fix(signals): derived store sync landing wakes parked readers

.changeset/derived-store-sync-landing-wakes-readers.md

@@ -0,0 +1,5 @@
+---
+"@solidjs/signals": patch
+---
+
+A derived store whose pending first flight is superseded by a synchronous landing now wakes the readers parked on that flight in the same flush, including readers whose store node the landing left unchanged (`"length" in store`, `Object.keys`). A landing staged by a transaction keeps the seed invisible until that transaction commits.

packages/signals/src/store/next/projection.ts

@@ -18,14 +18,17 @@ import {
   computed,
   CONFIG_AUTO_DISPOSE,
   getOwner,
+  GlobalQueue,
   handleAsync,
   isDisposed,
   STATUS_PENDING,
+  STATUS_UNINITIALIZED,
   type Computed,
   type Refreshable
 } from "../../core/index.js";
 
 import {
+  activeTransition,
   projectionWriteActive,
   scheduleWithheld,
   setProjectionWriteActive
@@ -239,6 +242,16 @@ function cloneState<T extends object>(v: T, shallow: boolean): T {
   return shallow ? (Array.isArray(v) ? (v.slice() as T) : { ...v }) : JSON.parse(JSON.stringify(v));
 }
 
+// Held past this flush: the transaction is parked by more than the node's own flight.
+function landingHeld(owner: Computed<any>): boolean {
+  const t = activeTransition;
+  if (t === null) return false;
+  if (t._actions.length || GlobalQueue._transitionBlocked?.(t)) return true;
+  for (const source of t._asyncReporters.keys())
+    if (source !== owner && source._x?._pendingSources?.size) return true;
+  return false;
+}
+
 export function runProjectionComputedNext<T extends object>(
   wrappedStore: Store<T>,
   fn: (draft: T) => void | T | Promise<void | T> | AsyncIterable<void | T>,
@@ -303,6 +316,9 @@ export function runProjectionComputedNext<T extends object>(
       };
       const sync = handleAsync(owner, result, commit);
       if (!owner._loading) commit(sync as void | T);
+      // Recompute's settle walk for a superseded flight skips an uninitialized node.
+      if (owner._x?._pendingSources?.has(owner) && !landingHeld(owner))
+        owner._statusFlags &= ~STATUS_UNINITIALIZED;
     },
     false
   );

packages/signals/tests/store/derived-presence-async-3726.test.ts

@@ -0,0 +1,120 @@
+import { describe, expect, it } from "vitest";
+import {
+  NotReadyError,
+  action,
+  createLoadingBoundary,
+  createRenderEffect,
+  createRoot,
+  createSignal,
+  createStore,
+  flush,
+  untrack,
+  type Store
+} from "../../src/index.js";
+
+const tick = () => new Promise(r => setTimeout(r, 0));
+
+describe("sync landing after a pending first flight wakes unchanged-node readers (#3726)", () => {
+  it("presence read inside a loading boundary shows the landed answer", async () => {
+    const [source, setSource] = createSignal<number[] | null>(null);
+    let resolve!: (v: number[]) => void;
+    let presence = "";
+    let sourceState = "";
+    createRoot(() => {
+      const [store] = createStore<number[]>(() => {
+        const s = source();
+        if (s) return s;
+        return new Promise<number[]>(r => (resolve = r));
+      }, []);
+      createLoadingBoundary(
+        () => {
+          createRenderEffect(
+            () => ("length" in store ? "present" : "missing"),
+            v => {
+              presence = v;
+            }
+          );
+          createRenderEffect(
+            () => (source() ? "resolved" : "pending"),
+            v => {
+              sourceState = v;
+            }
+          );
+        },
+        () => "fallback"
+      );
+    });
+    flush();
+    expect(presence).toBe("");
+    expect(sourceState).toBe("pending");
+
+    setSource([1]);
+    flush();
+    expect(sourceState).toBe("resolved");
+    expect(presence).toBe("present");
+
+    resolve([2, 2]);
+    await tick();
+    flush();
+    expect(presence).toBe("present");
+  });
+
+  it("readers of presence, length and keys wake when the landing leaves them unchanged", () => {
+    const [source, setSource] = createSignal<number[] | null>(null);
+    const seen: string[] = [];
+    createRoot(() => {
+      const [store] = createStore<number[]>(() => {
+        const s = source();
+        if (s) return s;
+        return new Promise<number[]>(() => {});
+      }, []);
+      createRenderEffect(
+        () => (0 in store ? "present" : "missing"),
+        v => {
+          seen.push(`in:${v}`);
+        }
+      );
+      createRenderEffect(
+        () => store.length,
+        v => {
+          seen.push(`length:${v}`);
+        }
+      );
+      createRenderEffect(
+        () => Object.keys(store).length,
+        v => {
+          seen.push(`keys:${v}`);
+        }
+      );
+    });
+    flush();
+    expect(seen).toEqual([]);
+
+    setSource([]);
+    flush();
+    expect(seen.sort()).toEqual(["in:missing", "keys:0", "length:0"]);
+  });
+
+  it("a landing held by a transaction keeps the seed invisible until the commit", async () => {
+    const [source, setSource] = createSignal<number[] | null>(null);
+    let release!: () => void;
+    let store!: Store<number[]>;
+    createRoot(() => {
+      [store] = createStore<number[]>(() => source() ?? new Promise(() => {}), [9, 9, 9]);
+    });
+    flush();
+
+    const done = action(function* () {
+      setSource([1]);
+      yield new Promise<void>(r => (release = r));
+    })();
+    flush();
+    expect(() => untrack(() => [...store])).toThrow(NotReadyError);
+
+    release();
+    await done;
+    await tick();
+    flush();
+    expect(untrack(() => [...store])).toEqual([1]);
+  });
+});