feat: support acl capabilities check

app/routes/acls/acl-loader.ts

@@ -0,0 +1,64 @@
+import { LoaderFunctionArgs } from 'react-router';
+import { LoadContext } from '~/server';
+import ResponseError from '~/server/headscale/api-error';
+import { Capabilities } from '~/server/web/roles';
+import { data403 } from '~/utils/res';
+
+// The logic for deciding policy factors is very complicated because
+// there are so many factors that need to be accounted for:
+// 1. Does the user have permission to read the policy?
+// 2. Does the user have permission to write to the policy?
+// 3. Is the Headscale policy in file or database mode?
+//    If database, we can read/write easily via the API.
+//    If in file mode, we can only write if context.config is available.
+// TODO: Consider adding back file editing mode instead of database
+export async function aclLoader({
+	request,
+	context,
+}: LoaderFunctionArgs<LoadContext>) {
+	const session = await context.sessions.auth(request);
+	const check = await context.sessions.check(request, Capabilities.read_policy);
+	if (!check) {
+		throw data403('You do not have permission to read the ACL policy.');
+	}
+
+	const flags = {
+		// Can the user write to the ACL policy
+		access: await context.sessions.check(request, Capabilities.write_policy),
+		writable: false,
+		policy: '',
+	};
+
+	// Try to load the ACL policy from the API.
+	try {
+		const { policy, updatedAt } = await context.client.get<{
+			policy: string;
+			updatedAt: string | null;
+		}>('v1/policy', session.get('api_key')!);
+
+		// Successfully loaded the policy, mark it as readable
+		// If `updatedAt` is null, it means the policy is in file mode.
+		flags.writable = updatedAt !== null;
+		flags.policy = policy;
+		return flags;
+	} catch (error) {
+		// This means Headscale returned a protobuf error to us
+		// It also means we 100% know this is in database mode
+		if (error instanceof ResponseError && error.responseObject?.message) {
+			const message = error.responseObject.message as string;
+			// This is stupid, refer to the link
+			// https://github.com/juanfont/headscale/blob/main/hscontrol/types/policy.go
+			if (message.includes('acl policy not found')) {
+				// This means the policy has never been initiated, and we can
+				// write to it to get it started or ignore it.
+				flags.policy = ''; // Start with an empty policy
+				flags.writable = true;
+			}
+
+			return flags;
+		}
+
+		// Otherwise, this is a Headscale error that we can just propagate.
+		throw error;
+	}
+}