stream: fix finished regression when working with legacy Stream

Signed-off-by: Matteo Collina <hello@matteocollina.com>

PR-URL: #40858
Reviewed-By: Robert Nagy <ronagy@icloud.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Antoine du Hamel <duhamelantoine1995@gmail.com>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>

lib/internal/streams/end-of-stream.js

@@ -165,13 +165,13 @@ function eos(stream, options, callback) {
   } else if (
     !readable &&
     (!willEmitClose || isReadable(stream)) &&
-    (writableFinished || !isWritable(stream))
+    (writableFinished || isWritable(stream) === false)
   ) {
     process.nextTick(onclose);
   } else if (
     !writable &&
     (!willEmitClose || isWritable(stream)) &&
-    (readableFinished || !isReadable(stream))
+    (readableFinished || isReadable(stream) === false)
   ) {
     process.nextTick(onclose);
   } else if ((rState && stream.req && stream.aborted)) {

test/parallel/test-stream-finished.js

@@ -7,7 +7,8 @@ const {
   Transform,
   finished,
   Duplex,
-  PassThrough
+  PassThrough,
+  Stream,
 } = require('stream');
 const assert = require('assert');
 const EE = require('events');
@@ -630,3 +631,11 @@ testClosed((opts) => new Writable({ write() {}, ...opts }));
     }));
   }));
 }
+
+{
+  // Legacy Streams do not inherit from Readable or Writable.
+  // We cannot really assume anything about them, so we cannot close them
+  // automatically.
+  const s = new Stream();
+  finished(s, common.mustNotCall());
+}