fix(metrics): NewZerologMetrics returns nil on file open failure causing 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() in internal/metrics/metrics.go returns nil instead of a safe fallback writer.The caller in cmd/urunc/main.go (line 142) assigns the return value directly to the global metrics variable without performing a nil check:

metrics = m.NewZerologMetrics(cfg.Timestamps.Enabled, cfg.Timestamps.Destination, "")

Since metrics is used throughout the codebase via metrics.Capture() calls (19+ occurrences across create.go, start.go, and unikontainers.go), any subsequent invocation results in a nil pointer dereference, crashing urunc silently.

This is especially impactful for new users who enable timestamps in their config but haven't yet created the log directory (e.g., /var/log/urunc/).

Root Cause

In internal/metrics/metrics.go, the NewZerologMetrics function:

file, err := os.OpenFile(target, os.O_CREATE|os.O_WRONLY|os.O_APPEND, 0666)
if err != nil {
    return nil  // ← this causes the panic
}

When enabled=false, the function correctly returns &mockWriter{} (a safe no-op). But when enabled=true and the file open fails, it returns nil instead of the same safe fallback.

Proposed Fix

Return a &mockWriter{} (no-op writer) instead of nil on file open failure, and log a warning to inform the user that metrics have been disabled due to the file error:

if err != nil {
    logrus.Warnf("Failed to open metrics file %s: %v. Metrics will be disabled.", target, err)
    return &mockWriter{}
}

This ensures urunc continues to function normally with metrics gracefully degraded, rather than crashing.

System info

• Urunc version: main (commit HEAD)
• Arch: x86_64 (amd64)
• VMM: Any (bug is in metrics initialization, not VMM-specific)
• Unikernel: Any (bug is in metrics initialization, not unikernel-specific)

Steps to reproduce

• Enable timestamps in /etc/urunc/config.toml:

[timestamps]
enabled = true
destination = "/var/log/urunc/timestamps.log"

• Make sure the destination directory does not exist:

sudo rm -rf /var/log/urunc/

• Run any urunc command (e.g., create a container):

sudo nerdctl run --rm --runtime io.containerd.urunc.v2 \
  harbor.nbfc.io/nubificus/urunc/nginx-qemu-unikraft-initrd:latest

• Expected behavior: urunc starts normally, logs a warning that metrics file could not be opened, and disables metrics gracefully.
• Actual behavior: urunc crashes with a nil pointer dereference panic at the first metrics.Capture() call.

[abhaygoudannavar]

@cmainas once approved, please assign it to me.