From 34acc990005dbba5283c37001b3c62010feec99a Mon Sep 17 00:00:00 2001
From: Alex Gleason <alex@alexgleason.me>
Date: Fri, 8 Sep 2023 15:48:00 -0500
Subject: [PATCH] Add a precheck file to throw when config is wrong

---
 src/precheck.ts | 22 ++++++++++++++++++++++
 src/server.ts   |  1 +
 2 files changed, 23 insertions(+)
 create mode 100644 src/precheck.ts

diff --git a/src/precheck.ts b/src/precheck.ts
new file mode 100644
index 0000000..40ab2fd
--- /dev/null
+++ b/src/precheck.ts
@@ -0,0 +1,22 @@
+import { Conf } from '@/config.ts';
+
+/** Ensure the media URL is not on the same host as the local domain. */
+function checkMediaHost() {
+  const { url, mediaDomain } = Conf;
+  const mediaUrl = new URL(mediaDomain);
+
+  if (url.host === mediaUrl.host) {
+    throw new PrecheckError('For security reasons, MEDIA_DOMAIN cannot be on the same host as LOCAL_DOMAIN.');
+  }
+}
+
+/** Error class for precheck errors. */
+class PrecheckError extends Error {
+  constructor(message: string) {
+    super(`${message}\nTo disable this check, set DITTO_PRECHECK="false"`);
+  }
+}
+
+if (Deno.env.get('DITTO_PRECHECK') !== 'false') {
+  checkMediaHost();
+}
diff --git a/src/server.ts b/src/server.ts
index 76cbc9f..e3cf7ca 100644
--- a/src/server.ts
+++ b/src/server.ts
@@ -1,3 +1,4 @@
+import './precheck.ts';
 import app from './app.ts';
 
 Deno.serve(app.fetch);