fs/promises: ensure options.flag is defaulted to 'r' in readFile

When passing {} or { encoding: 'utf8' } as options to readFile, the
flag is not defaulted to 'r' unlike normal fs. This fix makes
(fs/promises).readFile() act consistently with fs.readFile().

lib/internal/fs/promises.js

@@ -463,11 +463,12 @@ async function appendFile(path, data, options) {
 
 async function readFile(path, options) {
   options = getOptions(options, { flag: 'r' });
+  const flag = options.flag || 'r';
 
   if (path instanceof FileHandle)
     return readFileHandle(path, options);
 
-  const fd = await open(path, options.flag, 0o666);
+  const fd = await open(path, flag, 0o666);
   return readFileHandle(fd, options).finally(fd.close.bind(fd));
 }
 

test/parallel/test-fs-promises-readfile-empty.js

@@ -0,0 +1,17 @@
+'use strict';
+require('../common');
+
+const assert = require('assert');
+const { promises: fs } = require('fs');
+const fixtures = require('../common/fixtures');
+
+const fn = fixtures.path('empty.txt');
+
+fs.readFile(fn)
+  .then(assert.ok);
+
+fs.readFile(fn, 'utf8')
+  .then(assert.strictEqual.bind(this, ''));
+
+fs.readFile(fn, { encoding: 'utf8' })
+  .then(assert.strictEqual.bind(this, ''));

test/parallel/test-fs-readfile-empty.js

@@ -38,5 +38,9 @@ fs.readFile(fn, 'utf8', function(err, data) {
   assert.strictEqual('', data);
 });
 
+fs.readFile(fn, { encoding: 'utf8' }, function(err, data) {
+  assert.strictEqual('', data);
+});
+
 assert.ok(fs.readFileSync(fn));
 assert.strictEqual('', fs.readFileSync(fn, 'utf8'));