stream: the position of _read() is wrong

lib/internal/fs/streams.js

@@ -255,6 +255,10 @@ ReadStream.prototype._read = function(n) {
       if (er) {
         errorOrDestroy(this, er);
       } else if (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
+
+const common = require('../common');
+const tmpdir = require('../common/tmpdir');
+const fs = require('fs');
+const assert = require('assert');
+const path = require('path');
+
+tmpdir.refresh();
+
+const file = path.join(tmpdir.path, '/read_stream_pos_test.txt');
+
+fs.writeFileSync(file, '');
+
+let counter = 0;
+
+setInterval(() => {
+  counter = counter + 1;
+  const line = `hello at ${counter}\n`;
+  fs.writeFileSync(file, line, { flag: 'a' });
+}, 1);
+
+const hwm = 10;
+let bufs = [];
+let isLow = false;
+let cur = 0;
+let stream;
+
+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) {
+      const brokenLines = Buffer.concat(bufs).toString()
+        .split('\n')
+        .filter((line) => {
+          const s = 'hello at'.slice(0, line.length);
+          if (line && !line.startsWith(s)) {
+            return true;
+          }
+          return false;
+        });
+      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);