Fix keepRuntimeAlive in the case where EXIT_RUNTIME=0 and noExitRunti…

…me is not referenced

Essentially the `keepRuntimeAlive` was relying on the `noExitRuntime`
variable being set based on `EXIT_RUNTIME` but when `noExitRuntime` was
absent the `EXIT_RUNTIME` settings was being ignored and the runtime was
exiting even though `EXIT_RUNTIME=0` was set (the default).

Fixes: #20636

src/library.js

@@ -2142,10 +2142,24 @@ addToLibrary({
   $runtimeKeepaliveCounter__internal: true,
   $runtimeKeepaliveCounter: 0,
 
-  $keepRuntimeAlive__deps: ['$runtimeKeepaliveCounter'],
 #if isSymbolNeeded('$noExitRuntime')
+  // If the `noExitRuntime` symbol is included in the build then
+  // keepRuntimeAlive is always conditional since its state can change
+  // at runtime.
+  $keepRuntimeAlive__deps: ['$runtimeKeepaliveCounter'],
   $keepRuntimeAlive: () => noExitRuntime || runtimeKeepaliveCounter > 0,
+#elif !EXIT_RUNTIME
+  // When `noExitRuntime` is not include and EXIT_RUNTIME=0 then we know the
+  // runtime can never exit (i.e. should always be kept alive).
+  // However for pthreads we always default to allowing the runtime to exit
+  // otherwise threads never exit and are not joinable.
+#if PTHREADS
+  $keepRuntimeAlive: () => !ENVIRONMENT_IS_PTHREAD,
 #else
+  $keepRuntimeAlive: () => true,
+#endif
+#else
+  $keepRuntimeAlive__deps: ['$runtimeKeepaliveCounter'],
   $keepRuntimeAlive: () => runtimeKeepaliveCounter > 0,
 #endif
 

src/library_pthread.js

@@ -203,7 +203,7 @@ var LibraryPThread = {
       // worker pool as an unused worker.
       worker.pthread_ptr = 0;
 
-#if ENVIRONMENT_MAY_BE_NODE && PROXY_TO_PTHREAD
+#if ENVIRONMENT_MAY_BE_NODE
       if (ENVIRONMENT_IS_NODE) {
         // Once the proxied main thread has finished, mark it as weakly
         // referenced so that its existence does not prevent Node.js from

test/other/test_pthread_strict.c

@@ -0,0 +1,41 @@
+#include <stdio.h>
+#include <stdbool.h>
+#include <unistd.h>
+#include <pthread.h>
+#include <emscripten/console.h>
+
+void _emscripten_thread_set_strongref(pthread_t thread);
+
+pthread_mutex_t ready_lock = PTHREAD_MUTEX_INITIALIZER;
+pthread_cond_t ready_cond = PTHREAD_COND_INITIALIZER;
+_Atomic bool ready = false;
+
+void* thread_main(void* arg) {
+  pthread_mutex_lock(&ready_lock);
+  if (!ready) {
+    pthread_cond_wait(&ready_cond, &ready_lock);
+  }
+  pthread_mutex_unlock(&ready_lock);
+  emscripten_outf("Hello from thread");
+  return NULL;
+}
+
+int main() {
+  pthread_t t;
+  pthread_create(&t, NULL, thread_main, NULL);
+  pthread_detach(t);
+  emscripten_outf("thread created, returning from main");
+
+  // Make sure the thread output doesn't appear until after the above message
+  pthread_mutex_lock(&ready_lock);
+  ready = true;
+  pthread_cond_signal(&ready_cond);
+  pthread_mutex_unlock(&ready_lock);
+
+  // This needed under node to make sure the process doesn't exit before the
+  // thread is done.
+  // TODO(sbc): We probably want all running threads to keep node from exiting
+  // like they would under POSIX.
+  _emscripten_thread_set_strongref(t);
+  return 0;
+}

test/other/test_pthread_strict.out

@@ -0,0 +1,2 @@
+thread created, returning from main
+Hello from thread

test/test_other.py

@@ -12071,6 +12071,10 @@ def test_pthread_reuse(self):
   def test_pthread_relocatable(self):
     self.do_run_in_out_file_test('hello_world.c', emcc_args=['-sRELOCATABLE'])
 
+  @node_pthreads
+  def test_pthread_strict(self):
+    self.do_other_test('test_pthread_strict.c', emcc_args=['-pthread', '-sPTHREAD_POOL_SIZE=1', '-sSTRICT'])
+
   def test_stdin_preprocess(self):
     create_file('temp.h', '#include <string>')
     outputStdin = self.run_process([EMCC, '-x', 'c++', '-dM', '-E', '-'], input="#include <string>", stdout=PIPE).stdout

tools/link.py

@@ -658,9 +658,6 @@ def phase_linker_setup(options, state, newargs):
     # Add `#!` line to output JS and make it executable.
     options.executable = True
 
-  if 'noExitRuntime' in settings.INCOMING_MODULE_JS_API:
-    settings.DEFAULT_LIBRARY_FUNCS_TO_INCLUDE.append('$noExitRuntime')
-
   if settings.OPT_LEVEL >= 1:
     default_setting('ASSERTIONS', 0)
 
@@ -914,6 +911,9 @@ def phase_linker_setup(options, state, newargs):
     else:
       default_setting('INCOMING_MODULE_JS_API', [])
 
+  if 'noExitRuntime' in settings.INCOMING_MODULE_JS_API:
+    settings.DEFAULT_LIBRARY_FUNCS_TO_INCLUDE.append('$noExitRuntime')
+
   if not settings.MINIMAL_RUNTIME and not settings.STRICT:
     # Export the HEAP object by default, when not running in STRICT mode
     settings.EXPORTED_RUNTIME_METHODS.extend([