test-mountpoint-util: run test in private mount namespace

This creates a private mount namespace for test-mountpint-util, with all
propagation from the host turned off. This gives us the guarantee that
/proc/self/mountinfo remains fixed and constant while we operate,
removing potential races against other unrelated stuff running on the
system that changes the mount table.

Prompted-by: #17050

(I doubt this actually fixes 17050, this is mostly to make sure that we
aren't possibly affected by such races in our test)

src/test/test-mountpoint-util.c

@@ -1,5 +1,6 @@
 /* SPDX-License-Identifier: LGPL-2.1+ */
 
+#include <sched.h>
 #include <sys/mount.h>
 #include <unistd.h>
 
@@ -258,6 +259,16 @@ static void test_path_is_mount_point(void) {
 int main(int argc, char *argv[]) {
         test_setup_logging(LOG_DEBUG);
 
+        /* let's move into our own mount namespace with all propagation from the host turned off, so that
+         * /proc/self/mountinfo is static and constant for the whole time our test runs. */
+        if (unshare(CLONE_NEWNS) < 0) {
+                if (!ERRNO_IS_PRIVILEGE(errno))
+                        return log_error_errno(errno, "Failed to detach mount namespace: %m");
+
+                log_notice("Lacking privilege to create separate mount namespace, proceeding in originating mount namespace.");
+        } else
+                assert_se(mount(NULL, "/", NULL, MS_PRIVATE | MS_REC, NULL) >= 0);
+
         test_mount_propagation_flags("shared", 0, MS_SHARED);
         test_mount_propagation_flags("slave", 0, MS_SLAVE);
         test_mount_propagation_flags("private", 0, MS_PRIVATE);