From 1f1972d444463df8a451baae57ea1d21ab791939 Mon Sep 17 00:00:00 2001
From: Sam Clegg <sbc@chromium.org>
Date: Thu, 29 Oct 2026 10:45:23 -0700
Subject: [PATCH] [FS] Only generate `FS_<symbol>` library aliases for
 functions

In #24116, a loop was added at the end of `src/lib/libfs.js` to
generate `$FS_<symbol>` wrapper functions (`(...args) => FS.foo(...)`)
for `FS` API methods. While the comment noted that non-functions should
be skipped, the condition only checked `key[0] !== key[0].toLowerCase()`
(which filters out uppercase classes like `ErrnoError`, `FSStream`, and
`FSNode`), assuming all lowercase properties on `LibraryFS.$FS` were
functions.

In practice, `LibraryFS.$FS` also contains lowercase state and object
properties (`root`, `mounts`, `devices`, `streams`, `nextInode`,
`nameTable`, `currentPath`, `initialized`, `ignorePermissions`,
`syncFSRequests`, and `chrdev_stream_ops`). Wrapping these in
`(...args) => FS.<key>(...args)` is broken:

- Calling any of these generated aliases at runtime would unconditionally
  throw `TypeError: FS.<key> is not a function`.
- When compiling with `-sINCLUDE_FULL_LIBRARY` and `--closure=1` (such
  as with `-sEXPORT_ES6`), Closure Compiler rejects calls to these
  non-callable properties on `FS`.
- In unoptimized (`-O0` / `ASSERTIONS`) builds, these bogus symbols
  pollute `missingLibrarySymbols`, unnecessarily increasing JS code
  size.

Check `typeof LibraryFS.$FS[key] !== 'function'` alongside the existing
lowercase check so that only actual methods on `FS` receive function
aliases.

See: #27747
---
 src/lib/libfs.js                                   |  2 +-
 test/codesize/test_codesize_hello_O0.json          |  8 ++++----
 test/codesize/test_codesize_minimal_O0.expected.js | 11 -----------
 test/codesize/test_codesize_minimal_O0.json        |  8 ++++----
 test/codesize/test_unoptimized_code_size.json      | 12 ++++++------
 5 files changed, 15 insertions(+), 26 deletions(-)

diff --git a/src/lib/libfs.js b/src/lib/libfs.js
index af6f6f6728d43..a2da5eafcc6fb 100644
--- a/src/lib/libfs.js
+++ b/src/lib/libfs.js
@@ -1954,7 +1954,7 @@ FS.staticInit();`;
 for (let key in LibraryFS.$FS) {
   const alias = `$FS_${key}`;
   // Skip defining the alias if it already exists or if it's not an API function.
-  if (LibraryFS[alias] || key[0] !== key[0].toLowerCase()) {
+  if (LibraryFS[alias] || typeof LibraryFS.$FS[key] !== 'function' || key[0] !== key[0].toLowerCase()) {
     continue;
   }
   LibraryFS[alias] = `(...args) => FS.${key}(...args)`;
diff --git a/test/codesize/test_codesize_hello_O0.json b/test/codesize/test_codesize_hello_O0.json
index bb883c8e5b440..e4ee584222637 100644
--- a/test/codesize/test_codesize_hello_O0.json
+++ b/test/codesize/test_codesize_hello_O0.json
@@ -1,10 +1,10 @@
 {
-  "a.out.js": 23551,
-  "a.out.js.gz": 8559,
+  "a.out.js": 23395,
+  "a.out.js.gz": 8502,
   "a.out.nodebug.wasm": 14275,
   "a.out.nodebug.wasm.gz": 7098,
-  "total": 37826,
-  "total_gz": 15657,
+  "total": 37670,
+  "total_gz": 15600,
   "sent": [
     "fd_write"
   ],
diff --git a/test/codesize/test_codesize_minimal_O0.expected.js b/test/codesize/test_codesize_minimal_O0.expected.js
index 797b9905a86a5..e82c6c5015078 100644
--- a/test/codesize/test_codesize_minimal_O0.expected.js
+++ b/test/codesize/test_codesize_minimal_O0.expected.js
@@ -1023,16 +1023,6 @@ missingLibrarySymbols.forEach(missingLibrarySymbol)
   'FS_createDevice',
   'FS_readFile',
   'FS',
-  'FS_root',
-  'FS_mounts',
-  'FS_devices',
-  'FS_streams',
-  'FS_nextInode',
-  'FS_nameTable',
-  'FS_currentPath',
-  'FS_initialized',
-  'FS_ignorePermissions',
-  'FS_syncFSRequests',
   'FS_lookupPath',
   'FS_getPath',
   'FS_hashName',
@@ -1064,7 +1054,6 @@ missingLibrarySymbols.forEach(missingLibrarySymbol)
   'FS_closeStream',
   'FS_dupStream',
   'FS_doSetAttr',
-  'FS_chrdev_stream_ops',
   'FS_major',
   'FS_minor',
   'FS_makedev',
diff --git a/test/codesize/test_codesize_minimal_O0.json b/test/codesize/test_codesize_minimal_O0.json
index e52559434d575..497e2d8deb801 100644
--- a/test/codesize/test_codesize_minimal_O0.json
+++ b/test/codesize/test_codesize_minimal_O0.json
@@ -1,10 +1,10 @@
 {
-  "a.out.js": 18738,
-  "a.out.js.gz": 6730,
+  "a.out.js": 18582,
+  "a.out.js.gz": 6672,
   "a.out.nodebug.wasm": 1071,
   "a.out.nodebug.wasm.gz": 636,
-  "total": 19809,
-  "total_gz": 7366,
+  "total": 19653,
+  "total_gz": 7308,
   "sent": [],
   "imports": [],
   "exports": [
diff --git a/test/codesize/test_unoptimized_code_size.json b/test/codesize/test_unoptimized_code_size.json
index 20b14290502c3..49629a20e7232 100644
--- a/test/codesize/test_unoptimized_code_size.json
+++ b/test/codesize/test_unoptimized_code_size.json
@@ -1,16 +1,16 @@
 {
-  "hello_world.js": 53025,
-  "hello_world.js.gz": 16866,
+  "hello_world.js": 52814,
+  "hello_world.js.gz": 16807,
   "hello_world.wasm": 14275,
   "hello_world.wasm.gz": 7098,
   "no_asserts.js": 22074,
   "no_asserts.js.gz": 7847,
   "no_asserts.wasm": 11389,
   "no_asserts.wasm.gz": 5645,
-  "strict.js": 50155,
-  "strict.js.gz": 15884,
+  "strict.js": 49944,
+  "strict.js.gz": 15826,
   "strict.wasm": 14275,
   "strict.wasm.gz": 7096,
-  "total": 165193,
-  "total_gz": 60436
+  "total": 164771,
+  "total_gz": 60319
 }