child_process: add ChildProcess 'spawn' event

The new event signals that the subprocess has spawned successfully and
no 'error' event will be emitted from failing to spawn.

Fixes: #35288

doc/api/child_process.md

@@ -1033,6 +1033,21 @@ child process, the `message` argument can contain data that JSON is not able
 to represent.
 See [Advanced serialization][] for more details.
 
+### Event: `'spawn'`
+<!-- YAML
+added: REPLACEME
+-->
+
+The `'spawn'` event is emitted once the child process has spawned successfully.
+
+If emitted, the `'spawn'` event comes before all other events and before any
+data is received via `stdout` or `stderr`.
+
+The `'spawn'` event will fire regardless of whether an error occurs **within**
+the spawned process. For example, if `bash some-command` spawns successfully,
+the `'spawn'` event will fire, though `bash` may fail to spawn `some-command`.
+This caveat also applies when using `{ shell: true }`.
+
 ### `subprocess.channel`
 <!-- YAML
 added: v7.1.0

lib/internal/child_process.js

@@ -400,6 +400,8 @@ ChildProcess.prototype.spawn = function(options) {
     this._handle.close();
     this._handle = null;
     throw errnoException(err, 'spawn');
+  } else {
+    process.nextTick(onSpawnNT, this);
   }
 
   this.pid = this._handle.pid;
@@ -465,6 +467,11 @@ function onErrorNT(self, err) {
 }
 
 
+function onSpawnNT(self) {
+  self.emit('spawn');
+}
+
+
 ChildProcess.prototype.kill = function(sig) {
 
   const signal = sig === 0 ? sig :

test/parallel/test-child-process-spawn-error.js

@@ -41,6 +41,8 @@ assert.strictEqual(enoentChild.stdio[0], enoentChild.stdin);
 assert.strictEqual(enoentChild.stdio[1], enoentChild.stdout);
 assert.strictEqual(enoentChild.stdio[2], enoentChild.stderr);
 
+enoentChild.on('spawn', common.mustNotCall());
+
 enoentChild.on('error', common.mustCall(function(err) {
   assert.strictEqual(err.code, 'ENOENT');
   assert.strictEqual(getSystemErrorName(err.errno), 'ENOENT');

test/parallel/test-child-process-spawn-event.js

@@ -0,0 +1,27 @@
+'use strict';
+const common = require('../common');
+const spawn = require('child_process').spawn;
+const assert = require('assert');
+
+const subprocess = spawn('echo', ['ok']);
+
+let didSpawn = false;
+subprocess.on('spawn', function() {
+  didSpawn = true;
+});
+function mustCallAfterSpawn() {
+  return common.mustCall(function() {
+    assert.ok(didSpawn);
+  });
+}
+
+subprocess.on('error', common.mustNotCall());
+subprocess.on('spawn', common.mustCall());
+subprocess.stdout.on('data', mustCallAfterSpawn());
+subprocess.stdout.on('end', mustCallAfterSpawn());
+subprocess.stdout.on('close', mustCallAfterSpawn());
+subprocess.stderr.on('data', common.mustNotCall());
+subprocess.stderr.on('end', mustCallAfterSpawn());
+subprocess.stderr.on('close', mustCallAfterSpawn());
+subprocess.on('exit', mustCallAfterSpawn());
+subprocess.on('close', mustCallAfterSpawn());