From 1beb18dc9f939b37e8216aceeba1f72032f23146 Mon Sep 17 00:00:00 2001
From: Guy Bedford <guybedford@gmail.com>
Date: Thu, 23 Jul 2026 23:00:15 -0700
Subject: [PATCH] Fix embind exports with MODULARIZE=instance and AUTO_INIT

With AUTO_INIT the embind AOT export snippet was appended after the
top-level `await init()`, so its addOnPostCtor callback registered
after onPostCtors had already fired, leaving the ES module exports
undefined.

Emit the snippet via a placeholder ahead of the init logic in the
postamble instead, and assert against late addOnPostCtor registration.

Fixes #27411
---
 src/lib/libcore.js |  7 ++++++-
 src/postamble.js   |  5 +++++
 test/test_other.py | 19 +++++++++++++++++++
 tools/link.py      |  2 +-
 4 files changed, 31 insertions(+), 2 deletions(-)

diff --git a/src/lib/libcore.js b/src/lib/libcore.js
index ad1c1d48023d1..34c209b99fa6a 100644
--- a/src/lib/libcore.js
+++ b/src/lib/libcore.js
@@ -2402,7 +2402,12 @@ addToLibrary({
     ATPOSTCTORS.unshift('callRuntimeCallbacks(onPostCtors);');
   },
   $addOnPostCtor__deps: ['$onPostCtors'],
-  $addOnPostCtor: (cb) => onPostCtors.push(cb),
+  $addOnPostCtor: (cb) => {
+#if ASSERTIONS
+    assert(!runtimeInitialized, 'addOnPostCtor called too late: ctors have already run');
+#endif
+    onPostCtors.push(cb);
+  },
   // See ATMAINS in parseTools.mjs for more information.
   $onMains: [],
   $onMains__internal: true,
diff --git a/src/postamble.js b/src/postamble.js
index d130ded126fbf..a474aa57280f3 100644
--- a/src/postamble.js
+++ b/src/postamble.js
@@ -244,6 +244,11 @@ var wasmRawExports;
 #endif
 
 #if MODULARIZE == 'instance'
+#if EMBIND_AOT
+// The embind exports are declared here so that their post-ctor registration
+// precedes any self-initialization below.  See phase_embind_aot in link.py.
+<<< EMBIND_AOT_EXPORTS >>>
+#endif
 // In MODULARIZE=instance mode we delay most of the initialization work until
 // the `init` function is called.
 #if ASSERTIONS
diff --git a/test/test_other.py b/test/test_other.py
index 09df84c0ed5c5..4b9b6691d9de4 100644
--- a/test/test_other.py
+++ b/test/test_other.py
@@ -6273,6 +6273,25 @@ def test_modularize_instance_auto_init(self):
 
     self.assertContained('main1\nmain2\nfoo\nbar\nbaz\n', self.run_js('runner.mjs'))
 
+  def test_modularize_instance_auto_init_embind(self):
+    # Embind exports must be assigned when the module self-initializes on
+    # import.  See https://github.com/emscripten-core/emscripten/issues/27411
+    self.run_process([EMXX, test_file('modularize_instance_embind.cpp'),
+                      '-sMODULARIZE=instance', '-sAUTO_INIT',
+                      '-Wno-experimental',
+                      '-lembind', '-sEMBIND_AOT',
+                      '-o', 'modularize_instance_embind.mjs'] + self.get_cflags())
+
+    create_file('runner.mjs', '''
+      import { foo, Bar } from "./modularize_instance_embind.mjs";
+      foo();
+      const bar = new Bar();
+      bar.print();
+      bar.delete();
+    ''')
+
+    self.assertContained('main\nfoo\nbar\n', self.run_js('runner.mjs'))
+
   @also_with_pthreads
   @requires_node_25
   def test_esm_integration_auto_init(self):
diff --git a/tools/link.py b/tools/link.py
index c287e1311c2e6..e5389eab1b2e7 100644
--- a/tools/link.py
+++ b/tools/link.py
@@ -2126,7 +2126,7 @@ def phase_embind_aot(options, wasm_target, js_syms):
 addOnPostCtor(assignEmbindExports);
 {decls}
 // end embind exports'''
-    src += exports
+    src = do_replace(src, '<<< EMBIND_AOT_EXPORTS >>>', exports)
   write_file(final_js, src)
   if settings.WASM_ESM_INTEGRATION:
     # With ESM integration the embind exports also need to be exported by the main file.