fs: promisify exists correctly

PR-URL: https://github.com/nodejs/node/pull/13316
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Refael Ackermann <refack@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Vladimir Kurchatkin <vladimir.kurchatkin@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Timothy Gu <timothygu99@gmail.com>

lib/fs.js

@@ -28,7 +28,7 @@ const constants = process.binding('constants').fs;
 const { S_IFIFO, S_IFLNK, S_IFMT, S_IFREG, S_IFSOCK } = constants;
 const util = require('util');
 const pathModule = require('path');
-const { isUint8Array } = process.binding('util');
+const { isUint8Array, createPromise, promiseResolve } = process.binding('util');
 
 const binding = process.binding('fs');
 const fs = exports;
@@ -308,6 +308,15 @@ fs.exists = function(path, callback) {
   }
 };
 
+Object.defineProperty(fs.exists, internalUtil.promisify.custom, {
+  value: (path) => {
+    const promise = createPromise();
+    fs.exists(path, (exists) => promiseResolve(promise, exists));
+    return promise;
+  }
+});
+
+
 fs.existsSync = function(path) {
   try {
     handleError((path = getPathFromURL(path)));

test/parallel/test-fs-promisified.js

@@ -9,6 +9,7 @@ common.crashOnUnhandledRejection();
 
 const read = promisify(fs.read);
 const write = promisify(fs.write);
+const exists = promisify(fs.exists);
 
 {
   const fd = fs.openSync(__filename, 'r');
@@ -29,3 +30,9 @@ common.refreshTmpDir();
     fs.closeSync(fd);
   }));
 }
+
+{
+  exists(__filename).then(common.mustCall((x) => {
+    assert.strictEqual(x, true);
+  }));
+}