stream: the position of _read() is wrong

Fixes: #33940
PR-URL: #38292
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Robert Nagy <ronagy@icloud.com>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>

Author: helloyou2012

lib/internal/fs/streams.js

@@ -255,6 +255,10 @@ ReadStream.prototype._read = function(n) {
 if(er){
 errorOrDestroy(this,er);
 }elseif(bytesRead>0){
+if(this.pos!==undefined){
+this.pos+=bytesRead;
+}
+
 this.bytesRead+=bytesRead;
 
 if(bytesRead!==buf.length){
@@ -271,10 +275,6 @@ ReadStream.prototype._read = function(n) {
 this.push(null);
 }
 });
-
-if(this.pos!==undefined){
-this.pos+=n;
-}
 };
 
 ReadStream.prototype._destroy=function(err,cb){

test/parallel/test-fs-read-stream-pos.js

@@ -0,0 +1,69 @@
+'use strict';
+
+// Refs: https://github.com/nodejs/node/issues/33940
+
+constcommon=require('../common');
+consttmpdir=require('../common/tmpdir');
+constfs=require('fs');
+constassert=require('assert');
+constpath=require('path');
+
+tmpdir.refresh();
+
+constfile=path.join(tmpdir.path,'/read_stream_pos_test.txt');
+
+fs.writeFileSync(file,'');
+
+letcounter=0;
+
+setInterval(()=>{
+counter=counter+1;
+constline=`hello at ${counter}\n`;
+fs.writeFileSync(file,line,{flag: 'a'});
+},1);
+
+consthwm=10;
+letbufs=[];
+letisLow=false;
+letcur=0;
+letstream;
+
+setInterval(()=>{
+if(stream)return;
+
+stream=fs.createReadStream(file,{
+highWaterMark: hwm,
+start: cur
+});
+stream.on('data',common.mustCallAtLeast((chunk)=>{
+cur+=chunk.length;
+bufs.push(chunk);
+if(isLow){
+constbrokenLines=Buffer.concat(bufs).toString()
+.split('\n')
+.filter((line)=>{
+consts='hello at'.slice(0,line.length);
+if(line&&!line.startsWith(s)){
+returntrue;
+}
+returnfalse;
+});
+assert.strictEqual(brokenLines.length,0);
+process.exit();
+return;
+}
+if(chunk.length!==hwm){
+isLow=true;
+}
+}));
+stream.on('end',()=>{
+stream=null;
+isLow=false;
+bufs=[];
+});
+},10);
+
+// Time longer than 90 seconds to exit safely
+setTimeout(()=>{
+process.exit();
+},90000);