test: increase coverage for fs.promises.truncate

To increase test coverage, added tests for fs.promises.truncate
and FileHandle.truncate.

PR-URL: #20638
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>

Backport-PR-URL: #21172

test/parallel/test-fs-promises-file-handle-truncate.js

@@ -0,0 +1,26 @@
+'use strict';
+
+const common = require('../common');
+const assert = require('assert');
+const path = require('path');
+const { open, readFile } = require('fs').promises;
+const tmpdir = require('../common/tmpdir');
+
+tmpdir.refresh();
+common.crashOnUnhandledRejection();
+
+async function validateTruncate() {
+  const text = 'Hello world';
+  const filename = path.resolve(tmpdir.path, 'truncate-file.txt');
+  const fileHandle = await open(filename, 'w+');
+
+  const buffer = Buffer.from(text, 'utf8');
+  await fileHandle.write(buffer, 0, buffer.length);
+
+  assert.deepStrictEqual((await readFile(filename)).toString(), text);
+
+  await fileHandle.truncate(5);
+  assert.deepStrictEqual((await readFile(filename)).toString(), 'Hello');
+}
+
+validateTruncate().then(common.mustCall());

test/parallel/test-fs-promises.js

@@ -17,13 +17,15 @@ const {
   mkdir,
   mkdtemp,
   open,
+  readFile,
   readdir,
   readlink,
   realpath,
   rename,
   rmdir,
   stat,
   symlink,
+  truncate,
   unlink,
   utimes
 } = fsPromises;
@@ -99,6 +101,8 @@ function verifyStatObject(stat) {
     const ret2 = await handle.read(Buffer.alloc(buf2Len), 0, buf2Len, 0);
     assert.strictEqual(ret2.bytesRead, buf2Len);
     assert.deepStrictEqual(ret2.buffer, buf2);
+    await truncate(dest, 5);
+    assert.deepStrictEqual((await readFile(dest)).toString(), 'hello');
 
     await chmod(dest, 0o666);
     await handle.chmod(0o666);