test: Grab FORK_MTX when unmounting or forking in the mount tests (#2386)

* mount tests: take FORK_MTX when forking or unmounting

unmounting can fail if a child process inherited a file a file descriptor
we opened in temporary mount point

Should fix #2369

* tests: clarify the meaning of the FORK_MTX

* tests: clarify the meaning of FORK_MTX

Co-authored-by: SteveLauC <stevelauc@outlook.com>

* here no one accesses the file system to be unmounted, so no fork lock needed

---------

Co-authored-by: SteveLauC <stevelauc@outlook.com>

Author: TheJonny

test/mount/test_mount.rs

@@ -53,6 +53,8 @@ fn test_mount_tmpfs_without_flags_allows_rwx() {
         .unwrap_or_else(|e| panic!("read failed: {e}"));
     assert_eq!(buf, SCRIPT_CONTENTS);
 
+    // while forking and unmounting prevent other child processes
+    let _m = FORK_MTX.lock();
     // Verify execute.
     assert_eq!(
         EXPECTED_STATUS,
@@ -129,6 +131,8 @@ fn test_mount_noexec_disallows_exec() {
         &test_path
     );
 
+    // while forking and unmounting prevent other child processes
+    let _m = FORK_MTX.lock();
     // EACCES: Permission denied
     assert_eq!(
         EACCES,
@@ -168,6 +172,8 @@ fn test_mount_bind() {
             .and_then(|mut f| f.write(SCRIPT_CONTENTS))
             .unwrap_or_else(|e| panic!("write failed: {e}"));
 
+        // wait for child processes to prevent EBUSY
+        let _m = FORK_MTX.lock();
         umount(mount_point.path())
             .unwrap_or_else(|e| panic!("umount failed: {e}"));
     }

test/test.rs

@@ -57,8 +57,9 @@ fn read_exact<Fd: AsFd>(f: Fd, buf: &mut [u8]) {
     }
 }
 
-/// Any test that creates child processes must grab this mutex, regardless
-/// of what it does with those children.
+/// Any test that creates child processes or can be affected by child processes must grab this mutex, regardless
+/// of what it does with those children. It must hold the mutex until the
+/// child processes are waited upon.
 pub static FORK_MTX: Mutex<()> = Mutex::new(());
 /// Any test that changes the process's current working directory must grab
 /// the RwLock exclusively.  Any process that cares about the current