Fix loop in unstable_useBlocker when used with an unstable blocker function

.changeset/fix-blocker-loop.md

@@ -0,0 +1,5 @@
+---
+"react-router": patch
+---
+
+Fix loop in `unstable_useBlocker` when used with an unstable blocker function

packages/react-router-dom/__tests__/use-blocker-test.tsx

@@ -114,6 +114,50 @@ describe("navigation blocking with useBlocker", () => {
     act(() => root.unmount());
   });
 
+  it("handles unstable blocker function identities", async () => {
+    let count = 0;
+    router = createMemoryRouter([
+      {
+        element: React.createElement(() => {
+          // New function identity on each render
+          let b = useBlocker(() => false);
+          blocker = b;
+          if (++count > 50) {
+            throw new Error("useBlocker caused a re-render loop!");
+          }
+          return (
+            <div>
+              <Link to="/about">/about</Link>
+              <Outlet />
+            </div>
+          );
+        }),
+        children: [
+          {
+            path: "/",
+            element: <h1>Home</h1>,
+          },
+          {
+            path: "/about",
+            element: <h1>About</h1>,
+          },
+        ],
+      },
+    ]);
+
+    act(() => {
+      root = ReactDOM.createRoot(node);
+      root.render(<RouterProvider router={router} />);
+    });
+
+    expect(node.querySelector("h1")?.textContent).toBe("Home");
+
+    act(() => click(node.querySelector("a[href='/about']")));
+    expect(node.querySelector("h1")?.textContent).toBe("About");
+
+    act(() => root.unmount());
+  });
+
   describe("on <Link> navigation", () => {
     describe("blocker returns false", () => {
       beforeEach(() => {

packages/react-router/lib/hooks.tsx

@@ -972,12 +972,23 @@ export function useBlocker(shouldBlock: boolean | BlockerFunction): Blocker {
     [basename, shouldBlock]
   );
 
+  // This effect is in charge of blocker key assignment and deletion (which is
+  // tightly coupled to the key)
   React.useEffect(() => {
     let key = String(++blockerId);
-    setBlocker(router.getBlocker(key, blockerFunction));
     setBlockerKey(key);
     return () => router.deleteBlocker(key);
-  }, [router, setBlocker, setBlockerKey, blockerFunction]);
+  }, [router]);
+
+  // This effect handles assigning the blockerFunction.  This is to handle
+  // unstable blocker function identities, and happens only after the prior
+  // effect so we don't get an orphaned blockerFunction in the router with a
+  // key of "".  Until then we just have the IDLE_BLOCKER.
+  React.useEffect(() => {
+    if (blockerKey !== "") {
+      setBlocker(router.getBlocker(blockerKey, blockerFunction));
+    }
+  }, [router, blockerKey, blockerFunction]);
 
   // Prefer the blocker from state since DataRouterContext is memoized so this
   // ensures we update on blocker state updates