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: nodejs#9836

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 (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

@@ -390,3 +390,23 @@ assert.throws(() => {
   });
   buf.fill('');
 });
+
+assert.strictEqual(
+    Buffer.allocUnsafeSlow(16).fill('a', 'utf16le').toString('utf16le'),
+    'aaaaaaaa');
+assert.strictEqual(
+    Buffer.allocUnsafeSlow(16).fill('a', 'latin1').toString('latin1'),
+    'aaaaaaaaaaaaaaaa');
+assert.strictEqual(
+    Buffer.allocUnsafeSlow(16).fill('a', 'utf8').toString('utf8'),
+    'aaaaaaaaaaaaaaaa');
+
+assert.strictEqual(
+    Buffer.allocUnsafeSlow(16).fill('Љ', 'utf16le').toString('utf16le'),
+    'ЉЉЉЉЉЉЉЉ');
+assert.strictEqual(
+    Buffer.allocUnsafeSlow(16).fill('Љ', 'latin1').toString('latin1'),
+    '\t'.repeat(16));
+assert.strictEqual(
+    Buffer.allocUnsafeSlow(16).fill('Љ', 'utf8').toString('utf8'),
+    'ЉЉЉЉЉЉЉЉ');