test: make sure over truncation of file zero fills

If the file is over truncated, then the rest of the file should be
filled with null bytes. These tests ensure the same.

PR-URL: #7648
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: James M Snell <jasnell@gmail.com>

test/parallel/test-fs-truncate.js

@@ -106,3 +106,43 @@ function testFtruncate(cb) {
     });
   });
 }
+
+
+// Make sure if the size of the file is smaller than the length then it is
+// filled with zeroes.
+
+{
+  const file1 = path.resolve(tmp, 'truncate-file-1.txt');
+  fs.writeFileSync(file1, 'Hi');
+  fs.truncateSync(file1, 4);
+  assert(fs.readFileSync(file1).equals(Buffer.from('Hi\u0000\u0000')));
+}
+
+{
+  const file2 = path.resolve(tmp, 'truncate-file-2.txt');
+  fs.writeFileSync(file2, 'Hi');
+  const fd = fs.openSync(file2, 'r+');
+  process.on('exit', () => fs.closeSync(fd));
+  fs.ftruncateSync(fd, 4);
+  assert(fs.readFileSync(file2).equals(Buffer.from('Hi\u0000\u0000')));
+}
+
+{
+  const file3 = path.resolve(tmp, 'truncate-file-3.txt');
+  fs.writeFileSync(file3, 'Hi');
+  fs.truncate(file3, 4, common.mustCall(function(err) {
+    assert.ifError(err);
+    assert(fs.readFileSync(file3).equals(Buffer.from('Hi\u0000\u0000')));
+  }));
+}
+
+{
+  const file4 = path.resolve(tmp, 'truncate-file-4.txt');
+  fs.writeFileSync(file4, 'Hi');
+  const fd = fs.openSync(file4, 'r+');
+  process.on('exit', () => fs.closeSync(fd));
+  fs.ftruncate(fd, 4, common.mustCall(function(err) {
+    assert.ifError(err);
+    assert(fs.readFileSync(file4).equals(Buffer.from('Hi\u0000\u0000')));
+  }));
+}