[eventloop] Fix main loop keepalive cleanup when cancelling or pausing

src/lib/libeventloop.js

@@ -204,16 +204,18 @@ LibraryJSEventLoop = {
     Module['resumeMainLoop'] = MainLoop.resume;
     MainLoop.init();`,
   $MainLoop: {
-    running: false,
+    // The main loop tick function that will be called at each iteration.
+    // This will be non-null whenever a loop function is registered.
+    func: null,
+    // This will be non-null whenever a loop function is both registered and
+    // currently running.
     scheduler: null,
     // Each main loop is numbered with a ID in sequence order. Only one main
     // loop can run at a time. This variable stores the ordinal number of the
     // main loop that is currently allowed to run. All previous main loops
     // will quit themselves. This is incremented whenever a new main loop is
     // created.
     currentlyRunningMainloop: 0,
-    // The main loop tick function that will be called at each iteration.
-    func: null,
     // The argument that will be passed to the main loop. (of type void*)
     arg: 0,
     timingMode: 0,
@@ -224,9 +226,12 @@ LibraryJSEventLoop = {
     postMainLoop: [],
 
     pause() {
-      MainLoop.scheduler = null;
-      // Incrementing this signals the previous main loop that it's now become old, and it must return.
-      MainLoop.currentlyRunningMainloop++;
+      if (MainLoop.scheduler) {
+        MainLoop.scheduler = null;
+        // Incrementing this signals the previous main loop that it's now become old, and it must return.
+        MainLoop.currentlyRunningMainloop++;
+        {{{ runtimeKeepalivePop() }}}
+      }
     },
 
     resume() {
@@ -328,10 +333,13 @@ LibraryJSEventLoop = {
       return 1; // Return non-zero on failure, can't set timing mode when there is no main loop.
     }
 
-    if (!MainLoop.running) {
-      {{{ runtimeKeepalivePush() }}}
-      MainLoop.running = true;
+#if useRuntimeKeepaliveStack()
+    // If there is no existing scheduler then we are transitioning from
+    // inactive to active and we add to runtime keepalive counter.
+    if (!MainLoop.scheduler) {
+      runtimeKeepalivePush();
     }
+#endif
     if (mode == {{{ cDefs.EM_TIMING_SETTIMEOUT }}}) {
       MainLoop.scheduler = function MainLoop_scheduler_setTimeout() {
         var timeUntilNextTick = Math.max(0, MainLoop.tickStartTime + value - _emscripten_get_now())|0;
@@ -422,7 +430,6 @@ LibraryJSEventLoop = {
 #if RUNTIME_DEBUG
         dbg('main loop exiting');
 #endif
-        {{{ runtimeKeepalivePop() }}}
 #if !MINIMAL_RUNTIME
         maybeExit();
 #endif
@@ -433,10 +440,7 @@ LibraryJSEventLoop = {
 
     // We create the loop runner here but it is not actually running until
     // _emscripten_set_main_loop_timing is called (which might happen at a
-    // later time).  This member signifies that the current runner has not
-    // yet been started so that we can call runtimeKeepalivePush when it
-    // gets its timing set for the first time.
-    MainLoop.running = false;
+    // later time).
     MainLoop.runner = function MainLoop_runner() {
       if (ABORT) return;
       if (MainLoop.queue.length > 0) {

src/parseTools.mjs

@@ -1063,22 +1063,32 @@ function awaitIf(condition) {
   return condition ? 'await ' : '';
 }
 
+function useRuntimeKeepaliveStack() {
+  return !(MINIMAL_RUNTIME || (EXIT_RUNTIME == 0 && PTHREADS == 0));
+}
+
 // Adds a call to runtimeKeepalivePush, if needed by the current build
 // configuration.
 // We skip this completely in MINIMAL_RUNTIME and also in builds that
 // don't ever need to exit the runtime.
 function runtimeKeepalivePush() {
-  if (MINIMAL_RUNTIME || (EXIT_RUNTIME == 0 && PTHREADS == 0)) return '';
-  return 'runtimeKeepalivePush();';
+  if (useRuntimeKeepaliveStack()) {
+    return 'runtimeKeepalivePush();';
+  } else {
+    return '';
+  }
 }
 
 // Adds a call to runtimeKeepalivePush, if needed by the current build
 // configuration.
 // We skip this completely in MINIMAL_RUNTIME and also in builds that
 // don't ever need to exit the runtime.
 function runtimeKeepalivePop() {
-  if (MINIMAL_RUNTIME || (EXIT_RUNTIME == 0 && PTHREADS == 0)) return '';
-  return 'runtimeKeepalivePop();';
+  if (useRuntimeKeepaliveStack()) {
+    return 'runtimeKeepalivePop();';
+  } else {
+    return '';
+  }
 }
 
 // Some web APIs like TextDecoder.decode() and XMLHttpRequest.send() do not
@@ -1291,4 +1301,5 @@ addToCompileTimeContext({
   nodeWWDetection,
   wasmWorkerDetection,
   pthreadDetection,
+  useRuntimeKeepaliveStack,
 });

test/codesize/test_codesize_hello_dylink_all.json

@@ -1,7 +1,7 @@
 {
-  "a.out.js": 267539,
+  "a.out.js": 267525,
   "a.out.nodebug.wasm": 588311,
-  "total": 855850,
+  "total": 855836,
   "sent": [
     "IMG_Init",
     "IMG_Load",

test/test_browser.py

@@ -1798,6 +1798,12 @@ def test_emscripten_api_infloop(self):
   def test_emscripten_main_loop(self):
     self.btest_exit('test_emscripten_main_loop.c')
 
+  def test_emscripten_main_loop_cancel_exit(self):
+    self.btest_exit('test_emscripten_main_loop_cancel_exit.c', cflags=['-sASSERTIONS=2'])
+
+  def test_emscripten_main_loop_cancel_force_exit(self):
+    self.btest_exit('test_emscripten_main_loop_cancel_force_exit.c', cflags=['-sASSERTIONS=2'])
+
   @parameterized({
     '': ([],),
     # test pthreads + AUTO_JS_LIBRARIES mode as well

test/test_emscripten_main_loop_cancel_exit.c

@@ -0,0 +1,20 @@
+/*
+ * Copyright 2026 The Emscripten Authors.  All rights reserved.
+ * Emscripten is available under two separate licenses, the MIT license and the
+ * University of Illinois/NCSA Open Source License.  Both these licenses can be
+ * found in the LICENSE file.
+ */
+
+#include <stdio.h>
+#include <stdlib.h>
+#include <emscripten.h>
+
+void loop(void) {
+  emscripten_cancel_main_loop();
+  exit(0);
+}
+
+int main(void) {
+  emscripten_set_main_loop(loop, -1, 0);
+  return 99;
+}

test/test_emscripten_main_loop_cancel_force_exit.c

@@ -0,0 +1,20 @@
+/*
+ * Copyright 2026 The Emscripten Authors.  All rights reserved.
+ * Emscripten is available under two separate licenses, the MIT license and the
+ * University of Illinois/NCSA Open Source License.  Both these licenses can be
+ * found in the LICENSE file.
+ */
+
+#include <stdio.h>
+#include <stdlib.h>
+#include <emscripten.h>
+
+void loop(void) {
+  emscripten_cancel_main_loop();
+  emscripten_force_exit(0);
+}
+
+int main(void) {
+  emscripten_set_main_loop(loop, -1, 0);
+  return 99;
+}

test/test_other.py

@@ -13429,6 +13429,20 @@ def test_emscripten_set_immediate_loop(self):
   def test_emscripten_main_loop(self, args):
     self.do_runf('test_emscripten_main_loop.c', cflags=args)
 
+  @parameterized({
+    '': ([],),
+    'exit_runtime': (['-sEXIT_RUNTIME'],),
+  })
+  def test_emscripten_main_loop_cancel_exit(self, args):
+    self.do_runf('test_emscripten_main_loop_cancel_exit.c', cflags=['-sASSERTIONS=2'] + args)
+
+  @parameterized({
+    '': ([],),
+    'exit_runtime': (['-sEXIT_RUNTIME'],),
+  })
+  def test_emscripten_main_loop_cancel_force_exit(self, args):
+    self.do_runf('test_emscripten_main_loop_cancel_force_exit.c', cflags=['-sASSERTIONS=2'] + args)
+
   def test_emscripten_main_loop_and_blocker(self):
     self.do_runf('test_emscripten_main_loop_and_blocker.c')