feat: add fingerprint command to agent (#1726)

Co-authored-by: henrygd <hank@henrygd.me>

agent/fingerprint.go

@@ -0,0 +1,87 @@
+package agent
+
+import (
+	"crypto/sha256"
+	"encoding/hex"
+	"errors"
+	"os"
+	"path/filepath"
+	"strings"
+
+	"github.com/shirou/gopsutil/v4/cpu"
+	"github.com/shirou/gopsutil/v4/host"
+)
+
+const fingerprintFileName = "fingerprint"
+
+// knownBadUUID is a commonly known "product_uuid" that is not unique across systems.
+const knownBadUUID = "03000200-0400-0500-0006-000700080009"
+
+// GetFingerprint returns the agent fingerprint. It first tries to read a saved
+// fingerprint from the data directory. If not found (or dataDir is empty), it
+// generates one from system properties. The hostname and cpuModel parameters are
+// used as fallback material if host.HostID() fails. If either is empty, they
+// are fetched from the system automatically.
+//
+// If a new fingerprint is generated and a dataDir is provided, it is saved.
+func GetFingerprint(dataDir, hostname, cpuModel string) string {
+	if dataDir != "" {
+		if fp, err := readFingerprint(dataDir); err == nil {
+			return fp
+		}
+	}
+	fp := generateFingerprint(hostname, cpuModel)
+	if dataDir != "" {
+		_ = SaveFingerprint(dataDir, fp)
+	}
+	return fp
+}
+
+// generateFingerprint creates a fingerprint from system properties.
+// It tries host.HostID() first, falling back to hostname + cpuModel.
+// If hostname or cpuModel are empty, they are fetched from the system.
+func generateFingerprint(hostname, cpuModel string) string {
+	fingerprint, err := host.HostID()
+	if err != nil || fingerprint == "" || fingerprint == knownBadUUID {
+		if hostname == "" {
+			hostname, _ = os.Hostname()
+		}
+		if cpuModel == "" {
+			if info, err := cpu.Info(); err == nil && len(info) > 0 {
+				cpuModel = info[0].ModelName
+			}
+		}
+		fingerprint = hostname + cpuModel
+	}
+
+	sum := sha256.Sum256([]byte(fingerprint))
+	return hex.EncodeToString(sum[:24])
+}
+
+// readFingerprint reads the saved fingerprint from the data directory.
+func readFingerprint(dataDir string) (string, error) {
+	fp, err := os.ReadFile(filepath.Join(dataDir, fingerprintFileName))
+	if err != nil {
+		return "", err
+	}
+	s := strings.TrimSpace(string(fp))
+	if s == "" {
+		return "", errors.New("fingerprint file is empty")
+	}
+	return s, nil
+}
+
+// SaveFingerprint writes the fingerprint to the data directory.
+func SaveFingerprint(dataDir, fingerprint string) error {
+	return os.WriteFile(filepath.Join(dataDir, fingerprintFileName), []byte(fingerprint), 0o644)
+}
+
+// DeleteFingerprint removes the saved fingerprint file from the data directory.
+// Returns nil if the file does not exist (idempotent).
+func DeleteFingerprint(dataDir string) error {
+	err := os.Remove(filepath.Join(dataDir, fingerprintFileName))
+	if errors.Is(err, os.ErrNotExist) {
+		return nil
+	}
+	return err
+}