stream: avoid destroying writable source

User might still want to be able to use the writable side
of src. This is in the case where e.g. the Duplex input
is not directly connected to its output. Such a case could
happen when the Duplex is reading from a socket and then echos
the data back on the same socket.

Backport-PR-URL: #32212
PR-URL: #32198
Refs: 4d93e10#commitcomment-37751035
Reviewed-By: James M Snell <[email protected]>
Reviewed-By: Matteo Collina <[email protected]>

lib/internal/streams/pipeline.js

@@ -57,6 +57,10 @@ function destroyer(stream, reading, writing, final, callback) {
       return callback();
     }
 
+    if (!err && reading && !writing && stream.writable) {
+      return callback();
+    }
+
     if (err || !final || !stream.readable) {
       destroyImpl.destroyer(stream, err);
     }

test/parallel/test-stream-pipeline.js

@@ -1016,3 +1016,19 @@ const { promisify } = require('util');
     req.on('error', common.mustNotCall());
   });
 }
+
+{
+  // Might still want to be able to use the writable side
+  // of src. This is in the case where e.g. the Duplex input
+  // is not directly connected to its output. Such a case could
+  // happen when the Duplex is reading from a socket and then echos
+  // the data back on the same socket.
+  const src = new PassThrough();
+  assert.strictEqual(src.writable, true);
+  const dst = new PassThrough();
+  pipeline(src, dst, common.mustCall((err) => {
+    assert.strictEqual(src.writable, true);
+    assert.strictEqual(src.destroyed, false);
+  }));
+  src.push(null);
+}