router: dedupe DirnameStore interns so FileSystemRouter.reload() does not exhaust the store

src/router/lib.rs

@@ -1123,18 +1123,14 @@ impl Route {
                     let name_offset = name.as_ptr() as usize - public_path.as_ptr() as usize;
                     let name_len = name.len();
 
-                    // NOTE: DirnameStore::append returns `&'static [u8]` (process-
+                    // NOTE: `intern_route_path` returns `&'static [u8]` (process-
                     // lifetime arena), so rebinding here drops the borrow on
                     // `route_file_buf` and avoids needing lifetime transmutes
                     // below.
-                    let dirname_store = FileSystem::instance().dirname_store();
-                    let public_path: &'static [u8] =
-                        dirname_store.append(public_path).expect("unreachable");
+                    let public_path: &'static [u8] = intern_route_path(public_path);
                     let name: &'static [u8] = &public_path[name_offset..][0..name_len];
                     let match_name: &'static [u8] = if has_uppercase {
-                        dirname_store
-                            .append_lower_case(&name[1..])
-                            .expect("unreachable")
+                        intern_route_path_lower_case(&name[1..])
                     } else {
                         &name[1..]
                     };
@@ -1143,9 +1139,7 @@ impl Route {
                     debug_assert!(name[0] == b'/');
                     (public_path, name, match_name)
                 } else {
-                    let dirname_store = FileSystem::instance().dirname_store();
-                    let public_path: &'static [u8] =
-                        dirname_store.append(public_path).expect("unreachable");
+                    let public_path: &'static [u8] = intern_route_path(public_path);
                     (
                         public_path,
                         Route::INDEX_ROUTE_NAME,
@@ -1231,10 +1225,7 @@ impl Route {
                     }
                 };
 
-                abs_path_str = FileSystem::instance()
-                    .dirname_store()
-                    .append(_abs)
-                    .expect("unreachable");
+                abs_path_str = intern_route_path(_abs);
 
                 // SAFETY: sole mutation; `base_`/`extname` (which may borrow
                 // `(*entry).base_.remainder_buf`) are not used after this.
@@ -1249,11 +1240,7 @@ impl Route {
                     abs_path_str,
                     &mut bufs.normalized_abs_path_buf,
                 );
-                let interned: &'static [u8] = FileSystem::instance()
-                    .dirname_store()
-                    .append(normalized)
-                    .expect("unreachable");
-                Interned::from_static(interned)
+                Interned::from_static(intern_route_path(normalized))
             };
             #[cfg(not(windows))]
             let abs_path = Interned::from_static(abs_path_str);
@@ -1269,14 +1256,11 @@ impl Route {
             }
 
             // NOTE: name/match_name/public_path are already `&'static` via
-            // DirnameStore::append above. `entry.base()` borrows the entry (it
+            // `intern_route_path` above. `entry.base()` borrows the entry (it
             // may be inline-stored for ≤31-byte names); intern it
             // explicitly to get `&'static` without a lifetime transmute.
             // SAFETY: read-only reborrow; the `&mut` write above is dead.
-            let basename: &'static [u8] = FileSystem::instance()
-                .dirname_store()
-                .append(unsafe { &*entry }.base())
-                .expect("unreachable");
+            let basename: &'static [u8] = intern_route_path(unsafe { &*entry }.base());
 
             Some(Route {
                 name,
@@ -1381,6 +1365,42 @@ thread_local! {
         #[cfg(windows)]
         normalized_abs_path_buf: bun_sys::windows::PathBuffer::ZEROED,
     }));
+
+    static ROUTE_PATH_INTERN: RefCell<bun_collections::HashMap<&'static [u8], ()>> =
+        RefCell::new(bun_collections::HashMap::new());
+}
+
+/// Intern `value` into the process-lifetime `DirnameStore`, deduplicated by
+/// content. `BSSStringList::append` does not dedupe, so without this every
+/// `FileSystemRouter::reload()` would re-append each route's public path,
+/// absolute path and basename, eventually exhausting the store's slot capacity
+/// (`AllocError`) and leaking one heap buffer per append. Same pattern as
+/// `intern_transpile_path` in `jsc_hooks.rs`.
+fn intern_route_path(value: &[u8]) -> &'static [u8] {
+    let dirname_store = FileSystem::instance().dirname_store();
+    if dirname_store.exists(value) {
+        // SAFETY: `exists` is a pointer-range check against the store's
+        // process-lifetime backing buffer, so `value` is already `'static`.
+        return unsafe { core::slice::from_raw_parts(value.as_ptr(), value.len()) };
+    }
+    ROUTE_PATH_INTERN.with_borrow_mut(|set| {
+        if let Some((interned, ())) = set.get_key_value(value) {
+            return *interned;
+        }
+        let interned: &'static [u8] = bun_core::handle_oom(dirname_store.append(value));
+        set.insert(interned, ());
+        interned
+    })
+}
+
+fn intern_route_path_lower_case(value: &[u8]) -> &'static [u8] {
+    if value.len() <= 256 {
+        let mut scratch = [0u8; 256];
+        intern_route_path(strings::copy_lowercase(value, &mut scratch[..value.len()]))
+    } else {
+        let mut scratch = vec![0u8; value.len()];
+        intern_route_path(strings::copy_lowercase(value, &mut scratch))
+    }
 }
 
 pub struct Match<'a> {

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

@@ -402,6 +402,54 @@
   expect(router.match("/posts")!.name).toBe("/posts");
 });
 
+it("reload() does not leak route paths into the process-global intern store", async () => {
+  // Route::parse interns each route's public path, absolute path, and basename
+  // into the never-freed DirnameStore. Without content dedup, every reload()
+  // re-appended identical strings, leaking one heap buffer per append and
+  // eventually panicking with `unreachable: AllocError` when the store's slot
+  // capacity was exhausted. Nest the pages directory deeply so the absolute
+  // path (which dominates bytes) is long enough for the leak to show over the
+  // unrelated per-reload overhead.
+  const seg = Buffer.alloc(100, "d").toString();
+  const parts = Array.from({ length: 14 }, () => seg);
+  const files: Record<string, string> = {};
+  for (let i = 0; i < 200; i++) files[path.join(...parts, "pages", `Page${i}.tsx`)] = "export default 1;\n";
+  using dir = tempDir("fsr-reload-intern", files);
+  const pagesDir = path.join(String(dir), ...parts, "pages");
+
+  const code = /* ts */ `
+    const router = new Bun.FileSystemRouter({
+      dir: ${JSON.stringify(pagesDir)},
+      style: "nextjs",
+      fileExtensions: [".tsx"],
+    });
+    const m = router.match("/Page7");
+    if (!m || !m.filePath.endsWith("Page7.tsx")) throw new Error("match() broken: " + m?.filePath);
+    for (let i = 0; i < 5; i++) router.reload();
+    Bun.gc(true);
+    const before = process.memoryUsage.rss();
+    for (let i = 0; i < 400; i++) router.reload();
+    Bun.gc(true);
+    const m2 = router.match("/Page7");
+    if (!m2 || !m2.filePath.endsWith("Page7.tsx")) throw new Error("match() broken after reload: " + m2?.filePath);
+    const growthMB = (process.memoryUsage.rss() - before) / 1024 / 1024;
+    console.error("RSS growth: " + growthMB.toFixed(1) + "MB");
+    if (growthMB > 110) throw new Error("leaked " + growthMB.toFixed(1) + "MB");
+  `;
+
+  await using proc = Bun.spawn({
+    cmd: [bunExe(), "--smol", "-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).not.toContain("leaked");
+  expect(stderr).not.toContain("AllocError");
+  expect(stdout).toBe("");
+  expect(exitCode).toBe(0);
+}, 60_000);
+
 it("reload() works with new dirs/files", () => {
   const { dir } = make(["posts.tsx"]);