FileSystemRouter: build JS error before freeing arena in error paths

src/bun.js/api/filesystem_router.zig

@@ -109,10 +109,12 @@
         const path_to_use = (root_dir_path.cloneWithTrailingSlash(allocator) catch unreachable).slice();
 
         const root_dir_info = vm.transpiler.resolver.readDirInfo(path_to_use) catch {
+            // Build the JS error before freeing the arena: `log` is backed by the arena allocator.
+            const err_value = try log.toJS(globalThis, globalThis.allocator(), "reading root directory");
             origin_str.deinit();
             arena.deinit();
             globalThis.allocator().destroy(arena);
-            return globalThis.throwValue(try log.toJS(globalThis, globalThis.allocator(), "reading root directory"));
+            return globalThis.throwValue(err_value);
         } orelse {
             origin_str.deinit();
             arena.deinit();
@@ -127,10 +129,12 @@
         }) catch unreachable;
 
         router.loadRoutes(&log, root_dir_info, Resolver, &vm.transpiler.resolver, router.config.dir) catch {
+            // Build the JS error before freeing the arena: `log` is backed by the arena allocator.
+            const err_value = try log.toJS(globalThis, globalThis.allocator(), "loading routes");
             origin_str.deinit();
             arena.deinit();
             globalThis.allocator().destroy(arena);
-            return globalThis.throwValue(try log.toJS(globalThis, globalThis.allocator(), "loading routes"));
+            return globalThis.throwValue(err_value);
         };
 
         if (try argument.get(globalThis, "origin")) |origin| {
@@ -143,10 +147,12 @@
         }
 
         if (log.errors + log.warnings > 0) {
+            // Build the JS error before freeing the arena: `log` is backed by the arena allocator.
+            const err_value = try log.toJS(globalThis, globalThis.allocator(), "loading routes");
             origin_str.deinit();
             arena.deinit();
             globalThis.allocator().destroy(arena);
-            return globalThis.throwValue(try log.toJS(globalThis, globalThis.allocator(), "loading routes"));
+            return globalThis.throwValue(err_value);
         }
 
         var fs_router = globalThis.allocator().create(FileSystemRouter) catch unreachable;
@@ -246,9 +252,11 @@
             .asset_prefix_path = this.router.config.asset_prefix_path,
         }) catch unreachable;
         router.loadRoutes(&log, root_dir_info, Resolver, &vm.transpiler.resolver, router.config.dir) catch {
+            // Build the JS error before freeing the arena: `log` is backed by the arena allocator.
+            const err_value = try log.toJS(globalThis, globalThis.allocator(), "loading routes");
             arena.deinit();
             globalThis.allocator().destroy(arena);
-            return globalThis.throwValue(try log.toJS(globalThis, globalThis.allocator(), "loading routes"));
+            return globalThis.throwValue(err_value);
         };
 
         this.router.deinit();

test/js/bun/util/filesystem_router.test.ts

@@ -510,3 +510,37 @@ it("MatchedRoute.params does not leak", async () => {
   expect(stdout).toBe("");
   expect(exitCode).toBe(0);
 }, 60_000);
+
+it("throws a clean error for invalid route filenames (no use-after-free)", async () => {
+  // The constructor's log is backed by an arena allocator. When route loading
+  // produces errors (e.g. a filename like `[foo.tsx` missing its closing bracket),
+  // the arena must not be freed before log.toJS() reads the messages.
+  // Run in a subprocess so an ASAN crash doesn't take down the test runner.
+  using dir = tempDir("fsr-invalid-route", {
+    "pages/[foo.tsx": "export default 1;",
+  });
+
+  const code = /* ts */ `
+    try {
+      new Bun.FileSystemRouter({
+        style: "nextjs",
+        dir: ${JSON.stringify(path.join(String(dir), "pages"))},
+        fileExtensions: [".tsx"],
+      });
+      console.log("no-throw");
+    } catch (e) {
+      console.log("caught:" + (e?.message ?? String(e)));
+    }
+  `;
+
+  await using proc = Bun.spawn({
+    cmd: [bunExe(), "-e", code],
+    env: bunEnv,
+    stdout: "pipe",
+    stderr: "pipe",
+  });
+  const [stdout, stderr, exitCode] = await Promise.all([proc.stdout.text(), proc.stderr.text(), proc.exited]);
+  expect(stderr).toBe("");
+  expect(stdout.trim()).toBe("caught:Route is missing a closing bracket]");
+  expect(exitCode).toBe(0);
+});