child_process: attach child in promisification

This commit updates the custom exec() and execFile()
promisification to attach the ChildProcess instance to the
returned Promise.

PR-URL: https://github.com/nodejs/node/pull/28325
Fixes: https://github.com/nodejs/node/issues/28244
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Wyatt Preul <wpreul@gmail.com>

doc/api/child_process.md

@@ -214,10 +214,11 @@ Unlike the exec(3) POSIX system call, `child_process.exec()` does not replace
 the existing process and uses a shell to execute the command.
 
 If this method is invoked as its [`util.promisify()`][]ed version, it returns
-a `Promise` for an `Object` with `stdout` and `stderr` properties. In case of an
-error (including any error resulting in an exit code other than 0), a rejected
-promise is returned, with the same `error` object given in the callback, but
-with an additional two properties `stdout` and `stderr`.
+a `Promise` for an `Object` with `stdout` and `stderr` properties. The returned
+`ChildProcess` instance is attached to the `Promise` as a `child` property. In
+case of an error (including any error resulting in an exit code other than 0), a
+rejected promise is returned, with the same `error` object given in the
+callback, but with an additional two properties `stdout` and `stderr`.
 
 ```js
 const util = require('util');
@@ -295,9 +296,10 @@ stderr output. If `encoding` is `'buffer'`, or an unrecognized character
 encoding, `Buffer` objects will be passed to the callback instead.
 
 If this method is invoked as its [`util.promisify()`][]ed version, it returns
-a `Promise` for an `Object` with `stdout` and `stderr` properties. In case of an
-error (including any error resulting in an exit code other than 0), a rejected
-promise is returned, with the same `error` object given in the
+a `Promise` for an `Object` with `stdout` and `stderr` properties. The returned
+`ChildProcess` instance is attached to the `Promise` as a `child` property. In
+case of an error (including any error resulting in an exit code other than 0), a
+rejected promise is returned, with the same `error` object given in the
 callback, but with an additional two properties `stdout` and `stderr`.
 
 ```js

lib/child_process.js

@@ -149,17 +149,24 @@ function exec(command, options, callback) {
 
 const customPromiseExecFunction = (orig) => {
   return (...args) => {
-    return new Promise((resolve, reject) => {
-      orig(...args, (err, stdout, stderr) => {
-        if (err !== null) {
-          err.stdout = stdout;
-          err.stderr = stderr;
-          reject(err);
-        } else {
-          resolve({ stdout, stderr });
-        }
-      });
+    let resolve;
+    let reject;
+    const promise = new Promise((res, rej) => {
+      resolve = res;
+      reject = rej;
     });
+
+    promise.child = orig(...args, (err, stdout, stderr) => {
+      if (err !== null) {
+        err.stdout = stdout;
+        err.stderr = stderr;
+        reject(err);
+      } else {
+        resolve({ stdout, stderr });
+      }
+    });
+
+    return promise;
   };
 };
 

test/parallel/test-child-process-promisified.js

@@ -8,25 +8,37 @@ const exec = promisify(child_process.exec);
 const execFile = promisify(child_process.execFile);
 
 {
-  exec(`${process.execPath} -p 42`).then(common.mustCall((obj) => {
+  const promise = exec(`${process.execPath} -p 42`);
+
+  assert(promise.child instanceof child_process.ChildProcess);
+  promise.then(common.mustCall((obj) => {
     assert.deepStrictEqual(obj, { stdout: '42\n', stderr: '' });
   }));
 }
 
 {
-  execFile(process.execPath, ['-p', '42']).then(common.mustCall((obj) => {
+  const promise = execFile(process.execPath, ['-p', '42']);
+
+  assert(promise.child instanceof child_process.ChildProcess);
+  promise.then(common.mustCall((obj) => {
     assert.deepStrictEqual(obj, { stdout: '42\n', stderr: '' });
   }));
 }
 
 {
-  exec('doesntexist').catch(common.mustCall((err) => {
+  const promise = exec('doesntexist');
+
+  assert(promise.child instanceof child_process.ChildProcess);
+  promise.catch(common.mustCall((err) => {
     assert(err.message.includes('doesntexist'));
   }));
 }
 
 {
-  execFile('doesntexist', ['-p', '42']).catch(common.mustCall((err) => {
+  const promise = execFile('doesntexist', ['-p', '42']);
+
+  assert(promise.child instanceof child_process.ChildProcess);
+  promise.catch(common.mustCall((err) => {
     assert(err.message.includes('doesntexist'));
   }));
 }