Fix #4486 (#4487)

src/misc/convert-host.ts

@@ -1,5 +1,6 @@
 import config from '../config';
 import { toUnicode, toASCII } from 'punycode';
+import { URL } from 'url';
 
 export function getFullApAccount(username: string, host: string) {
 	return host ? `${username}@${toApHost(host)}` : `${username}@${toApHost(config.host)}`;
@@ -10,6 +11,11 @@ export function isSelfHost(host: string) {
 	return toApHost(config.host) === toApHost(host);
 }
 
+export function extractDbHost(uri: string) {
+	const url = new URL(uri);
+	return toDbHost(url.hostname);
+}
+
 export function toDbHost(host: string) {
 	if (host == null) return null;
 	return toUnicode(host.toLowerCase());