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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

  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.

## 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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Affects/CppNix

Probably affects CppNix. This is a best-effort tag for troubles in old code; it exists to inform the CppNix team about problems they might care about also.

  
Affects/Nightly

Found in Lix nightly.

  
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Tag for regressions in Lix nightly. We intend to fix all of these prior to releases.

  
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Affects the stable release of Lix. Usually such issues affect nightly too but there is no guarantee.

  
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regarding the building or packaging of Lix itself

  
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The command line interface of Lix

  
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libexpr and anything else related to the evaluation of Nix expressions

  
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Fetching sources or other materials from the World Wide Web in Lix

  
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nix-eval-jobs for Lix

  
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profiles: nix-env, nix profile, and installer flavors all alike

  
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The protocol, used for communicating between the client and the daemon

  
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remote builds

  
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This issue might require some context that isn't written down. It might be possible to handle as a person who already worked on this area.

  
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This issue needs discussion from multiple maintainers and interested parties: it's a Request For Discussion.

  
crash 💥

Lix crashes (possibly bad error handling)

  
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Downstream Dependents

regarding an external tool's usage with Lix (e.g. nix-eval-jobs)

  
E/easy

bug is relatively simple and could simply be fixed with some minor effort

  
E/hard

this issue should probably be solved with close involvement of an expert

  
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maintainers may not get to this soon, but we want it fixed

  
E/reproducible

This bug is reproducible and obvious.

  
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this bug cannot be fixed properly without fixing a known architectural flaw

  
imported

Issue imported from upstream nix (please do not remove this label, it will cause dupes if import is rerun)

  
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Affects specifically Lix on Linux

  
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Affects specifically Lix on macOS

  
performance

make lix go faster

  
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Blocks public release

  
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We chose not to change this (see issue comments for more detail).

  
testing

Tests that should be added

  
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Tests that are unreliable/flakey (not to be confused with flakes that test)

  
Topic/Large Scale Installations

Large scale installations of Lix, e.g. NixOS Hydra, Lix CI, big corporate installations etc.

  
ux

makes the user experience better :)

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Reference: lix-project/lix#563
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