streams: set default encoding for writable streams

Add API Writable#setDefaultEncoding().

PR-URL: #8483
Fixes: #7159
Reviewed-by: Trevor Norris <trev.norris@gmail.com>

doc/api/stream.markdown

@@ -564,6 +564,14 @@ Buffered data will be flushed either at `.uncork()` or at `.end()` call.
 
 Flush all data, buffered since `.cork()` call.
 
+#### writable.setDefaultEncoding(encoding)
+
+* `encoding` {String} The new default encoding
+* Return: `Boolean`
+
+Sets the default encoding for a writable stream. Returns `true` if the encoding
+is valid and is set. Otherwise returns `false`.
+
 #### writable.end([chunk][, encoding][, callback])
 
 * `chunk` {String | Buffer} Optional data to write

lib/_stream_writable.js

@@ -221,6 +221,17 @@ Writable.prototype.uncork = function() {
   }
 };
 
+Writable.prototype.setDefaultEncoding = function setDefaultEncoding(encoding) {
+  if (typeof encoding !== 'string')
+    return false;
+  // node::ParseEncoding() requires lower case.
+  encoding = encoding.toLowerCase();
+  if (!Buffer.isEncoding(encoding))
+    return false;
+  this._writableState.defaultEncoding = encoding;
+  return true;
+};
+
 function decodeChunk(state, chunk, encoding) {
   if (!state.objectMode &&
       state.decodeStrings !== false &&

test/simple/test-stream-writable-change-default-encoding.js

@@ -0,0 +1,76 @@
+// Copyright Joyent, Inc. and other Node contributors.
+//
+// Permission is hereby granted, free of charge, to any person obtaining a
+// copy of this software and associated documentation files (the
+// "Software"), to deal in the Software without restriction, including
+// without limitation the rights to use, copy, modify, merge, publish,
+// distribute, sublicense, and/or sell copies of the Software, and to permit
+// persons to whom the Software is furnished to do so, subject to the
+// following conditions:
+//
+// The above copyright notice and this permission notice shall be included
+// in all copies or substantial portions of the Software.
+//
+// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS
+// OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
+// MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN
+// NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM,
+// DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR
+// OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE
+// USE OR OTHER DEALINGS IN THE SOFTWARE.
+
+var common = require('../common');
+var assert = require('assert');
+
+var stream = require('stream');
+var util = require('util');
+
+function MyWritable(fn, options) {
+  stream.Writable.call(this, options);
+  this.fn = fn;
+};
+
+util.inherits(MyWritable, stream.Writable);
+
+MyWritable.prototype._write = function (chunk, encoding, callback) {
+  this.fn(Buffer.isBuffer(chunk), typeof chunk, encoding);
+  callback();
+};
+
+(function defaultCondingIsUtf8() {
+  var m = new MyWritable(function(isBuffer, type, enc) {
+    assert.equal(enc, 'utf8');
+  }, { decodeStrings: false });
+  m.write('foo');
+  m.end();
+}());
+
+(function changeDefaultEncodingToAscii() {
+  var m = new MyWritable(function(isBuffer, type, enc) {
+    assert.equal(enc, 'ascii');
+  }, { decodeStrings: false });
+  var status = m.setDefaultEncoding('ascii');
+  assert.equal(status, true);
+  m.write('bar');
+  m.end();
+}());
+
+(function changeDefaultEncodingToInvalidValue() {
+  var m = new MyWritable(function(isBuffer, type, enc) {
+    assert.equal(enc, 'utf8');
+  }, { decodeStrings: false });
+  var status = m.setDefaultEncoding({});
+  assert.equal(status, false);
+  m.write('bar');
+  m.end();
+}());
+
+(function checkVairableCaseEncoding() {
+  var m = new MyWritable(function(isBuffer, type, enc) {
+    assert.equal(enc, 'ascii');
+  }, { decodeStrings: false });
+  var status = m.setDefaultEncoding('AsCii');
+  assert.equal(status, true);
+  m.write('bar');
+  m.end();
+}());