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fix(signals): publish latest errors with their readers #4705

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151cc83
fix(signals): preserve latest error publication through boundaries
GabbeV Nov 6, 2026
6e282e2
docs(signals): refresh error publication rule coverage
GabbeV Nov 6, 2026
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5 changes: 5 additions & 0 deletions .changeset/latest-error-publication.md
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@@ -0,0 +1,5 @@
---
"@solidjs/signals": patch
---

Publish errors exposed through `latest` with their reader's update, including error fallback changes and recovery, instead of accidentally holding them with the input update.
2 changes: 1 addition & 1 deletion packages/signals/docs/RULES-INDEX.md
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Expand Up @@ -105,7 +105,7 @@ Status legend: **live** stated and standing · **ruled** carries an explicit rul
| D-38 | live | `docs/RULES-DERIVED.md:245` | — | errored-reader-4486.test.ts×1 | Effect error phases; a comparator throw is a compute error; an escaping error halts |
| D-39 | live | `docs/RULES-DERIVED.md:253` | boundaries.ts×9 core.ts×1 lanes.ts×1 scheduler.ts×3 types.ts×1 store.ts×1 | derived-store-recovery-wake.test.ts×1 error-boundary-superseded-staged-error-4527.test.ts×1 error-is-an-answer.test.ts×2 error-staged-recovery.test.ts×1 errored-heal-memo-reader-4644.test.ts×2 errored-list-recovery-held-4526.test.ts×1 errored-memo-over-output-held-reject-4640.test.ts×1 errored-over-rearmed-loading-4384.test.ts×1 errored-reader-4486.test.ts×3 errored-recovery-fresh-loading-4627.test.ts×2 errored-recovery-stale-memo-4592.test.ts×2 errored-recovery-wedge-4403-4404-4405.test.ts×5 latest-memo-inline-parity.test.ts×2 latest-uninitialized-over-latest-4362.test.ts×1 createProjection.async.test.ts×1 derived-store-heal-4451.test.ts×1 | An error fallback is the display; it stays until a new answer replaces the error |
| D-40 | live | `docs/RULES-DERIVED.md:293` | constants.ts×1 core.ts×1 deferred.ts×2 signals.ts×1 | create-deferred-3710.test.ts×1 create-deferred.test.ts×1 | A deferred source lags its readers and never leads them |
| D-41 | live | `docs/RULES-DERIVED.md:257` | boundaries.ts×5 async.ts×5 constants.ts×1 core.ts×11 scheduler.ts×3 seat.ts×10 types.ts×1 verdict.ts×4 signals.ts×1 projection.ts×3 store.ts×2 | derived-store-error-delivered-once.test.ts×2 derived-store-recovery-wake.test.ts×1 error-boundary-superseded-staged-error-4527.test.ts×3 error-is-an-answer.test.ts×3 error-staged-recovery.test.ts×1 errored-held-mount-staged-error-4480.test.ts×3 errored-latest-obsolete-flight-4568.test.ts×1 errored-list-recovery-held-4526.test.ts×2 errored-memo-over-output-held-reject-4640.test.ts×1 errored-reader-4486.test.ts×1 errored-recovery-fresh-loading-4627.test.ts×4 errored-recovery-wedge-4403-4404-4405.test.ts×6 errored-staged-source-4364.test.ts×1 held-sync-throw-4109.test.ts×2 hold-recovers-landing-reruns-4469.test.ts×3 latest-beside-staged-read-4365.test.ts×1 latest-derived-held-error-4106.test.ts×1 latest-derived-held-error-guess-lane-4106.test.ts×1 latest-derived-sync-error-4377.test.ts×1 loading-reveal-held-outside-4593.test.ts×1 optimistic-held-error-untracked-read-4489.test.ts×3 settle-diamond-revisit.test.ts×1 staged-error-derivation.test.ts×1 staged-error-rerun-4109.test.ts×1 staged-error-through-async-memo-4640.test.ts×1 writable-memo-write-clears-error-4110.test.ts×1 | An error is an answer: it ends the wait like a value, and it shows when a value would |
| D-41 | live | `docs/RULES-DERIVED.md:257` | boundaries.ts×5 async.ts×5 constants.ts×1 core.ts×11 scheduler.ts×3 seat.ts×10 types.ts×1 verdict.ts×4 signals.ts×1 projection.ts×3 store.ts×2 | derived-store-error-delivered-once.test.ts×2 derived-store-recovery-wake.test.ts×1 error-boundary-superseded-staged-error-4527.test.ts×3 error-is-an-answer.test.ts×3 error-staged-recovery.test.ts×1 errored-held-mount-staged-error-4480.test.ts×3 errored-latest-obsolete-flight-4568.test.ts×1 errored-list-recovery-held-4526.test.ts×2 errored-memo-over-output-held-reject-4640.test.ts×1 errored-reader-4486.test.ts×1 errored-recovery-fresh-loading-4627.test.ts×4 errored-recovery-wedge-4403-4404-4405.test.ts×6 errored-staged-source-4364.test.ts×1 held-sync-throw-4109.test.ts×2 hold-recovers-landing-reruns-4469.test.ts×3 latest-beside-staged-read-4365.test.ts×1 latest-derived-held-error-4106.test.ts×1 latest-derived-held-error-guess-lane-4106.test.ts×1 latest-derived-sync-error-4377.test.ts×2 loading-reveal-held-outside-4593.test.ts×1 optimistic-held-error-untracked-read-4489.test.ts×3 settle-diamond-revisit.test.ts×1 staged-error-derivation.test.ts×1 staged-error-rerun-4109.test.ts×1 staged-error-through-async-memo-4640.test.ts×1 writable-memo-write-clears-error-4110.test.ts×1 | An error is an answer: it ends the wait like a value, and it shows when a value would |
| D-42 | live | `docs/RULES-DERIVED.md:275` | boundaries.ts×2 async.ts×3 core.ts×5 dev.ts×1 effect.ts×2 owner.ts×1 verdict.ts×1 index.ts×1 signals.ts×1 | caught-reactive-read-diagnostic.test.ts×1 errored-reader-4486.test.ts×2 try-catch-reactive-read.test.ts×2 | A reactive read is not an `await`: `try/catch` around it is not a channel |

## A — spec propositions
Expand Down
21 changes: 21 additions & 0 deletions packages/signals/src/boundaries.ts
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Expand Up @@ -114,6 +114,8 @@ interface Boundary {
* beside the held frame — not the frame's. `_lane` is set when the arm
* flips the boundary and consumed by the swap pass. */
_armLane: Transaction | null;
/** Next swap's lane. An error caught ahead carries the reader's lane;
* recovery of that fallback stays in its output's lane until it ends. */
_lane: Transaction | null;
/** DEV: the arm was display-ahead (a lane) — the fallback shows now,
* whatever holds the frame; no LOADING_ON_OUTSIDE_HOLD report. */
Expand Down Expand Up @@ -193,6 +195,11 @@ function record(b: Boundary, node: Computed<any>, source = node): void {
* decides). */
function caught(b: Boundary, node: Computed<any>, error: unknown, source = node): void {
if (b._type === STATUS_ERROR) {
// The error's source is the retry target, but the reader's selected
// outcome owns publication. Do not lose a latest/optimistic lane when
// converting its error into fallback content.
const lane = passLane ?? (node._x?._transaction?._lane ? txOf(node) : null);
if (!b._fallback) b._lane = lane;
// An error boundary waits on what THREW (`StatusError.source`), not on
// the reader that saw it, while the error is settled only when its
// source is neither errored nor in flight and the reader is not pending
Expand All @@ -205,6 +212,9 @@ function caught(b: Boundary, node: Computed<any>, error: unknown, source = node)
source = (error as { source?: Computed<any> })?.source ?? node;
if (source._flags & REACTIVE_DISPOSED) source = node;
const caught = unwrapStatusError(error);
// Subsequent errors update the existing fallback through this signal;
// its message must publish with the swap, including after a first show.
if (lane) laneStageHook!(b._error! as Computed<any>, lane, true, false);
setSignal(b._error!, caught);
reportClientError(caught, b._owner, source);
}
Expand Down Expand Up @@ -276,6 +286,7 @@ function catchStatus(
b._readers.delete(node) &&
!prune(b, 0)
) {
if (b._type === STATUS_ERROR) b._lane = passLane;
if (b._fallback) redraw(b);
if (revealHooks && b._reveal !== null) revealHooks._changed(b._reveal);
}
Expand Down Expand Up @@ -792,6 +803,16 @@ function createBoundary<T>(
// re-arm's swap is the change's work (`arm`), until that swap commits
// (D-13, #3540).
const self = getOwner() as Computed<any>;
// Reading the retained tree need not read the leaf that recovered.
// Keep the ahead swap's lane for recovery as well as for the fallback;
// otherwise that successful answer is parked in the input's hold.
if (b._type === STATUS_ERROR && b._fallback && self._config & CONFIG_OVERRIDE)
b._lane ??= txOf(self);
if (b._type === STATUS_ERROR && b._lane !== null) {
setPassLane(b._lane);
self._flags |= REACTIVE_LANE_READ;
b._lane = null;
}
if (!b._armed && !(self._config & CONFIG_HELD)) self._config |= CONFIG_FRAME;
// Reveal order (reveal.ts): a gated slot shows its fallback — or, in a
// collapsed tail, nothing — whatever its content's state; the
Expand Down
7 changes: 6 additions & 1 deletion packages/signals/src/core/lanes.ts
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Expand Up @@ -740,7 +740,12 @@ export function laneRead(c: Computed<any> | null, el: Signal<any> | Computed<any
}
const status = (el as Computed<any>)._statusFlags;
// An errored member's lane value predates its error; the error is the answer (#4106).
if (!guess && status & STATUS_ERROR) return NOT_PENDING;
if (!guess && status & STATUS_ERROR) {
// The error still belongs to this lane. A downstream memo must select
// the same update before read() throws, as it does for a lane value.
if (c !== null && tracking && !(c._config & CONFIG_CHILDREN_FORBIDDEN)) enterLane(l, c);
return NOT_PENDING;
}
if (c === null) {
if (__TEST__) shadowActual(SLOT_LV);
return laneValueOf(el);
Expand Down
11 changes: 6 additions & 5 deletions packages/signals/tests/latest-derived-sync-error-4377.test.ts
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Expand Up @@ -6,11 +6,12 @@
* the error before `m` did, so `<Errored>`, which waits on what threw, let it
* go as settled; at the commit the leaf re-derived and reached the boundary
* again, the memo's reader did not (an equal status re-notifies nobody) and
* kept `m`'s last value. With a held write the same: the leaf shows the
* error at the reveal, the derivation never did. Fixed by #4364's `prune`
* kept `m`'s last value. With a held write the leaf originally showed the
* error at the reveal, while the derivation never did. Fixed by #4364's `prune`
* change (#4387): a staged error on an error boundary's source keeps the
* boundary waiting, so the commit that publishes it finds the boundary still
* waiting on `m`. These pin it.
* waiting on `m`. The held cases now also pin D-41's value/error symmetry:
* latest publishes the error ahead; an ordinary read waits for the reveal.
*/
import { describe, expect, it } from "vitest";
import {
Expand Down Expand Up @@ -66,7 +67,7 @@ describe("a memo over latest(m) shows m's sync error as the leaf does (#4377)",
expect(s.screen()).toBe("[ERR]");
});

it(`${shape}: a held write shows the error at the reveal`, async () => {
it(`${shape}: a held error follows the selected value publication`, async () => {
const s = scene(shape);
let release!: () => void;
action(function* () {
Expand All @@ -75,7 +76,7 @@ describe("a memo over latest(m) shows m's sync error as the leaf does (#4377)",
})();
flush();
await tick();
expect(s.screen()).toBe(0);
expect(s.screen()).toBe(shape === "plain" ? 0 : "[ERR]");
release();
await tick();
await tick();
Expand Down
4 changes: 3 additions & 1 deletion packages/signals/tests/latest-uninitialized-over-latest-4362.test.ts
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Expand Up @@ -307,6 +307,8 @@ describe("uninitialized async memo over latest(s) (#4362, #4363)", () => {
// shape #4365 found): the pass `latest(a) + b()` runs ahead of the hold
// that holds `b`, throws over b's screen value (2 + 0 + 1), and its error
// is its own. The landing re-derives it on the landed world (2 + 3 + 1).
// Like a successful answer to that ahead pass, the error publishes ahead;
// the recovery then replaces it when the ordinary update lands.
it.each(["memo", "effect"])(
"a pass that threw over a held write re-derives at the landing (%s)",
async reader => {
Expand Down Expand Up @@ -353,7 +355,7 @@ describe("uninitialized async memo over latest(s) (#4362, #4363)", () => {
setB(3);
setC(2);
flush();
expect(out).toBe("title=2");
expect(out).toBe("[E m:3]");
resolve();
for (let i = 0; i < 8; i++) await Promise.resolve();
flush();
Expand Down
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