child_process: fix data loss with readable event

This commit prevents child process stdio streams from being
automatically flushed on child process exit/close if a 'readable'
event handler has been attached at the time of exit.

Without this, child process stdio data can be lost if the process
exits quickly and a `read()` (e.g. from a 'readable' handler)
hasn't had the chance to get called yet.

Fixes: #5034

lib/child_process.js

@@ -1094,8 +1094,11 @@ util.inherits(ChildProcess, EventEmitter);
 function flushStdio(subprocess) {
   if (subprocess.stdio == null) return;
   subprocess.stdio.forEach(function(stream, fd, stdio) {
-    if (!stream || !stream.readable || stream._consuming ||
-        stream._readableState.flowing)
+    if (!stream ||
+        !stream.readable ||
+        stream._consuming ||
+        stream._readableState.flowing ||
+        stream._readableState.readableListening)
       return;
     stream.resume();
   });

test/simple/test-child-process-flush-stdio.js

@@ -0,0 +1,17 @@
+'use strict';
+var cp = require('child_process');
+var common = require('../common');
+var assert = require('assert');
+
+var buffer = [];
+var p = cp.spawn('echo', ['123']);
+p.on('close', common.mustCall(function(code, signal) {
+  assert.strictEqual(code, 0);
+  assert.strictEqual(signal, null);
+  assert.strictEqual(Buffer.concat(buffer).toString().trim(), '123');
+}));
+p.stdout.on('readable', function() {
+  var buf;
+  while (buf = this.read())
+    buffer.push(buf);
+});