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>

1 files changed, 27 insertions, 1 deletions

diff --git a/test/parallel/test-buffer-fill.js b/test/parallel/test-buffer-fill.js
index 68b8bbd69c..eecb14abb0 100644
--- a/test/parallel/test-buffer-fill.js
+++ b/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));