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>

Author: addaleax

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){
-varcode=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&&typeofencoding!=='string'){
 thrownewTypeError('encoding must be a string');
 }
-if(typeofencoding==='string'&&!Buffer.isEncoding(encoding)){
+varnormalizedEncoding=internalUtil.normalizeEncoding(encoding);
+if(normalizedEncoding===undefined){
 thrownewTypeError('Unknown encoding: '+encoding);
 }
 
+if(val.length===0){
+// Previously, if val === '', the Buffer would not fill,
+// which is rather surprising.
+val=0;
+}elseif(val.length===1){
+varcode=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;
+}
+}
 }elseif(typeofval==='number'){
 val=val&255;
 }

test/parallel/test-buffer-fill.js

@@ -395,11 +395,37 @@ assert.throws(() => {
 buf.fill('');
 },/^RangeError:outofrangeindex$/);
 
-
 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));