buffer: fix single-character string filling

Fix the fast path for `buffer.fill()` with a single-character string.

The fast path only works for strings that are equivalent to a
single-byte buffer, but that condition was not checked properly
for the `utf8` or `utf16le` encodings and is always true for the
`latin1` encoding.

This change fixes these problems.

Fixes: #9836
PR-URL: #9837
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Trevor Norris <trev.norris@gmail.com>

lib/buffer.js

@@ -664,23 +664,27 @@ Buffer.prototype.fill = function fill(val, start, end, encoding) {
  encoding = end;
  end = this.length;
  }
- if (val.length === 1) {
- var code = val.charCodeAt(0);
- if (code < 256)
- val = code;
- }
- if (val.length === 0) {
- // Previously, if val === '', the Buffer would not fill,
- // which is rather surprising.
- val = 0;
- }
+
  if (encoding !== undefined && typeof encoding !== 'string') {
  throw new TypeError('encoding must be a string');
  }
- if (typeof encoding === 'string' && !Buffer.isEncoding(encoding)) {
+ var normalizedEncoding = internalUtil.normalizeEncoding(encoding);
+ if (normalizedEncoding === undefined) {
  throw new TypeError('Unknown encoding: ' + encoding);
  }
 
+ if (val.length === 0) {
+ // Previously, if val === '', the Buffer would not fill,
+ // which is rather surprising.
+ val = 0;
+ } else if (val.length === 1) {
+ var code = val.charCodeAt(0);
+ if ((normalizedEncoding === 'utf8' && code < 128) ||
+ normalizedEncoding === 'latin1') {
+ // Fast path: If `val` fits into a single byte, use that numeric value.
+ val = code;
+ }
+ }
  } else if (typeof val === 'number') {
  val = val & 255;
  }

test/parallel/test-buffer-fill.js

@@ -395,11 +395,37 @@ assert.throws(() => {
  buf.fill('');
 }, /^RangeError: out of range index$/);
 
-
 assert.deepStrictEqual(
  Buffer.allocUnsafeSlow(16).fill('ab', 'utf16le'),
  Buffer.from('61006200610062006100620061006200', 'hex'));
 
 assert.deepStrictEqual(
  Buffer.allocUnsafeSlow(15).fill('ab', 'utf16le'),
  Buffer.from('610062006100620061006200610062', 'hex'));
+
+assert.deepStrictEqual(
+ Buffer.allocUnsafeSlow(16).fill('ab', 'utf16le'),
+ Buffer.from('61006200610062006100620061006200', 'hex'));
+assert.deepStrictEqual(
+ Buffer.allocUnsafeSlow(16).fill('a', 'utf16le'),
+ Buffer.from('61006100610061006100610061006100', 'hex'));
+
+assert.strictEqual(
+ Buffer.allocUnsafeSlow(16).fill('a', 'utf16le').toString('utf16le'),
+ 'a'.repeat(8));
+assert.strictEqual(
+ Buffer.allocUnsafeSlow(16).fill('a', 'latin1').toString('latin1'),
+ 'a'.repeat(16));
+assert.strictEqual(
+ Buffer.allocUnsafeSlow(16).fill('a', 'utf8').toString('utf8'),
+ 'a'.repeat(16));
+
+assert.strictEqual(
+ Buffer.allocUnsafeSlow(16).fill('Љ', 'utf16le').toString('utf16le'),
+ 'Љ'.repeat(8));
+assert.strictEqual(
+ Buffer.allocUnsafeSlow(16).fill('Љ', 'latin1').toString('latin1'),
+ '\t'.repeat(16));
+assert.strictEqual(
+ Buffer.allocUnsafeSlow(16).fill('Љ', 'utf8').toString('utf8'),
+ 'Љ'.repeat(8));