minor server-sent events fixes (mastodon#12945)

* Send output on the server-sent events stream immediately so the client sees that it was successfully opened even if it doesn't have any messages.

Fix transparent SSE streaming for the public:local and hashtag:local stream types.

* Tell caches to never store server-sent events.

app/javascript/mastodon/stream.js

@@ -92,6 +92,7 @@ export default function getStream(streamingAPIBaseURL, accessToken, stream, { co
  return ws;
  }
 
+ stream = stream.replace(/:/g, '/');
  params.push(`access_token=${accessToken}`);
  const es = new EventSource(`${streamingAPIBaseURL}/api/v1/streaming/${stream}?${params.join('&')}`);
 

streaming/index.js

@@ -436,8 +436,11 @@ const startWorker = (workerId) => {
  const accountId = req.accountId || req.remoteAddress;
 
  res.setHeader('Content-Type', 'text/event-stream');
+ res.setHeader('Cache-Control', 'no-store');
  res.setHeader('Transfer-Encoding', 'chunked');
 
+ res.write(':)\n');
+
  const heartbeat = setInterval(() => res.write(':thump\n'), 15000);
 
  req.on('close', () => {