stream: handle setEncoding after buffered data

mcollina · aduh95 · commit 315ca426d8ac · 2026-06-20T16:58:28.000+02:00
Signed-off-by: Matteo Collina <hello@matteocollina.com> PR-URL: #63973 Refs: #63570 Refs: nodejs/undici#5002 Reviewed-By: Jithil P Ponnan <jithil@outlook.com> Reviewed-By: James M Snell <jasnell@gmail.com>
diff --git a/lib/internal/streams/readable.js b/lib/internal/streams/readable.js
@@ -604,6 +604,8 @@ Readable.prototype.setEncoding = function(enc) {
   for (const data of state.buffer.slice(state.bufferIndex)) {
     content += decoder.write(data);
   }
+  if ((state[kState] & kEnded) !== 0)
+    content += decoder.end();
   state.buffer.length = 0;
   state.bufferIndex = 0;
 
diff --git a/test/parallel/test-stream-readable-setEncoding-existing-buffers.js b/test/parallel/test-stream-readable-setEncoding-existing-buffers.js
@@ -58,3 +58,48 @@ const assert = require('assert');
     assert.deepStrictEqual(chunks, ['🎉']);
   });
 }
+
+{
+  // Call .setEncoding() before the stream starts flowing, after _read()
+  // has buffered the first chunk of a split character.
+  const chunks = [
+    Buffer.from([0xf0, 0x9f]),
+    Buffer.from([0x8e, 0x89]),
+    null,
+  ];
+  const r = new Readable({
+    read() {
+      this.push(chunks.shift());
+    },
+  });
+
+  r.read(0);
+  assert.strictEqual(r.readableFlowing, null);
+  assert.strictEqual(r.readableLength, 2);
+
+  r.setEncoding('utf8');
+  const received = [];
+  r.on('data', (chunk) => received.push(chunk));
+
+  process.nextTick(() => {
+    assert.deepStrictEqual(received, ['🎉']);
+  });
+}
+
+{
+  // Call .setEncoding() after EOF while the buffer contains an
+  // incomplete character.
+  const r = new Readable({ read() {} });
+
+  r.push(Buffer.from([0xf0]));
+  r.push(Buffer.from([0x9f]));
+  r.push(null);
+
+  r.setEncoding('utf8');
+  const chunks = [];
+  r.on('data', (chunk) => chunks.push(chunk));
+
+  process.nextTick(() => {
+    assert.deepStrictEqual(chunks, ['�']);
+  });
+}

Original file line number	Diff line number	Diff line change
`@@ -604,6 +604,8 @@ Readable.prototype.setEncoding = function(enc) {`
`604`	`604`	`for (const data of state.buffer.slice(state.bufferIndex)) {`
`605`	`605`	`content += decoder.write(data);`
`606`	`606`	`}`
	`607`	`+ if ((state[kState] & kEnded) !== 0)`
	`608`	`+ content += decoder.end();`
`607`	`609`	`state.buffer.length = 0;`
`608`	`610`	`state.bufferIndex = 0;`
`609`	`611`