feat(core): add graceful shutdown and concurrency safety

[hajnalmt]

Summary

• Implements graceful shutdown via signal.NotifyContext (SIGTERM/SIGINT)
• Adds sync.RWMutex protection for package-level mutable state (numaMap, config)
• Extracts hardcoded environment variable name into a named constant
• Improves error messages for missing configuration

Stacked PR

This PR is stacked on top of PR #16. The relevant commit for review is b77e504.

Related Issues

• Improve code quality: graceful shutdown and concurrency safety #17 (Improve code quality: graceful shutdown and concurrency safety)

Design

Graceful shutdown

The main loop now uses signal.NotifyContext to catch SIGTERM and SIGINT. When a signal is received:

1. The context is cancelled
2. The ticker is stopped (via defer tick.Stop())
3. klog is flushed (via defer klog.Flush())
4. A shutdown message is logged before the process exits

This is critical for Kubernetes where pods receive SIGTERM before being forcefully killed. Without signal handling, the process would be mid-operation when killed, potentially leaving stale state.

Concurrency safety

Package-level mutable state is now protected with sync.RWMutex:

• numaMap in framework.go — holds registered topology info providers and their latest state
• config in kubeletconfig.go — holds kubelet policy and reservation data

Write operations (RegisterNumaType, TopoInfoUpdate, TryUpdatingResourceReservation) take a write lock. Read operations (GetPolicy, GetResReserved, GetAllResAllocatableInfo, GetCpusDetail) take a read lock.

While the current implementation is single-goroutine, this makes the code safe for future concurrency (e.g., adding an HTTP health endpoint on a separate goroutine) and prevents subtle bugs.

Constants

The MY_NODE_NAME environment variable string is extracted into a const envNodeName in both main.go and update.go, eliminating magic strings and improving error messages.

AI Assistance Disclosure

This PR was developed with AI guidance. The author reviewed, directed, and validated all changes carefully.

Signals come, the daemon hears,
no more dying mid-career.
Mutex guards the shared terrain,
future threads may run in vain—
but never race, and never clash,
for locks prevent the data crash.

Closes

• Closes Improve code quality: graceful shutdown and concurrency safety #17

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[gemini-code-assist]

Code Review

This pull request introduces several structural improvements to the resource-exporter, including the addition of internal packages for capacity, checkpoint, cpuset, and eviction logic. It also enhances the build process via Makefile updates, adds graceful shutdown handling with signal notifications, and ensures thread-safety for shared state by introducing mutexes. The code review identified an opportunity to improve the robustness of the countCPUs function by providing more descriptive error handling for malformed input strings, which has been acknowledged as a valid improvement.

[gemini-code-assist]

The current implementation of countCPUs silently ignores invalid range parts, such as 0-1-2 in a string like 0-1-2,3. This could lead to an incorrect CPU count without any indication of an error. It would be more robust to return an error for such malformed range parts. Using a switch statement can make the logic clearer and handle invalid formats explicitly. This change also improves error messages for better diagnostics.

		switch len(parts) {
		case 1:
			if _, err := strconv.Atoi(strings.TrimSpace(parts[0])); err != nil {
				return 0, fmt.Errorf("invalid CPU ID in range part %q: %w", r, err)
			}
			count++
		case 2:
			start, err := strconv.Atoi(strings.TrimSpace(parts[0]))
			if err != nil {
				return 0, fmt.Errorf("invalid start CPU ID in range part %q: %w", r, err)
			}
			end, err := strconv.Atoi(strings.TrimSpace(parts[1]))
			if err != nil {
				return 0, fmt.Errorf("invalid end CPU ID in range part %q: %w", r, err)
			}
			if end < start {
				return 0, fmt.Errorf("invalid CPU range: start %d > end %d in %q", start, end, r)
			}
			count += end - start + 1
		default:
			return 0, fmt.Errorf("invalid CPU range format: %q", r)
		}