# Cgroup Freezing for .scope Cgroups - IMPLEMENTED

## Status: COMPLETE

Both major enhancements have been implemented:
1. **Configuration system refactor** - DONE (was already implemented)
2. **Cgroup-based freezing** - DONE (implemented in this session)

## Summary

The cgroup freezer is now used for any process in a `.scope` cgroup. Scopes are
process-specific cgroups created by systemd-run, tmux, screen, and similar tools.
This is more reliable than SIGSTOP because:
- It's atomic (all processes freeze together)
- Can't be bypassed by parent processes or terminal multiplexers
- No race conditions with job control

### How it works

```bash
# Find process cgroup
cat /proc/<pid>/cgroup
# 0::/user.slice/user-7385.slice/user@7385.service/tmux-spawn-xxx.scope

# Freeze entire cgroup (atomic, no race conditions)
echo 1 > /sys/fs/cgroup/.../cgroup.freeze

# Unfreeze
echo 0 > /sys/fs/cgroup/.../cgroup.freeze
```

### Benefits
- Atomic freezing of all processes in the cgroup
- No race conditions with job control
- tmux/screen can't resume cgroup-frozen processes
- Works with any terminal multiplexer
- Fallback to SIGSTOP for regular processes

## Implementation Details

### New Functions Added (thrash_protect.py)

1. `get_cgroup_path(pid)` - Gets the cgroup v2 path for a process
2. `is_cgroup_freezable(cgroup_path)` - Checks if cgroup supports freezing
3. `freeze_cgroup(cgroup_path)` - Freezes all processes in a cgroup
4. `unfreeze_cgroup(cgroup_path)` - Unfreezes all processes in a cgroup
5. `should_use_cgroup_freeze(pid)` - Determines if cgroup freezing should be used
6. `get_all_frozen_pids()` - Returns combined list of all frozen pids

### Modified Functions

1. `freeze_something()` - Now checks for cgroup freezing first for tmux/screen sessions
2. `unfreeze_something()` - Handles cgroup unfreezing before SIGCONT
3. `cleanup()` - Unfreezes cgroups on exit
4. `log_frozen()` / `log_unfrozen()` - Updated to show all frozen processes

### New Global State

```python
# Unified list of frozen items - each entry is one of:
#   ('cgroup', cgroup_path, pids) - frozen via cgroup freezer
#   ('sigstop', pids)             - frozen via SIGSTOP
frozen_items = []
```

## Tests Added

- `test_get_cgroup_path_self` - Tests cgroup detection for current process
- `test_get_cgroup_path_nonexistent` - Tests handling of non-existent processes
- `test_is_cgroup_freezable` - Tests freezability detection
- `test_should_use_cgroup_freeze_tmux` - Tests tmux detection
- `test_should_use_cgroup_freeze_screen` - Tests screen detection
- `test_should_not_use_cgroup_freeze_regular` - Tests regular process handling
- `test_get_all_frozen_pids_empty` - Tests empty state
- `test_get_all_frozen_pids_with_frozen` - Tests combined pid listing
- `test_cleanup_with_cgroups` - Tests cleanup with both cgroups and regular pids

## Notes

- Cgroup freezing requires write access to `/sys/fs/cgroup/...`
- thrash-protect typically runs as root, so this works
- For non-root operation, appropriate cgroup permissions are needed
- When a cgroup is frozen, new processes spawned into it are also frozen
