async: stop using threads for asynchronous_close()

Let's work towards PID1 being purely single threaded again. Let's rework
asynchronous_close() on top of clone() with CLONE_FILES (so that we
can manipulate PID1's fd table correctly).

One less use of pthread_create() in PID 1.

src/test/test-async.c

@@ -1,12 +1,14 @@
 /* SPDX-License-Identifier: LGPL-2.1-or-later */
 
 #include <fcntl.h>
+#include <sys/prctl.h>
 #include <unistd.h>
 
 #include "async.h"
 #include "fs-util.h"
-#include "tmpfile-util.h"
+#include "process-util.h"
 #include "tests.h"
+#include "tmpfile-util.h"
 
 static bool test_async = false;
 
@@ -17,21 +19,44 @@ static void *async_func(void *arg) {
 }
 
 TEST(test_async) {
+        assert_se(asynchronous_job(async_func, NULL) >= 0);
+        assert_se(asynchronous_sync(NULL) >= 0);
+
+        assert_se(test_async);
+}
+
+TEST(asynchronous_close) {
         _cleanup_(unlink_tempfilep) char name[] = "/tmp/test-asynchronous_close.XXXXXX";
-        int fd;
+        int fd, r;
 
         fd = mkostemp_safe(name);
         assert_se(fd >= 0);
         asynchronous_close(fd);
 
-        assert_se(asynchronous_job(async_func, NULL) >= 0);
-        assert_se(asynchronous_sync(NULL) >= 0);
-
         sleep(1);
 
         assert_se(fcntl(fd, F_GETFD) == -1);
         assert_se(errno == EBADF);
-        assert_se(test_async);
+
+        r = safe_fork("(subreaper)", FORK_RESET_SIGNALS|FORK_CLOSE_ALL_FDS|FORK_DEATHSIG|FORK_LOG|FORK_WAIT, NULL);
+        assert(r >= 0);
+
+        if (r == 0) {
+                /* child */
+
+                assert(prctl(PR_SET_CHILD_SUBREAPER, 1, 0, 0, 0) >= 0);
+
+                fd = open("/dev/null", O_RDONLY|O_CLOEXEC);
+                assert_se(fd >= 0);
+                asynchronous_close(fd);
+
+                sleep(1);
+
+                assert_se(fcntl(fd, F_GETFD) == -1);
+                assert_se(errno == EBADF);
+
+                _exit(EXIT_SUCCESS);
+        }
 }
 
 DEFINE_TEST_MAIN(LOG_DEBUG);