server: prefer IPv6

> wild, it seems they had a bug about A/AAAA fallback a while ago but the
> way they fixed it is "v6 if v4 fails", not the other way around
>
> https://github.com/szmarczak/cacheable-lookup/issues/27
> b2348d5aed
>
> javascript community pls
-- @sn0w@cofe.rocks

packages/backend/src/misc/fetch.ts

@@ -1,6 +1,7 @@
 import * as http from 'node:http';
 import * as https from 'node:https';
 import { URL } from 'node:url';
+import * as dns from 'node:dns';
 import CacheableLookup from 'cacheable-lookup';
 import fetch from 'node-fetch';
 import { HttpProxyAgent, HttpsProxyAgent } from 'hpagent';
@@ -78,13 +79,38 @@ const cache = new CacheableLookup({
 	lookup: false,	// nativeのdns.lookupにfallbackしない
 });
 
+// because `cacheable-lookup` is annoying and prefers IPv4 by default,
+// we need a little wrapper setting some extra options
+const cacheLookup = (host, _2, _3) => {
+	// do all the weird parameter shenanigans that nodejs's dns.lookup does.
+	let options = {}, callback;
+	if (_3) {
+		options = _2;
+		if (typeof options === 'number') {
+			options = { family: options };
+		}
+		callback = _3;
+	} else {
+		callback = _2;
+	}
+
+	// here come the shenanigans, trying to be careful not to mess up
+	// intentionally different behaviour
+	if (options.family == null && (options.hints ?? 0) === 0) {
+		options.family = 6;
+		options.hints = dns.V4MAPPED;
+	}
+
+	return cache.lookup(host, options, callback);
+};
+
 /**
  * Get http non-proxy agent
  */
 const _http = new http.Agent({
 	keepAlive: true,
 	keepAliveMsecs: 30 * SECOND,
-	lookup: cache.lookup,
+	lookup: cacheLookup,
 } as http.AgentOptions);
 
 /**
@@ -93,7 +119,7 @@ const _http = new http.Agent({
 const _https = new https.Agent({
 	keepAlive: true,
 	keepAliveMsecs: 30 * SECOND,
-	lookup: cache.lookup,
+	lookup: cacheLookup,
 } as https.AgentOptions);
 
 const maxSockets = Math.max(256, config.deliverJobConcurrency);