fix(metrics): fallback to mock writer on file open failure to prevent panic

[abhaygoudannavar]

Description

When timestamps are enabled in the urunc configuration (/etc/urunc/config.toml) but the target file path is invalid or the parent directory does not exist, NewZerologMetrics() returns nil. Because the caller assigns this directly without a nil check, any subsequent metrics.Capture() calls result in a nil pointer dereference, crashing urunc silently.

This PR fixes the issue by returning a &mockWriter{} (safe no-op fallback) and logging a warning instead of returning nil on file open failure. This ensures urunc gracefully disables metrics rather than panicking.

Related issues

• Fixes fix(metrics): NewZerologMetrics returns nil on file open failure causing panic #595

How was this tested?

1. Enabled timestamps in /etc/urunc/config.toml with enabled = true and destination set to a non-existent directory (/var/log/urunc/timestamps.log).
2. Ran sudo nerdctl run --rm --runtime io.containerd.urunc.v2 harbor.nbfc.io/nubificus/urunc/nginx-qemu-unikraft-initrd:latest.
3. Verified that urunc starts normally, logs a warning about the file, and gracefully disables metrics without crashing.
4. Ran make unittest to ensure no existing tests were broken.

LLM usage

No usage

Checklist

• I have read the contribution guide.
• The linter passes locally (make lint).
• The e2e tests of at least one tool pass locally (make test_ctr, make test_nerdctl, make test_docker, make test_crictl).
• If LLMs were used: I have read the llm policy.

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[cmainas]

Hello @abhaygoudannavar ,

please do not overwrite the PR description. Read the contribution guide for more information.

[abhaygoudannavar]

@cmainas i have updated the PR description as asked for now it can be merged.

[cmainas]

Hello @abhaygoudannavar ,

you have linked a PR instead of the issue in the description. Also, I left a comment for the unit test.

[abhaygoudannavar]

@cmainas i have done the changes asked for.

• Corrected the PR description to correctly link the original issue (fix(metrics): NewZerologMetrics returns nil on file open failure causing panic #595).
• Updated the unit test to use require.NotNil as suggested.

[cmainas]

Hello @abhaygoudannavar ,

thank you for the changes. I have added a comment.

[cmainas]

Hello @abhaygoudannavar ,

it seems there was a misunderstanding. Maybe I di not place the comment in the correct line. The check for the nil return should remain, the call later to Capture should get removed. We should not test by checking if the test crashes.

[abhaygoudannavar]

@cmainas Thanks for the clarification, I've updated the test to explicitly check for a non-nil writer instead of relying on the panic recovery. The PR has been updated.