Fix bug with fetcher submission ancestor-thrown middleware errors

.changeset/forty-bugs-turn.md

@@ -0,0 +1,5 @@
+---
+"react-router": patch
+---
+
+Properly handle ancestor thrown middleware errors before `next()` on fetcher submissions

packages/react-router/__tests__/router/context-middleware-test.tsx

@@ -1633,6 +1633,56 @@ describe("context/middleware", () => {
           e: new Error("E ERROR"),
         });
       });
+
+      it("throwing from a fetcher action middleware before next bubbles up to the boundary", async () => {
+        router = createRouter({
+          history: createMemoryHistory(),
+          routes: [
+            {
+              path: "/",
+            },
+            {
+              id: "a",
+              path: "/a",
+              hasErrorBoundary: true,
+              children: [
+                {
+                  id: "b",
+                  path: "b",
+                  hasErrorBoundary: false,
+                  middleware: [
+                    ({ request }) => {
+                      if (request.method === "POST") {
+                        throw new Error("B ERROR");
+                      }
+                    },
+                  ],
+                  action: () => "B",
+                },
+              ],
+            },
+          ],
+        });
+
+        // Bubbles to B because it's the initial load and it's loader hasn't run
+        await router.navigate("/a/b");
+        expect(router.state.loaderData).toEqual({});
+        expect(router.state.errors).toBeNull();
+
+        let data;
+        router.subscribe((state) => {
+          data ??= state.fetchers.get("key")?.data;
+        });
+
+        await router.fetch("key", "b", "/a/b", {
+          formMethod: "post",
+          formData: createFormData({}),
+        });
+        expect(data).toBeUndefined();
+        expect(router.state.errors).toEqual({
+          a: new Error("B ERROR"),
+        });
+      });
     });
   });
 

packages/react-router/lib/router/router.ts

@@ -2465,6 +2465,17 @@ export function createRouter(init: RouterInit): Router {
     );
     let actionResult = actionResults[match.route.id];
 
+    if (!actionResult) {
+      // If this error came from a parent middleware before the action ran,
+      // then it won't be tied to the action route
+      for (let match of fetchMatches) {
+        if (actionResults[match.route.id]) {
+          actionResult = actionResults[match.route.id];
+          break;
+        }
+      }
+    }
+
     if (fetchRequest.signal.aborted) {
       // We can delete this so long as we weren't aborted by our own fetcher
       // re-submit which would have put _new_ controller is in fetchControllers