resolver: dedupe DirnameStore interns across bust+reread and Route::parse

src/codegen/generate-js2native.ts

@@ -59,6 +59,7 @@ const rustIdentifierPaths: Record<string, string> = {
   "escapeRegExp.rs": "string/escapeRegExp.rs",
   "event_loop.rs": "jsc/event_loop.rs",
   "ffi.rs": "runtime/ffi/ffi.rs",
+  "filesystem_router.rs": "runtime/api/filesystem_router.rs",
   "h2_frame_parser.rs": "runtime/api/bun/h2_frame_parser.rs",
   "hosted_git_info.rs": "install/hosted_git_info.rs",
   "http/H2Client.rs": "http/H2Client.rs",

src/js/internal-for-testing.ts

@@ -52,6 +52,8 @@ const shellParse = $newRustFunction("shell.rs", "TestingAPIs.shellParse", 2);
 
 export const sslCtxLiveCount = $newRustFunction("SecureContext.rs", "jsLiveCount", 0);
 
+export const dirnameStoreAppendCount = $newRustFunction("filesystem_router.rs", "jsDirnameStoreAppendCount", 0);
+
 export const napiThreadsafeFunctionLiveCount = $newRustFunction("napi_body.rs", "jsThreadsafeFunctionLiveCount", 0);
 
 export const escapeRegExp = $newRustFunction("escapeRegExp.rs", "jsEscapeRegExp", 1);

src/resolver/lib.rs

@@ -122,12 +122,83 @@
                     // SAFETY: see `append_slice`.
                     unsafe { &*$backing() }.exists(value)
                 }
+                /// Total number of `append*` calls (inline + overflow). Exposed via
+                /// `bun:internal-for-testing` so leak tests can assert "N reloads
+                /// performed zero new appends".
+                pub fn append_count(&self) -> u32 {
+                    // SAFETY: see `append_slice`.
+                    let b = unsafe { &*$backing() };
+                    let _g = b.mutex.lock();
+                    b.slice_buf_used as u32 + b.overflow_list.count
+                }
             }
         };
     }
     string_store_impl!(DirnameStore, DIRNAME_STORE_ZST, dirname_store_backing);
     string_store_impl!(FilenameStore, FILENAME_STORE_ZST, filename_store_backing);
 
+    thread_local! {
+        static DIRNAME_INTERN: core::cell::RefCell<
+            bun_collections::HashMap<&'static [u8], ()>,
+        > = core::cell::RefCell::new(bun_collections::HashMap::new());
+    }
+
+    impl DirnameStore {
+        /// Content-deduplicated [`append_slice`]. `BSSStringList::append` does not
+        /// dedupe, so a bust-then-reread cycle (hot reload, `FileSystemRouter.reload`)
+        /// would otherwise re-append the same directory path on every miss, exhausting
+        /// the store's slot capacity and leaking one heap buffer per overflow append.
+        pub fn intern_slice(&self, value: &[u8]) -> crate::CrateResult<&'static [u8]> {
+            if self.exists(value) {
+                // SAFETY: `exists` is a pointer-range check — `value` lies wholly
+                // within the process-lifetime backing buffer, so widening is sound.
+                return Ok(unsafe { core::slice::from_raw_parts(value.as_ptr(), value.len()) });
+            }
+            DIRNAME_INTERN.with_borrow_mut(|set| {
+                if let Some((interned, ())) = set.get_key_value(value) {
+                    return Ok(*interned);
+                }
+                let interned = self.append_slice(value)?;
+                set.insert(interned, ());
+                Ok(interned)
+            })
+        }
+
+        /// Content-deduplicated [`append_parts`]. See [`intern_slice`].
+        pub fn intern_parts(&self, parts: &[&[u8]]) -> crate::CrateResult<&'static [u8]> {
+            const STACK: usize = 512;
+            let total: usize = parts.iter().map(|p| p.len()).sum();
+            let mut stack = [0u8; STACK];
+            let mut heap: Vec<u8>;
+            let scratch: &mut [u8] = if total <= STACK {
+                &mut stack[..total]
+            } else {
+                heap = vec![0u8; total];
+                &mut heap[..]
+            };
+            let mut at = 0;
+            for p in parts {
+                scratch[at..at + p.len()].copy_from_slice(p);
+                at += p.len();
+            }
+            self.intern_slice(&scratch[..at])
+        }
+
+        /// Content-deduplicated [`append_lower_case`]. See [`intern_slice`].
+        pub fn intern_lower_case(&self, value: &[u8]) -> crate::CrateResult<&'static [u8]> {
+            const STACK: usize = 256;
+            let mut stack = [0u8; STACK];
+            let mut heap: Vec<u8>;
+            let scratch: &mut [u8] = if value.len() <= STACK {
+                &mut stack[..value.len()]
+            } else {
+                heap = vec![0u8; value.len()];
+                &mut heap[..]
+            };
+            self.intern_slice(bun_core::strings::copy_lowercase(value, scratch))
+        }
+    }
+
     macro_rules! string_store_append_impl {
         ($t:ty, $backing:ident) => {
             impl $t {

src/resolver/resolver.rs

@@ -3509,7 +3509,7 @@ impl<'a> Resolver<'a> {
                     unsafe { &*existing }.dir
                 } else {
                     Fs::file_system::DirnameStore::instance()
-                        .append_slice(dir_path)
+                        .intern_slice(dir_path)
                         .expect("unreachable")
                 },
                 self.generation,
@@ -4517,9 +4517,9 @@ impl<'a> Resolver<'a> {
                     let input = &path[..input_path_len];
                     if input[input.len() - 1] != SEP {
                         let parts: [&[u8]; 2] = [input, SEP_STR.as_bytes()];
-                        _safe_path = Some(self.fs_ref().dirname_store.append_parts(&parts)?);
+                        _safe_path = Some(self.fs_ref().dirname_store.intern_parts(&parts)?);
                     } else {
-                        _safe_path = Some(self.fs_ref().dirname_store.append_slice(input)?);
+                        _safe_path = Some(self.fs_ref().dirname_store.intern_slice(input)?);
                     }
                 }
 
@@ -4578,7 +4578,7 @@ impl<'a> Resolver<'a> {
                         unsafe { &*existing }.dir
                     } else {
                         Fs::file_system::DirnameStore::instance()
-                            .append_slice(dir_path)
+                            .intern_slice(dir_path)
                             .expect("unreachable")
                     },
                     self.generation,

src/router/lib.rs

@@ -1123,17 +1123,17 @@ 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: DirnameStore::intern_* 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");
+                        dirname_store.intern_slice(public_path).expect("unreachable");
                     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..])
+                            .intern_lower_case(&name[1..])
                             .expect("unreachable")
                     } else {
                         &name[1..]
@@ -1145,7 +1145,7 @@ impl Route {
                 } else {
                     let dirname_store = FileSystem::instance().dirname_store();
                     let public_path: &'static [u8] =
-                        dirname_store.append(public_path).expect("unreachable");
+                        dirname_store.intern_slice(public_path).expect("unreachable");
                     (
                         public_path,
                         Route::INDEX_ROUTE_NAME,
@@ -1233,7 +1233,7 @@ impl Route {
 
                 abs_path_str = FileSystem::instance()
                     .dirname_store()
-                    .append(_abs)
+                    .intern_slice(_abs)
                     .expect("unreachable");
 
                 // SAFETY: sole mutation; `base_`/`extname` (which may borrow
@@ -1251,7 +1251,7 @@ impl Route {
                 );
                 let interned: &'static [u8] = FileSystem::instance()
                     .dirname_store()
-                    .append(normalized)
+                    .intern_slice(normalized)
                     .expect("unreachable");
                 Interned::from_static(interned)
             };
@@ -1269,13 +1269,13 @@ impl Route {
             }
 
             // NOTE: name/match_name/public_path are already `&'static` via
-            // DirnameStore::append above. `entry.base()` borrows the entry (it
-            // may be inline-stored for ≤31-byte names); intern it
+            // DirnameStore::intern_slice 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())
+                .intern_slice(unsafe { &*entry }.base())
                 .expect("unreachable");
 
             Some(Route {

src/runtime/api/filesystem_router.rs

@@ -53,6 +53,18 @@ pub use crate::bake::framework_router::JSFrameworkRouter as FrameworkFileSystemR
 
 pub(crate) const DEFAULT_EXTENSIONS: &[&[u8]] = &[b"tsx", b"jsx", b"ts", b"mjs", b"cjs", b"js"];
 
+/// Exposed via `bun:internal-for-testing` so leak tests can assert
+/// `reload()` performed zero new `DirnameStore` appends.
+#[bun_jsc::host_fn]
+pub fn js_dirname_store_append_count(
+    _global: &JSGlobalObject,
+    _callframe: &CallFrame,
+) -> JsResult<JSValue> {
+    Ok(JSValue::js_number(
+        Fs::DirnameStore::instance().append_count() as f64,
+    ))
+}
+
 // ── local shims ───────────────────────────────────────────────────────────
 // `to_js` lives on the `bun_jsc::ZigStringJsc` extension trait; `from_bytes`
 // auto-detects UTF-8.

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

@@ -402,6 +402,35 @@ it("reload() works", () => {
   expect(router.match("/posts")!.name).toBe("/posts");
 });
 
+it("reload() does not re-intern directory paths into DirnameStore on every bust+reread", () => {
+  const { dirnameStoreAppendCount } = require("bun:internal-for-testing") as {
+    dirnameStoreAppendCount: () => number;
+  };
+  const files: Record<string, string> = { "pages/sub/nested.tsx": "export default 1;\n" };
+  for (let i = 0; i < 20; i++) files[`pages/p${i}.tsx`] = "export default 1;\n";
+  using dir = tempDir("fsr-reload-dirname-intern", files);
+
+  const router = new Bun.FileSystemRouter({
+    dir: path.join(String(dir), "pages"),
+    style: "nextjs",
+    fileExtensions: [".tsx"],
+  });
+  // First reload primes the intern cache for the bust+reread paths.
+  router.reload();
+  const before = dirnameStoreAppendCount();
+  for (let i = 0; i < 50; i++) router.reload();
+  const delta = dirnameStoreAppendCount() - before;
+  // Without the fix each reload re-appended identical bytes into the never-freed
+  // store: `dir_info_cached_miss` re-interned safe_path + DirEntry.dir per dir
+  // (6 per reload here) and `Route::parse` re-interned public_path/abs_path/
+  // basename per route (63 per reload here), so 50 reloads grew the store by
+  // ~3450 slots and eventually overflowed it with `AllocError`.
+  expect({ delta, match: router.match("/sub/nested")?.filePath.endsWith("nested.tsx") }).toEqual({
+    delta: 0,
+    match: true,
+  });
+});
+
 it("reload() works with new dirs/files", () => {
   const { dir } = make(["posts.tsx"]);