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Lix is stuck

🐛 Debugging a stuck Lix invocation

If you're experiencing an issue where a lix command appears to hang or make no visible progress, this page outlines multiple methods to investigate what’s going on.

Step 1: Increase Verbosity

Start by rerunning the Lix command with high verbosity to get a better sense of whether it's doing useful work.

nix build -vvv <your-command>

Check the output for any signs of activity such as:

  • Downloads or substitutions
  • Evaluations
  • Store interactions

If logs show it's idle or stalled, move on to the next steps.

Step 2: strace the client

Attach strace to the stuck lix process to see where it is spending its time:

strace -yy -f nix build <your-command>

Watch for long periods spent on blocking syscalls like:

  • poll
  • read
  • recvmsg
  • futex

This means that it's stuck on communicating with the daemon, move on to the next steps.

Step 3: strace the subdaemon

Lix launches subdaemons to handle client requests. You can identify the correct subdaemon by inspecting processes:

ps ax | grep nix-daemon

Look for a line like:

356981 ?        Ssl    0:00 nix-daemon 67890

Here, 67890 is the PID of the client Lix process. The daemon serving it is 356981.

Attach strace to the subdaemon:

strace -f -p 356981 -yy

This lets you inspect whether the daemon is doing work, waiting on a lock, or itself stalled.

Step 4: Investigate lock contention

If any of the traced processes are stuck waiting for file or directory locks (common with .lock files in the Nix store), you can identify who is holding the lock:

lsof <path to the lockfile>

Alternatively, you can ask all locks held by a process: lsof -p <PID of the target process>.

Another handy tool is lslocks for this.

Check whether the holder is another nix-daemon (or nix-<something>) process. If so, strace that one as well.

If that second process is:

  • Doing useful work: this may be expected behavior.
  • Also stuck on a lock or polling something indefinitely: you may be looking at a Lix bug.

Try to reduce the chain of waiting processes to the minimal reproducible set and report the issue with detailed diagnostics.

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