fs: ensure readFile[Sync] reads from the beginning

It would previously read from the current file position, which can be
non-zero if the `fd` has been read from or written to. This contradicts
the documentation which states that it "reads the entire contents of a
file".

PR-URL: nodejs#9699
Fixes: nodejs#9671
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Sam Roberts <vieuxtech@gmail.com>

lib/fs.js

@@ -310,7 +310,7 @@ ReadFileContext.prototype.read = function() {
   req.oncomplete = readFileAfterRead;
   req.context = this;
 
-  binding.read(this.fd, buffer, offset, length, -1, req);
+  binding.read(this.fd, buffer, offset, length, this.pos, req);
 };
 
 ReadFileContext.prototype.close = function(err) {
@@ -447,11 +447,11 @@ function tryCreateBuffer(size, fd, isUserFd) {
   return buffer;
 }
 
-function tryReadSync(fd, isUserFd, buffer, pos, len) {
+function tryReadSync(fd, isUserFd, buffer, pos, len, offset) {
   var threw = true;
   var bytesRead;
   try {
-    bytesRead = fs.readSync(fd, buffer, pos, len);
+    bytesRead = fs.readSync(fd, buffer, pos, len, offset);
     threw = false;
   } finally {
     if (threw && !isUserFd) fs.closeSync(fd);
@@ -480,15 +480,15 @@ fs.readFileSync = function(path, options) {
 
   if (size !== 0) {
     do {
-      bytesRead = tryReadSync(fd, isUserFd, buffer, pos, size - pos);
+      bytesRead = tryReadSync(fd, isUserFd, buffer, pos, size - pos, pos);
       pos += bytesRead;
     } while (bytesRead !== 0 && pos < size);
   } else {
     do {
       // the kernel lies about many files.
       // Go ahead and try to read some bytes.
       buffer = Buffer.allocUnsafe(8192);
-      bytesRead = tryReadSync(fd, isUserFd, buffer, 0, 8192);
+      bytesRead = tryReadSync(fd, isUserFd, buffer, 0, 8192, pos);
       if (bytesRead !== 0) {
         buffers.push(buffer.slice(0, bytesRead));
       }

test/parallel/test-fs-readfile-fd-offset.js

@@ -0,0 +1,23 @@
+'use strict';
+const common = require('../common');
+const assert = require('assert');
+const fs = require('fs');
+const path = require('path');
+
+const filename = path.join(common.tmpDir, 'readfile.txt');
+const dataExpected = 'a'.repeat(100);
+fs.writeFileSync(filename, dataExpected);
+const fileLength = dataExpected.length;
+
+['r', 'a+'].forEach((mode) => {
+  const fd = fs.openSync(filename, mode);
+  assert.strictEqual(fs.readFileSync(fd).length, fileLength);
+
+  // Reading again should result in the same length.
+  assert.strictEqual(fs.readFileSync(fd).length, fileLength);
+
+  fs.readFile(fd, common.mustCall((err, buf) => {
+    assert.ifError(err);
+    assert.strictEqual(buf.length, fileLength);
+  }));
+});