37.
 
-Pending is per node — per store key: a held write to a plain store pends exactly the touched keys; a derived store's leaves report the derive's new-question refetch like any async memo, in both forms, and a quiet re-ask (`refresh(store)`) is quiet on them too. A derived store's landing resumes the frame that asked it: when a consumer of its leaves goes async, the landed leaves are held with that frame and read stale and pending until it reveals (signal parity); an untouched sibling key is final. A derived store's seed is a draft for the derive, never an observable value: until the first resolution (first yield for a generator) every outside read — tracked, untracked, `in`, enumeration — throws `NotReadyError` (DEV in a component body: `PENDING_ASYNC_UNTRACKED_READ`); write-path reads see it. A seedless derived store (`createStore(fn)` with a derive that takes no draft) starts from an empty record, the same unobservable draft. `loadingValue` / `seedLoadingValue` promote a first value to commit #0: reads serve it, no boundary trips, no hold starts or extends, `isPending` is `false` on every axis until the first real landing — a hydration invariant (the server always answers `false`) — after which the node is an ordinary memo.
+Pending is per node — per store key: a held write to a plain store pends exactly the touched keys; a derived store's leaves report the derive's new-question refetch like any async memo, in both forms, and a quiet re-ask (`refresh(store)`) is quiet on them too. So do the leaves of a derive that is a verdict reader only through what it reads (D-25) and was served a flight's committed value: a tracked reader of them reads the flight as the memo's reader would — one that mounts mid-flight is held (F-2) and shows the flight's answer, never the old question's — and the derive computes that landing plainly, as the hold's work (D-25) (#4491, `tests/unread-derive-midflight-mount-4491.test.ts`). A derived store's landing resumes the frame that asked it: when a consumer of its leaves goes async, the landed leaves are held with that frame and read stale and pending until it reveals (signal parity); an untouched sibling key is final. A derived store's seed is a draft for the derive, never an observable value: until the first resolution (first yield for a generator) every outside read — tracked, untracked, `in`, enumeration — throws `NotReadyError` (DEV in a component body: `PENDING_ASYNC_UNTRACKED_READ`); write-path reads see it. A seedless derived store (`createStore(fn)` with a derive that takes no draft) starts from an empty record, the same unobservable draft. `loadingValue` / `seedLoadingValue` promote a first value to commit #0: reads serve it, no boundary trips, no hold starts or extends, `isPending` is `false` on every axis until the first real landing — a hydration invariant (the server always answers `false`) — after which the node is an ordinary memo.
 
 **Collapses:** [A9](./SPEC-ASYNC-SEMANTICS.md#a9-store-leaves-behind-a-firewall-report-the-firewalls-new-question-refetch), [A22](./SPEC-ASYNC-SEMANTICS.md#a22-pending-is-per-node-store-wide-only-for-the-firewalls-own-work) (amended 2026-10-03), [A25](./SPEC-ASYNC-SEMANTICS.md#a25-a-derived-stores-seed-is-a-draft-never-an-observable-value), [A27](./SPEC-ASYNC-SEMANTICS.md#a27-the-commit-0-loading-window-is-loading-class-and-verdict-quiet), the L2 stores ruling (statement 7), OS-R30–R36, PJ-R23/R30/R31. **Status:** standing; one adoption shape pinned `it.fails` (`adoption-unchanged-key-read-3706.test.ts`, a row first read after the adoption).
 

packages/signals/docs/RULES-INDEX.md

@@ -59,7 +59,7 @@ Status legend: **live** stated and standing · **ruled** carries an explicit rul
 | A12 | amended | `docs/SPEC-ASYNC-SEMANTICS.md:199` | — | createOptimistic.test.ts×2 spec-async-semantics.test.ts×1 | [ruled, amended in place] Resting optimistic nodes report pending like a plain memo — A resting optimistic node reports pending via exactly the causes a plain async memo does (A19) — a reverting optim… |
 | A13 | ruled | `docs/SPEC-ASYNC-SEMANTICS.md:207` | — | spec-async-semantics.test.ts×7 | [ruled 2026-07-06 (promoted from B1)] Resting optimistic ≡ plain async memo at every checkpoint — (was B1) A resting optimistic node (no active override) is observationally identical to a plain async … |
 | A14 | amended | `docs/SPEC-ASYNC-SEMANTICS.md:215` | — | spec-async-semantics.test.ts×1 | [ruled, amended in place 2026-07-06 (promoted from B2)] Companion nodes get child lanes that do not merge with the owner — (was B2) `isPending`/`latest` companion nodes get child lanes that do not mer… |
-| A15 | amended | `docs/SPEC-ASYNC-SEMANTICS.md:251` | async.ts×2 constants.ts×1 core.ts×4 lanes.ts×6 scheduler.ts×16 seat.ts×1 verdict.ts×4 | async-chain-supersession.test.ts×1 boundary-output-frame-reader.test.ts×1 create-deferred-3710.test.ts×2 first-observer-stale-reader.test.ts×1 fuzz-findings-l2.test.ts×17 held-restore.test.ts×1 hold-recovers-landing-reruns-4469.test.ts×1 l2-contract.test.ts×7 l2-fuzz-existing-rules.test.ts×2 lane-contract.test.ts×1 lane-hold-on-observation.test.ts×1 lane-outside-view.test.ts×1 lane-pass-stamped-effect-3662.test.ts×1 latest-memo-inline-parity.test.ts×1 latest-uninitialized-over-latest-4362.test.ts×1 loading-fallback-in-flush-3540.test.ts×4 mount-over-foreign-hold-3761.test.ts×2 overlapping-flights.test.ts×3 posture-born-held-and-observation.test.ts×4 posture-store-parity.test.ts×2 reveal-carve-out.test.ts×2 shared-effect-no-entangle.test.ts×1 show-stale-child-in-action-4137.test.ts×2 spec-async-semantics.test.ts×2 stale-read-uninitialized-cross-transition.test.ts×1 superseded-source-blocks-3462.test.ts×2 tick-scoped-pass-transaction.test.ts×1 treeshake.test.ts×4 verdict-memo-flight-no-reask.test.ts×1 visibility-oracle-store.states.ts×6 visibility-oracle.states.ts×7 visibility-oracle.test.ts×1 write-proposals-3494.test.ts×1 | [ruled, amended in place 2026-07-06 (promoted from B3)] Transition entanglement is graph-driven; lanes settle as one reveal — (was B3) Transition entanglement is graph-driven: writes whose async work … |
+| A15 | amended | `docs/SPEC-ASYNC-SEMANTICS.md:251` | async.ts×2 constants.ts×1 core.ts×4 lanes.ts×6 scheduler.ts×16 seat.ts×1 verdict.ts×4 store.ts×1 | async-chain-supersession.test.ts×1 boundary-output-frame-reader.test.ts×1 create-deferred-3710.test.ts×2 first-observer-stale-reader.test.ts×1 fuzz-findings-l2.test.ts×17 held-restore.test.ts×1 hold-recovers-landing-reruns-4469.test.ts×1 l2-contract.test.ts×7 l2-fuzz-existing-rules.test.ts×2 lane-contract.test.ts×1 lane-hold-on-observation.test.ts×1 lane-outside-view.test.ts×1 lane-pass-stamped-effect-3662.test.ts×1 latest-memo-inline-parity.test.ts×1 latest-uninitialized-over-latest-4362.test.ts×1 loading-fallback-in-flush-3540.test.ts×4 mount-over-foreign-hold-3761.test.ts×2 overlapping-flights.test.ts×3 posture-born-held-and-observation.test.ts×4 posture-store-parity.test.ts×2 reveal-carve-out.test.ts×2 shared-effect-no-entangle.test.ts×1 show-stale-child-in-action-4137.test.ts×2 spec-async-semantics.test.ts×2 stale-read-uninitialized-cross-transition.test.ts×1 superseded-source-blocks-3462.test.ts×2 tick-scoped-pass-transaction.test.ts×1 treeshake.test.ts×4 unread-derive-midflight-mount-4491.test.ts×1 verdict-memo-flight-no-reask.test.ts×1 visibility-oracle-store.states.ts×6 visibility-oracle.states.ts×7 visibility-oracle.test.ts×1 write-proposals-3494.test.ts×1 | [ruled, amended in place 2026-07-06 (promoted from B3)] Transition entanglement is graph-driven; lanes settle as one reveal — (was B3) Transition entanglement is graph-driven: writes whose async work … |
 | A16 | amended | `docs/SPEC-ASYNC-SEMANTICS.md:223` | scheduler.ts×1 | latest-uninitialized-over-latest-4362.test.ts×2 spec-async-semantics.test.ts×1 strict-read-pending-store.test.ts×2 uninitialized-visibility.test.ts×1 visibility-oracle-store.states.ts×2 visibility-oracle.states.ts×2 visibility-oracle.test.ts×1 | [ruled, amended in place 2026-07-06 (promoted from B5)] `isPending` never throws in untracked contexts — (was B5) `isPending` never throws in untracked contexts — thunks that throw real errors or read… |
 | A17 | amended | `docs/SPEC-ASYNC-SEMANTICS.md:71` | async.ts×3 constants.ts×1 lanes.ts×7 scheduler.ts×2 seat.ts×1 verdict.ts×1 map.ts×1 optimistic.ts×1 store.ts×3 | create-deferred-3710.test.ts×1 fuzz-findings-l2.test.ts×6 lane-uninitialized-landing-3648.test.ts×5 optimistic-over-held-row-3796.test.ts×1 optimistic-read-lane-not-transaction-3698.test.ts×2 optimistic-undefined-override.test.ts×1 posture-store-parity.test.ts×1 refresh-await.test.ts×1 reveal-gating-contract.test.ts×3 spec-async-semantics.test.ts×10 createOptimisticStore.test.ts×2 kanban-a17-fixture.test.ts×3 optimistic-list-mutation-matrix.test.ts×1 optimistic-maparray-index-frame-f1.test.ts×1 optimistic-untracked-reads-f3-f5.test.ts×1 signal-store-twins-qd.test.ts×1 treeshake.test.ts×1 until.test.ts×1 visibility-oracle-store.states.ts×24 visibility-oracle-store.test.ts×1 visibility-oracle.states.ts×20 visibility-oracle.test.ts×1 | [ruled, amended in place 2026-07-06 (promoted from C4)] An active override is the displayed value until its transaction commits, and the graph's value until its own source answers — **Statement (curre… |
 | A18 | amended | `docs/SPEC-ASYNC-SEMANTICS.md:81` | action.ts×1 async.ts×2 core.ts×3 lanes.ts×8 scheduler.ts×2 types.ts×1 verdict.ts×1 map.ts×1 optimistic.ts×1 projection.ts×1 | body-end-supersession-visibility.test.ts×4 createOptimistic.test.ts×3 l2-fuzz-existing-rules.test.ts×2 lane-contract.test.ts×1 lane-frame-deferred-run-3662.test.ts×1 lane-outside-view.test.ts×1 lane-uninitialized-landing-3648.test.ts×5 optimistic-move-duplicate-3548.test.ts×2 optimistic-read-lane-not-transaction-3698.test.ts×4 posture-store-parity.test.ts×5 spec-async-semantics.test.ts×5 flight-owned-transaction.test.ts×1 lane-authority-twins.test.ts×1 optimistic-list-mutation-matrix.test.ts×1 optimistic-untracked-reads-f3-f5.test.ts×1 signal-store-twins-qd.test.ts×1 unchanged-presence-no-hold-3743.test.ts×2 superseded-before-first-commit.test.ts×4 visibility-oracle-store.states.ts×8 visibility-oracle-store.test.ts×1 visibility-oracle.states.ts×19 visibility-oracle.test.ts×1 | [ruled, amended in place 2026-07-07 (promoted from B4)] An override lives exactly as long as its own transaction; a newer truth from the source supersedes it in the graph immediately, on screen at com… |

packages/signals/src/store/projection.ts

@@ -19,6 +19,7 @@
 import { attrHooks } from "../core/attribution-hooks.js";
 import {
   CONFIG_AUTO_DISPOSE,
+  CONFIG_VERDICT,
   STATUS_ERROR,
   STATUS_PENDING,
   STATUS_UNINITIALIZED
@@ -34,7 +35,13 @@ import {
 } from "../core/async.js";
 import { enqueueSub } from "../core/heap.js";
 import { getOwner, isDisposed } from "../core/owner.js";
-import { insertSubs, question, schedule, setQuestion } from "../core/scheduler.js";
+import {
+  insertSubs,
+  observeFlightHook,
+  question,
+  schedule,
+  setQuestion
+} from "../core/scheduler.js";
 import type { Computed, Signal } from "../core/types.js";
 import type { Refreshable } from "../core/index.js";
 import { optHooks } from "./hooks.js";
@@ -239,6 +246,18 @@ export function runProjectionComputed<T extends object>(
   const target: StoreTarget = (wrappedStore as any)[$TARGET];
   const fam = target.fam!;
   const run = (fam.run = (fam.run || 0) + 1);
+  // A verdict reader's pass that is the hold's work reads plainly, as a
+  // pending node's is (core `recompute`, #3884, D-25), so the landing it
+  // computes is the one a reader held on it shows, not the screen a frame
+  // reader is served (#4491; D-30, F-2): its pending is on its leaves'
+  // readers (`wakeFamily`) with no landing to settle them yet — the settle
+  // walk clears the derive's own pending before the landing re-runs it — or
+  // a reader of the leaves reads a flight it was served the committed value
+  // of (`fam.w`, store.ts `pullFamily`; this pass consumes the mark). For
+  // this pass only: its reads set the posture again at its end, and a window
+  // in it re-stamps it (`markVerdictReader`). Only the verdict engine makes
+  // a verdict reader (`observeFlightHook`).
+  if (observeFlightHook && (fam.woke || fam.w)) ((fam.w = 0), (owner._config &= ~CONFIG_VERDICT));
   let result: void | T | Promise<void | T> | AsyncIterable<void | T>;
   // Open loading window (seedLoadingValue): the observable store IS commit #0
   // for the whole first flight — the derive works a detached shadow of the

packages/signals/src/store/store.ts

@@ -95,6 +95,7 @@ import {
   joinPassTx,
   list,
   laneStageHook,
+  observeFlightHook,
   passLane,
   queuePendingNode,
   schedule,
@@ -1871,7 +1872,38 @@ function pullFamily(target: StoreTarget): void {
     // (optimistic.ts `stands`, #4336).
     if (verdict === null && fw._statusFlags & STATUS_PENDING && optHooks?.stands(target)) return;
     readNode(fw);
-  } else untrack(() => readNode(fw));
+  } else {
+    untrack(() => readNode(fw));
+    // #4491: a derive that is a verdict reader only through what it reads (a
+    // verdict memo's value, D-25 / F-6; no window of its own) is served the
+    // committed value of a flight it reads (`observeFlight`, A15's stale
+    // reader) and settles over it: its leaves are the screen, not the
+    // flight's answer. A derived store reads like its memo (D-30): a tracked
+    // reader of the leaves reads that flight as the memo's reader would, so
+    // one that mounts mid-flight consumes unanswered async and is held (F-2)
+    // instead of shown the old question's answer. The derive is that hold's
+    // work until it answers (D-25): the read marks its next pass, which reads
+    // plainly (`fam.w`, projection.ts), so the landing it computes is the one
+    // the held reader shows. The mark is all it writes — the derive's posture
+    // stays its passes' own (core `recompute`) — and a read that finds no
+    // flight up clears it: a landing equal to the screen re-runs nothing, and
+    // the mark does not outlive the flight. Only the verdict engine makes a
+    // verdict reader (`observeFlightHook`: without it the branch folds away).
+    // (The window flag is REACTIVE_WINDOW, spelled from EFFECT_RENDER: an
+    // import of it keeps a dead copy in bundles without the store.)
+    if (
+      observeFlightHook &&
+      fw._config & CONFIG_VERDICT &&
+      !(fw._flags & (EFFECT_RENDER << 21)) &&
+      tracking
+    ) {
+      const fam = target.fam!;
+      fam.w = 0;
+      for (let l = fw._deps; l; l = l._nextDep)
+        if ((l._dep as Computed<any>)._statusFlags & STATUS_PENDING)
+          ((fam.w = 1), readNode(l._dep));
+    }
+  }
 }
 
 /** A projection's writes are its derive's: a staging made while the derive

packages/signals/src/store/target.ts

@@ -49,6 +49,10 @@ export interface StoreFamily {
    * landing has settled them yet: the next commit walks the family
    * (`settleFamily`). A sync commit with no wake behind it walks nothing. */
   woke?: boolean;
+  /** A tracked reader of the leaves reads a flight the derive was served
+   * the committed value of (store.ts `pullFamily`, #4491): the derive's next
+   * pass reads plainly (projection.ts). */
+  w?: number;
 }
 
 /** One setter's written keys; `u` is the batch's earlier setters' (the