buffer: strip high bits when converting to ascii

Consider the following example:

  console.log(Buffer('ú').toString('ascii'));

Before this commit, the contents of the buffer was used as-is and hence it
prints 'ú'.

Now, it prints 'C:'. Perhaps not much of an improvement but it conforms to what
the documentation says it does: strip off the high bits.

Fixes #4371.

doc/api/buffer.markdown

@@ -19,9 +19,10 @@ encoding method.  Here are the different string encodings.
 
 * `'ascii'` - for 7 bit ASCII data only.  This encoding method is very fast, and
   will strip the high bit if set.
-  Note that this encoding converts a null character (`'\0'` or `'\u0000'`) into
-  `0x20` (character code of a space). If you want to convert a null character
-  into `0x00`, you should use `'utf8'`.
+
+  Note that when converting from string to buffer, this encoding converts a null
+  character (`'\0'` or `'\u0000'`) into `0x20` (character code of a space). If
+  you want to convert a null character into `0x00`, you should use `'utf8'`.
 
 * `'utf8'` - Multibyte encoded Unicode characters. Many web pages and other
   document formats use UTF-8.

src/node_buffer.cc

@@ -247,15 +247,38 @@ Handle<Value> Buffer::BinarySlice(const Arguments &args) {
 }
 
 
+static bool contains_non_ascii(const char* buf, size_t len) {
+  for (size_t i = 0; i < len; ++i) {
+    if (buf[i] & 0x80) return true;
+  }
+  return false;
+}
+
+
+static void force_ascii(const char* src, char* dst, size_t len) {
+  for (size_t i = 0; i < len; ++i) {
+    dst[i] = src[i] & 0x7f;
+  }
+}
+
+
 Handle<Value> Buffer::AsciiSlice(const Arguments &args) {
   HandleScope scope;
   Buffer *parent = ObjectWrap::Unwrap<Buffer>(args.This());
   SLICE_ARGS(args[0], args[1])
 
   char* data = parent->data_ + start;
-  Local<String> string = String::New(data, end - start);
+  size_t len = end - start;
 
-  return scope.Close(string);
+  if (contains_non_ascii(data, len)) {
+    char* out = new char[len];
+    force_ascii(data, out, len);
+    Local<String> rc = String::New(out, len);
+    delete[] out;
+    return scope.Close(rc);
+  }
+
+  return scope.Close(String::New(data, len));
 }
 
 
@@ -268,6 +291,7 @@ Handle<Value> Buffer::Utf8Slice(const Arguments &args) {
   return scope.Close(string);
 }
 
+
 Handle<Value> Buffer::Ucs2Slice(const Arguments &args) {
   HandleScope scope;
   Buffer *parent = ObjectWrap::Unwrap<Buffer>(args.This());

test/simple/test-buffer-ascii.js

@@ -0,0 +1,27 @@
+// 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');
+
+// ASCII conversion in node.js simply masks off the high bits,
+// it doesn't do transliteration.
+assert.equal(Buffer('hérité').toString('ascii'), 'hC)ritC)');