Explicitly create the proxied main thread as detached (#14880)

Allowing musl to know whether it can immediately dispose of the
thread resources once the thread is exited, instead of keeping
it around (for a possible `pthread_join` call, which will never
occur for this thread).

This change can probably be considered non-functional, because
the proxied main thread was previously disposed anyhow during
`PThread.terminateAllThreads`.

Resolves: #9467.

Author: kleisauke

system/lib/pthread/emscripten_proxy_main.c

@@ -25,6 +25,7 @@ static void* _main_thread(void* param) {
 int emscripten_proxy_main(int argc, char** argv) {
   pthread_attr_t attr;
   pthread_attr_init(&attr);
+  pthread_attr_setdetachstate(&attr, PTHREAD_CREATE_DETACHED);
   // Use the size of the current stack, which is the normal size of the stack
   // that main() would have without PROXY_TO_PTHREAD.
   pthread_attr_setstacksize(&attr, emscripten_stack_get_base() - emscripten_stack_get_end());

tests/other/test_pthread_self_join_detach.c

@@ -0,0 +1,24 @@
+#include <assert.h>
+#include <errno.h>
+#include <pthread.h>
+#include <stdio.h>
+
+#include <emscripten/threading.h>
+
+int main() {
+  pthread_t self = pthread_self();
+
+  /* Join the calling thread. This should fail with EDEADLK. */
+  assert(pthread_join(self, NULL) == EDEADLK);
+
+  /* We should be able to detach the main thread,
+   * but not the proxied main thread (as that one
+   * is already detached).
+   */
+  int ret = emscripten_is_main_browser_thread() ? 0 : EINVAL;
+  assert(pthread_detach(self) == ret);
+
+  puts("passed");
+
+  return 0;
+}

tests/other/test_pthread_self_join_detach.out

@@ -0,0 +1 @@
+passed

tests/test_other.py

@@ -9759,6 +9759,19 @@ def test_pthread_stub(self):
     # libpthread_stub.a
     self.do_other_test('test_pthread_stub.c')
 
+  @node_pthreads
+  def test_main_pthread_join_detach(self):
+    # Verify that we're unable to join the main thread
+    self.set_setting('EXIT_RUNTIME')
+    self.do_run_in_out_file_test('other/test_pthread_self_join_detach.c')
+
+  @node_pthreads
+  def test_proxy_pthread_join_detach(self):
+    # Verify that we're unable to detach or join the proxied main thread
+    self.set_setting('PROXY_TO_PTHREAD')
+    self.set_setting('EXIT_RUNTIME')
+    self.do_run_in_out_file_test('other/test_pthread_self_join_detach.c')
+
   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