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: #27932 PR-URL: #27936 Reviewed-By: Matteo Collina <[email protected]> Reviewed-By: Luigi Pinca <[email protected]> Reviewed-By: Ruben Bridgewater <[email protected]> Reviewed-By: Rich Trott <[email protected]>

Author: addaleax

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=newStringDecoder(enc);
+constdecoder=newStringDecoder(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:
+letp=this._readableState.buffer.head;
+letcontent='';
+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;
 returnthis;
 };
 

test/parallel/test-stream-readable-setEncoding-existing-buffers.js

@@ -0,0 +1,60 @@
+'use strict';
+require('../common');
+const{ Readable }=require('stream');
+constassert=require('assert');
+
+{
+// Call .setEncoding() while there are bytes already in the buffer.
+constr=newReadable({read(){}});
+
+r.push(Buffer.from('a'));
+r.push(Buffer.from('b'));
+
+r.setEncoding('utf8');
+constchunks=[];
+r.on('data',(chunk)=>chunks.push(chunk));
+
+process.nextTick(()=>{
+assert.deepStrictEqual(chunks,['ab']);
+});
+}
+
+{
+// Call .setEncoding() while the buffer contains a complete,
+// but chunked character.
+constr=newReadable({read(){}});
+
+r.push(Buffer.from([0xf0]));
+r.push(Buffer.from([0x9f]));
+r.push(Buffer.from([0x8e]));
+r.push(Buffer.from([0x89]));
+
+r.setEncoding('utf8');
+constchunks=[];
+r.on('data',(chunk)=>chunks.push(chunk));
+
+process.nextTick(()=>{
+assert.deepStrictEqual(chunks,['🎉']);
+});
+}
+
+{
+// Call .setEncoding() while the buffer contains an incomplete character,
+// and finish the character later.
+constr=newReadable({read(){}});
+
+r.push(Buffer.from([0xf0]));
+r.push(Buffer.from([0x9f]));
+
+r.setEncoding('utf8');
+
+r.push(Buffer.from([0x8e]));
+r.push(Buffer.from([0x89]));
+
+constchunks=[];
+r.on('data',(chunk)=>chunks.push(chunk));
+
+process.nextTick(()=>{
+assert.deepStrictEqual(chunks,['🎉']);
+});
+}