fix(router): don't cache file descriptors in Route.parse to prevent stale fd reuse (#27164)

## Summary
- `FileSystemRouter.Route.parse()` was caching file descriptors in the
global entry cache (`entry.cache.fd`). When `Bun.build()` later closed
these fds during `ParseTask`, the cache still referenced them.
Subsequent `Bun.build()` calls would find these stale fds, pass them to
`readFileWithAllocator`, and `seekTo(0)` would fail with EBADF (errno
9).
- The fix ensures `Route.parse` always closes the file it opens for
`getFdPath` instead of caching it in the shared entry. The fd was only
used to resolve the absolute path via `getFdPath`, so caching was
unnecessary and harmful.

Closes #18242

## Test plan
- [x] Added regression test `test/regression/issue/18242.test.ts` that
creates a `FileSystemRouter` and runs `Bun.build()` three times
sequentially
- [x] Test passes with `bun bd test test/regression/issue/18242.test.ts`
- [x] Test fails with `USE_SYSTEM_BUN=1 bun test
test/regression/issue/18242.test.ts` (system bun v1.3.9)
- [x] Verified 5 sequential builds work correctly after the fix

🤖 Generated with [Claude Code](https://claude.com/claude-code)

---------

Co-authored-by: Claude Bot <claude-bot@bun.sh>
Co-authored-by: Claude <noreply@anthropic.com>
Co-authored-by: Jarred Sumner <jarred@jarredsumner.com>

src/router.zig

@@ -731,23 +731,22 @@ pub const Route = struct {
 
         if (abs_path_str.len == 0) {
             var file: std.fs.File = undefined;
-            var needs_close = false;
+            var needs_close = true;
             defer if (needs_close) file.close();
             if (entry.cache.fd.unwrapValid()) |valid| {
                 file = valid.stdFile();
+                needs_close = false;
             } else {
                 var parts = [_]string{ entry.dir, entry.base() };
                 abs_path_str = FileSystem.instance.absBuf(&parts, &route_file_buf);
                 route_file_buf[abs_path_str.len] = 0;
                 const buf = route_file_buf[0..abs_path_str.len :0];
                 file = std.fs.openFileAbsoluteZ(buf, .{ .mode = .read_only }) catch |err| {
+                    needs_close = false;
                     log.addErrorFmt(null, Logger.Loc.Empty, allocator, "{s} opening route: {s}", .{ @errorName(err), abs_path_str }) catch unreachable;
                     return null;
                 };
                 FileSystem.setMaxFd(file.handle);
-
-                needs_close = FileSystem.instance.fs.needToCloseFiles();
-                if (!needs_close) entry.cache.fd = .fromStdFile(file);
             }
 
             const _abs = bun.getFdPath(.fromStdFile(file), &route_file_buf) catch |err| {

test/regression/issue/18242.test.ts

@@ -0,0 +1,62 @@
+import { expect, test } from "bun:test";
+import { bunEnv, bunExe, tempDir } from "harness";
+
+test("Bun.build works multiple times after FileSystemRouter is created", async () => {
+  using dir = tempDir("issue-18242", {
+    "pages/index.ts": `console.log("Hello via Bun!");`,
+    "build.ts": `
+import path from "path";
+
+const PAGES_DIR = path.resolve(process.cwd(), "pages");
+
+const srcRouter = new Bun.FileSystemRouter({
+  dir: PAGES_DIR,
+  style: "nextjs",
+});
+
+const entrypoints = Object.values(srcRouter.routes);
+
+const result1 = await Bun.build({
+  entrypoints,
+  outdir: "dist/browser",
+});
+
+const result2 = await Bun.build({
+  entrypoints,
+  outdir: "dist/bun",
+  target: "bun",
+});
+
+const result3 = await Bun.build({
+  entrypoints,
+  outdir: "dist/third",
+});
+
+console.log(JSON.stringify({
+  build1: result1.success,
+  build2: result2.success,
+  build3: result3.success,
+  build2Logs: result2.logs.map(String),
+  build3Logs: result3.logs.map(String),
+}));
+`,
+  });
+
+  await using proc = Bun.spawn({
+    cmd: [bunExe(), "build.ts"],
+    env: bunEnv,
+    cwd: String(dir),
+    stderr: "pipe",
+  });
+
+  const [stdout, stderr, exitCode] = await Promise.all([proc.stdout.text(), proc.stderr.text(), proc.exited]);
+
+  const result = JSON.parse(stdout.trim());
+
+  expect(result.build1).toBe(true);
+  expect(result.build2).toBe(true);
+  expect(result.build3).toBe(true);
+  expect(result.build2Logs).toEqual([]);
+  expect(result.build3Logs).toEqual([]);
+  expect(exitCode).toBe(0);
+});