fix: createReaction crash when rerun tracks zero dependencies

dispose() used a do/while that called unlinkSubs once even when
node._deps was null, dereferencing a null link. Guard the loop so a
reaction whose invalidating rerun reads no dependencies disposes
cleanly. Fixes #2763.

Co-authored-by: Cursor <cursoragent@cursor.com>

Author: ryansolid

.changeset/fix-reaction-zero-dep-dispose.md

@@ -0,0 +1,5 @@
+---
+"@solidjs/signals": patch
+---
+
+Fix `createReaction` crashing with `Cannot read properties of null (reading '_dep')` when the invalidating rerun of the tracked callback reads zero dependencies. `dispose()` now guards its dependency-unlink loop instead of unconditionally calling `unlinkSubs` once.

packages/solid-signals/src/core/owner.ts

@@ -36,10 +36,10 @@ export function markDisposal(el: Owner): void {
 }
 
 export function dispose(node: Computed<unknown>): void {
-  let toRemove = node._deps || null;
-  do {
-    toRemove = unlinkSubs(toRemove!);
-  } while (toRemove !== null);
+  let toRemove = node._deps;
+  while (toRemove !== null) {
+    toRemove = unlinkSubs(toRemove);
+  }
   node._deps = null;
   node._depsTail = null;
   disposeChildren(node, true);

packages/solid-signals/tests/createReaction.test.ts

@@ -27,6 +27,28 @@ describe("createReaction", () => {
     expect(count).toBe(2);
   });
 
+  test("fires and disposes cleanly when the rerun tracks zero dependencies", () => {
+    let fired = 0;
+    let readDep = true;
+    const [sign, setSign] = createSignal("a");
+    createRoot(() => {
+      const track = createReaction(() => {
+        fired++;
+      });
+      track(() => {
+        // initial run reads a dep; the invalidating rerun reads none
+        if (readDep) sign();
+      });
+    });
+    flush();
+    expect(fired).toBe(0);
+
+    readDep = false;
+    setSign("b");
+    expect(() => flush()).not.toThrow();
+    expect(fired).toBe(1);
+  });
+
   test("throws on invalid cleanup values", () => {
     const [sign, setSign] = createSignal("thoughts");
     const track = createRoot(() =>