Fix keepRuntimeAlive in the case where EXIT_RUNTIME=0 and noExitRuntime is not referenced (#22542)

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

Author: sbc100

src/library.js

@@ -2136,10 +2136,24 @@ 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
 

test/other/test_no_exit_runtime_strict.c

@@ -0,0 +1,24 @@
+#include <assert.h>
+#include <emscripten/emscripten.h>
+#include <emscripten/em_js.h>
+#include <stdio.h>
+
+// Verify that `keepRuntimeAlive()` always returns true by default (i.e. when
+// EXIT_RUNTIME=0).
+
+EM_JS_DEPS(deps, "$keepRuntimeAlive");
+
+EM_JS(void, timeout_func, (), {
+  console.log("timeout_func: keepRuntimeAlive() ->", keepRuntimeAlive());
+  // once this timeout is done the node process should exit with 0
+});
+
+int main() {
+  int keep_alive = EM_ASM_INT({
+    setTimeout(timeout_func);
+    return keepRuntimeAlive();
+  });
+  printf("main done: %d\n", keep_alive);
+  assert(keep_alive == 1);
+  return 0;
+}

test/other/test_no_exit_runtime_strict.out

@@ -0,0 +1,2 @@
+main done: 1
+timeout_func: keepRuntimeAlive() -> true

test/other/test_unoptimized_code_size_strict.js.size

@@ -1 +1 @@
-53834
+53730

test/test_other.py

@@ -4997,6 +4997,9 @@ def test(cxx, no_exit, assertions, flush=0, keepalive=0, filesystem=1):
           for flush in (0, 1):
             test(cxx, no_exit, assertions, flush)
 
+  def test_no_exit_runtime_strict(self):
+    self.do_other_test('test_no_exit_runtime_strict.c', emcc_args=['-sSTRICT'])
+
   def test_extra_opt_levels(self):
     # Opt levels that we don't tend to test elsewhere
     for opt in ('-Og', '-Ofast'):

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)
 
@@ -918,6 +915,9 @@ 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([