`KillMode=process` may leave subdaemons which hold locks to paths a user want to build later #563

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opened 2024-10-30 18:36:28 +00:00 by raito · 0 comments

raito commented

2024-10-30 18:36:28 +00:00

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Describe the bug

Consider a bug in Lix that would cause a subdaemon to deadlock somewhere, e.g. while downloading stuff, building paths, whatever.
Assume that subdaemon locks path A via flock.

Build a new Lix, install it on your system, restart nix-daemon. Observe that subdaemon didn't go away.

Retry what you wanted to have, i.e. rebuild path A, same bug but the new subdaemon is hanging on the flock call because the previous subdaemon still hold the path A flock.

Steps To Reproduce

Introduce a bug that will deadlock your daemon during a build of path A.
Install a new Lix, restart a new nix-daemon with it (KillMode=process will leave the subdaemons around)
Rebuild path A.

How to recognize this?

Find the PID of your Nix client which is deadlocked.
Find the PID of your Nix subdaemon counterpart (nix-daemon $CLIENT_PID in the process list)
sudo strace -fp $DAEMON_PID
If you observe flock syscalls, you may be running in an instance of this issue.

Expected behavior

Rebuilding path A should work.

Additional context

Fixing this would probably require moving to KillMode=mixed and removing progressively the existence of subdaemons à la inetd.

## Describe the bug Consider a bug in Lix that would cause a subdaemon to deadlock somewhere, e.g. while downloading stuff, building paths, whatever. Assume that subdaemon locks path A via flock. Build a new Lix, install it on your system, restart nix-daemon. Observe that subdaemon didn't go away. Retry what you wanted to have, i.e. rebuild path A, same bug but the new subdaemon is hanging _on_ the flock call because the previous subdaemon still hold the path A flock. ## Steps To Reproduce 1. Introduce a bug that will deadlock your daemon during a build of path A. 2. Install a new Lix, restart a new nix-daemon with it (`KillMode=process` will leave the subdaemons around) 3. Rebuild path A. ## How to recognize this? 1. Find the PID of your Nix client which is deadlocked. 2. Find the PID of your Nix subdaemon counterpart (`nix-daemon $CLIENT_PID` in the process list) 3. `sudo strace -fp $DAEMON_PID` 4. If you observe `flock` syscalls, you may be running in an instance of this issue. ## Expected behavior Rebuilding path A should work. ## Additional context Fixing this would probably require moving to `KillMode=mixed` and removing progressively the existence of subdaemons à la inetd.

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