stream: do not emit readable if the stream ended

Fixes a regression introduced by the once-per-microtick 'readable'
event emission.

See: https://github.com/nodejs/node/pull/17979
PR-URL: https://github.com/nodejs/node/pull/18372
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>

lib/_stream_readable.js

@@ -520,7 +520,9 @@ function emitReadable(stream) {
 function emitReadable_(stream) {
   var state = stream._readableState;
   debug('emit readable');
-  stream.emit('readable');
+  if (!state.destroyed && (state.length || state.ended)) {
+    stream.emit('readable');
+  }
   state.needReadable = !state.flowing && !state.ended;
   flow(stream);
 }

test/parallel/test-stream-readable-no-unneeded-readable.js

@@ -0,0 +1,39 @@
+'use strict';
+const common = require('../common');
+const { Readable, PassThrough } = require('stream');
+
+const source = new Readable({
+  read: () => {}
+});
+
+source.push('foo');
+source.push('bar');
+source.push(null);
+
+const pt = source.pipe(new PassThrough());
+
+const wrapper = new Readable({
+  read: () => {
+    let data = pt.read();
+
+    if (data) {
+      wrapper.push(data);
+      return;
+    }
+
+    pt.once('readable', function() {
+      data = pt.read();
+      if (data) {
+        wrapper.push(data);
+      }
+      // else the end event should fire
+    });
+  }
+});
+
+pt.once('end', function() {
+  wrapper.push(null);
+});
+
+wrapper.resume();
+wrapper.once('end', common.mustCall());