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: https://github.com/nodejs/node/issues/9836
PR-URL: https://github.com/nodejs/node/pull/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

@@ -672,23 +672,27 @@ 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 (encoding !== undefined && typeof encoding !== 'string') {
+      throw new TypeError('encoding must be a string');
+    }
+    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;
       }
-    if (encoding !== undefined && typeof encoding !== 'string') {
-      throw new TypeError('encoding must be a string');
-    }
-    if (typeof encoding === 'string' && !Buffer.isEncoding(encoding)) {
-      throw new TypeError('Unknown encoding: ' + encoding);
     }
-
   } else if (typeof val === 'number') {
     val = val & 255;
   }

test/parallel/test-buffer-fill.js

@@ -395,7 +395,6 @@ assert.throws(() => {
   buf.fill('');
 }, /^RangeError: out of range index$/);
 
-
 assert.deepStrictEqual(
     Buffer.allocUnsafeSlow(16).fill('ab', 'utf16le'),
     Buffer.from('61006200610062006100620061006200', 'hex'));
@@ -403,3 +402,30 @@ assert.deepStrictEqual(
 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));