umount_hook never executes under Python 3 due to lazy map()

[shaoyihao]

Runner.umount() intends to execute cleanup callbacks stored in self.umount_hook (e.g., deinit_mem_disk()), but under Python 3 the hooks are never invoked because the code uses map(...) without consuming the iterator.

Affected code

In Runner.umount():

(umount_hook, self.umount_hook) = (self.umount_hook, [])
# In Python 3, map() returns an iterator and does nothing until iterated.
map(lambda hook: hook(), umount_hook)

Expected behavior

All functions appended to self.umount_hook should be executed when Runner.umount() is called (after unmount attempts), and then the hook list should be cleared.

Actual behavior

Hooks are not executed on Python 3, since map() is lazy and the returned iterator is not consumed anywhere.

Why it matters

For the mem media path, init_mem_disk() appends deinit_mem_disk() into self.umount_hook after a successful losetup. Because these hooks never run, loop devices and temporary mounts leak across runs (e.g., /dev/loopX remains attached and the backing file stays mounted), which can lead to setup failures in subsequent runs or resource exhaustion.

Steps to reproduce (minimal)

1. Run a configuration that triggers init_mem_disk() (e.g., media=mem with ext4 or other block-based filesystems).
2. Observe that cleanup actions expected in deinit_mem_disk() are not performed after the run (the loop device is not detached).

Proposed fix

Replace the lazy map() call with an explicit loop:

for hook in umount_hook:
    hook()

Side Note: The usage of if p.returncode is not 0: in the same function is also incorrect for checking values and triggers a SyntaxWarning in Python 3.8+. It should be updated to if p.returncode != 0:.

I'm happy to submit a pull request with this fix if you'd like.