fs: keep fs.promises.readFile read until EOF is reached

Author: kylo5aby

lib/internal/fs/promises.js

@@ -535,6 +535,7 @@ async function readFileHandle(filehandle, options) {
     throw new ERR_FS_FILE_TOO_LARGE(size);
 
   let totalRead = 0;
+  const noSize = size === 0;
   let buffer = Buffer.allocUnsafeSlow(length);
   let result = '';
   let offset = 0;
@@ -557,7 +558,7 @@ async function readFileHandle(filehandle, options) {
 
     if (bytesRead === 0 ||
         totalRead === size ||
-        (bytesRead !== buffer.length && !chunkedRead)) {
+        (bytesRead !== buffer.length && !chunkedRead && !noSize)) {
       const singleRead = bytesRead === totalRead;
 
       const bytesToCheck = chunkedRead ? totalRead : bytesRead;
@@ -567,7 +568,7 @@ async function readFileHandle(filehandle, options) {
       }
 
       if (!encoding) {
-        if (size === 0 && !singleRead) {
+        if (noSize && !singleRead) {
           ArrayPrototypePush(buffers, buffer);
           return Buffer.concat(buffers, totalRead);
         }
@@ -582,7 +583,8 @@ async function readFileHandle(filehandle, options) {
     }
 
     if (encoding) {
-      result += decoder.write(buffer);
+      result += decoder.write(noSize && bytesRead !== kReadFileUnknownBufferLength ?
+        buffer.subarray(0, bytesRead) : buffer);
     } else if (size !== 0) {
       offset = totalRead;
     } else {