fix(agent): unify agent and openresty runtime user as openflare

Introduce the shared openflare service account for the agent process and
OpenResty workers, normalize data_dir ownership on startup, and ensure
managed paths are chowned with 0755/0644 during sync and apply. Docker
entrypoint fixes volume ownership before dropping privileges; local systemd
install runs the service as openflare with CAP_NET_BIND_SERVICE.

internal/apps/agent/runtimeuser/runtimeuser.go

@@ -0,0 +1,111 @@
+// Package runtimeuser defines the shared OS account used by the agent process
+// and OpenResty worker processes so file ownership stays aligned.
+package runtimeuser
+
+import (
+	"errors"
+	"fmt"
+	"log/slog"
+	"os"
+	"os/user"
+	"path/filepath"
+	"strconv"
+	"strings"
+
+	openrestyrender "github.com/Rain-kl/Wavelet/pkg/render/openresty"
+)
+
+// Name is the dedicated service account shared by the agent and OpenResty workers.
+const Name = openrestyrender.OpenFlareRuntimeUser
+
+const (
+	// DefaultDirPerm is the normalized permission for runtime directories.
+	DefaultDirPerm = 0o755
+	// DefaultFilePerm is the normalized permission for runtime files.
+	DefaultFilePerm = 0o644
+)
+
+// Account holds the resolved UID/GID for Name on the current host.
+type Account struct {
+	Name string
+	UID  int
+	GID  int
+}
+
+// Lookup resolves the runtime account on the current host.
+func Lookup() (*Account, error) {
+	record, err := user.Lookup(Name)
+	if err != nil {
+		return nil, fmt.Errorf("lookup %s: %w", Name, err)
+	}
+	uid, err := strconv.Atoi(record.Uid)
+	if err != nil {
+		return nil, fmt.Errorf("parse uid for %s: %w", Name, err)
+	}
+	gid, err := strconv.Atoi(record.Gid)
+	if err != nil {
+		return nil, fmt.Errorf("parse gid for %s: %w", Name, err)
+	}
+	return &Account{Name: Name, UID: uid, GID: gid}, nil
+}
+
+// CurrentEUID returns the effective UID of the current process.
+func CurrentEUID() int {
+	return os.Geteuid()
+}
+
+// IsRuntimeUser reports whether the current process runs as Name.
+func IsRuntimeUser() bool {
+	account, err := Lookup()
+	if err != nil {
+		return false
+	}
+	return os.Geteuid() == account.UID
+}
+
+// EnsureProcessUser drops from root to Name when possible so the agent writes
+// files with the same ownership OpenResty workers read.
+func EnsureProcessUser() error {
+	account, err := Lookup()
+	if err != nil {
+		slog.Warn("runtime user unavailable, agent continues as current user", "user", Name, "euid", os.Geteuid(), "error", err)
+		return nil
+	}
+	if os.Geteuid() == account.UID {
+		slog.Info("agent running as runtime user", "user", Name, "uid", account.UID)
+		return nil
+	}
+	if os.Geteuid() != 0 {
+		slog.Warn("agent is not running as runtime user", "expected", Name, "euid", os.Geteuid())
+		return nil
+	}
+	if dropErr := dropTo(account); dropErr != nil {
+		return dropErr
+	}
+	slog.Info("agent dropped privileges to runtime user", "user", Name, "uid", account.UID)
+	return nil
+}
+
+// EnsurePathOwnership makes root and its ancestors traversable, assigns runtime
+// ownership when running as root, and normalizes directory/file modes.
+func EnsurePathOwnership(root string, dirPerm os.FileMode, filePerm os.FileMode) error {
+	root = filepath.Clean(strings.TrimSpace(root))
+	if root == "" || root == "." {
+		return nil
+	}
+	if err := ensureWorldTraversablePath(root); err != nil {
+		return err
+	}
+	if _, statErr := os.Stat(root); os.IsNotExist(statErr) {
+		return nil
+	}
+	account, lookupErr := Lookup()
+	if lookupErr != nil {
+		var unknown user.UnknownUserError
+		if errors.As(lookupErr, &unknown) {
+			return ensureModesOnly(root, dirPerm, filePerm)
+		}
+		return lookupErr
+	}
+	return applyOwnershipAndModes(root, account, dirPerm, filePerm)
+}