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fs: make sure readFile[Sync] reads from the beginning

Rodrigo Muino Tomonari requested to merge github/fork/seishun/readfile-offset into master
Checklist
  • make -j8 test (UNIX), or vcbuild test nosign (Windows) passes
  • tests and/or benchmarks are included
  • commit message follows commit guidelines
Affected core subsystem(s)

fs

Description of change

Currently, fs.readFile[Sync] starts reading from the current file position, which can be non-zero if the fd was previously read or written from/to. This conflicts with the documentation which states that it "reads the entire contents of a file".

Fixes: https://github.com/nodejs/node/issues/9671

Provisional testcase for fs.readFile:

const assert = require('assert');
const fs = require('fs');

const filleLength = 2;
const path = 'test-read-file';
const fd = fs.openSync(path, 'w+');

fs.writeSync(fd, Buffer.alloc(filleLength, 1), 0, filleLength);

fs.readFile(fd, (err, buf) => {
  if (err) throw err;
  assert.strictEqual(buf.length, filleLength);
});

For fs.readFileSync:

'use strict';

const assert = require('assert');
const fs = require('fs');

const fileLength = 2;
const path = 'test-read-file';
const fd = fs.openSync(path, 'w+');

fs.writeSync(fd, Buffer.alloc(fileLength, 1), 0, fileLength);

assert.strictEqual(fs.readFileSync(fd).length, fileLength);

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