stream: convert existing buffer when calling .setEncoding

Convert already-stored chunks when `.setEncoding()` is called
so that subsequent `data` events will receive decoded strings,
as they expect.

Fixes: https://github.com/nodejs/node/issues/27932

PR-URL: https://github.com/nodejs/node/pull/27936
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Rich Trott <rtrott@gmail.com>

1 files changed, 14 insertions, 1 deletions

diff --git a/lib/_stream_readable.js b/lib/_stream_readable.js
index 5905c56bd4..163a042f08 100644
--- a/lib/_stream_readable.js
+++ b/lib/_stream_readable.js
@@ -321,9 +321,22 @@ Readable.prototype.isPaused = function() {
 Readable.prototype.setEncoding = function(enc) {
   if (!StringDecoder)
     StringDecoder = require('string_decoder').StringDecoder;
-  this._readableState.decoder = new StringDecoder(enc);
+  const decoder = new StringDecoder(enc);
+  this._readableState.decoder = decoder;
   // If setEncoding(null), decoder.encoding equals utf8
   this._readableState.encoding = this._readableState.decoder.encoding;
+
+  // Iterate over current buffer to convert already stored Buffers:
+  let p = this._readableState.buffer.head;
+  let content = '';
+  while (p !== null) {
+    content += decoder.write(p.data);
+    p = p.next;
+  }
+  this._readableState.buffer.clear();
+  if (content !== '')
+    this._readableState.buffer.push(content);
+  this._readableState.length = content.length;
   return this;
 };