blockdev: Fix loopback device resource leak on signal interruption

[gursewak1997]

This commit implements issue #799 by creating a signal-safe cleanup helper for loopback devices to prevent resource leaks when bootc install --via-loopback is interrupted by signals like SIGINT (Ctrl-C).

The solution uses an 'out-of-process drop' helper that:

• Forks a cleanup helper process when creating a loopback device
• Uses PR_SET_PDEATHSIG to detect when the parent process dies
• Masks most signals to avoid being killed accidentally
• Automatically cleans up leaked loopback devices if the parent dies
• Gracefully terminates when the parent performs normal cleanup

This prevents the common issue where interrupting bootc install --via-loopback with Ctrl-C would leave /dev/loopN devices allocated on the system.

Fixes: #799

[gemini-code-assist]

Code Review

This pull request introduces a signal-safe cleanup mechanism for loopback devices to prevent resource leaks on signal interruption. It uses an out-of-process helper to clean up leaked loopback devices. I've added comments to enhance error logging for better debugging.

[cgwalters]

Thanks for working on this! While it will be a bit more awkward can you try doing it this way #799 (comment) - that should 100% avoid all the unsafe code.

Basically instead of a raw fork() (source of basically all the unsafe in general) we fork+exec our own binary /proc/self/exe. Look at e.g. reexec_with_guardenv.

[cgwalters]

Thanks! Looking closer

[cgwalters]

Thanks, looking a lot closer!

[cgwalters]

This is looking pretty good to me now! Is there a reason it's still in draft? Can you also mark issues in the discussion as resolved when they're fixed?

[gursewak1997]

Can you also mark issues in the discussion as resolved when they're fixed?

Every issue in discussion should be fixed now.

[cgwalters]

LGTM aside from one last bit

[cgwalters]

Fallout in #1439