cmd/tailscale/cli, client/tailscale, ipn/localapi: add tailscale syspolicy {list,reload} commands

In this PR, we add the tailscale syspolicy command with two subcommands: list, which displays
policy settings, and reload, which forces a reload of those settings. We also update the LocalAPI
and LocalClient to facilitate these additions.

Updates #12687

Signed-off-by: Nick Khyl <nickk@tailscale.com>

ipn/localapi/localapi.go

@@ -62,6 +62,8 @@ import (
 	"tailscale.com/util/osdiag"
 	"tailscale.com/util/progresstracking"
 	"tailscale.com/util/rands"
+	"tailscale.com/util/syspolicy/rsop"
+	"tailscale.com/util/syspolicy/setting"
 	"tailscale.com/version"
 	"tailscale.com/wgengine/magicsock"
 )
@@ -76,6 +78,7 @@ var handler = map[string]localAPIHandler{
 	"cert/":     (*Handler).serveCert,
 	"file-put/": (*Handler).serveFilePut,
 	"files/":    (*Handler).serveFiles,
+	"policy/":   (*Handler).servePolicy,
 	"profiles/": (*Handler).serveProfiles,
 
 	// The other /localapi/v0/NAME handlers are exact matches and contain only NAME
@@ -1332,6 +1335,53 @@ func (h *Handler) servePrefs(w http.ResponseWriter, r *http.Request) {
 	e.Encode(prefs)
 }
 
+func (h *Handler) servePolicy(w http.ResponseWriter, r *http.Request) {
+	if !h.PermitRead {
+		http.Error(w, "policy access denied", http.StatusForbidden)
+		return
+	}
+
+	suffix, ok := strings.CutPrefix(r.URL.EscapedPath(), "/localapi/v0/policy/")
+	if !ok {
+		http.Error(w, "misconfigured", http.StatusInternalServerError)
+		return
+	}
+
+	var scope setting.PolicyScope
+	if suffix == "" {
+		scope = setting.DefaultScope()
+	} else if err := scope.UnmarshalText([]byte(suffix)); err != nil {
+		http.Error(w, fmt.Sprintf("%q is not a valid scope", suffix), http.StatusBadRequest)
+		return
+	}
+
+	policy, err := rsop.PolicyFor(scope)
+	if err != nil {
+		http.Error(w, err.Error(), http.StatusInternalServerError)
+		return
+	}
+
+	var effectivePolicy *setting.Snapshot
+	switch r.Method {
+	case "GET":
+		effectivePolicy = policy.Get()
+	case "POST":
+		effectivePolicy, err = policy.Reload()
+		if err != nil {
+			http.Error(w, err.Error(), http.StatusInternalServerError)
+			return
+		}
+	default:
+		http.Error(w, "unsupported method", http.StatusMethodNotAllowed)
+		return
+	}
+
+	w.Header().Set("Content-Type", "application/json")
+	e := json.NewEncoder(w)
+	e.SetIndent("", "\t")
+	e.Encode(effectivePolicy)
+}
+
 type resJSON struct {
 	Error string `json:",omitempty"`
 }