stream: fix dropped first chunk in Utf8Stream buffer mode

Signed-off-by: Daijiro Wachi <daijiro.wachi@gmail.com>
PR-URL: #63833
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Mattias Buelens <mattias@buelens.com>

Author: watilde

lib/internal/streams/fast-utf8-stream.js

@@ -818,7 +818,7 @@ class Utf8Stream extends EventEmitter {
 bufs.length===0||
 lens[lens.length-1]+data.length>this.#maxWrite
 ){
-ArrayPrototypePush(bufs,[]);
+ArrayPrototypePush(bufs,[data]);
 ArrayPrototypePush(lens,data.length);
 }else{
 ArrayPrototypePush(bufs[bufs.length-1],data);

test/parallel/test-fastutf8stream-write-buffer.js

@@ -0,0 +1,72 @@
+'use strict';
+
+// Regression test for buffer content mode: the first buffer of every
+// batch used to be dropped, corrupting the output and eventually
+// crashing in mergeBuf(). Refs: https://github.com/nodejs/node/pull/58897
+
+constcommon=require('../common');
+consttmpdir=require('../common/tmpdir');
+constassert=require('node:assert');
+const{
+ readFile,
+ Utf8Stream,
+}=require('node:fs');
+const{ join }=require('node:path');
+
+tmpdir.refresh();
+letfileCounter=0;
+
+functiongetTempFile(){
+returnjoin(tmpdir.path,`fastutf8stream-${process.pid}-${Date.now()}-${fileCounter++}.log`);
+}
+
+runTests(false);
+runTests(true);
+
+functionrunTests(sync){
+{
+// A single buffer write must end up in the file.
+constdest=getTempFile();
+conststream=newUtf8Stream({ dest, sync,contentMode: 'buffer'});
+
+stream.on('ready',common.mustCall(()=>{
+assert.ok(stream.write(Buffer.from('hello world\n')));
+stream.end();
+
+stream.on('finish',common.mustCall(()=>{
+readFile(dest,'utf8',common.mustSucceed((data)=>{
+assert.strictEqual(data,'hello world\n');
+}));
+}));
+}));
+}
+
+{
+// Writes that exceed maxWrite start a new batch; data must survive
+// the batch boundary and be written in order.
+constdest=getTempFile();
+conststream=newUtf8Stream({
+ dest,
+ sync,
+contentMode: 'buffer',
+minLength: 60,
+maxWrite: 64,
+});
+
+stream.on('ready',common.mustCall(()=>{
+stream.write(Buffer.from('a'.repeat(40)));
+stream.write(Buffer.from('b'.repeat(40)));
+stream.write(Buffer.from('c'.repeat(40)));
+stream.end();
+
+stream.on('finish',common.mustCall(()=>{
+readFile(dest,'utf8',common.mustSucceed((data)=>{
+assert.strictEqual(
+data,
+'a'.repeat(40)+'b'.repeat(40)+'c'.repeat(40),
+);
+}));
+}));
+}));
+}
+}