child_process: flush consuming streams

When a client calls read() with a nonzero argument
on a Socket, that Socket sets this._consuming to true.
It never sets this._consuming back to false.
ChildProcess.flushStdio() currently doesn't flush
any streams where _consuming is truthy. But, that means
that it never flushes any stream that has ever been read from.
This prevents a child process from ever closing if one of
its streams has been read from, causing issue #4049. This
commit allows consuming streams to be flushed, and the
child process to emit a close event.

Fixes: #4049
PR-URL: #4071
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>

Author: Dave

lib/internal/child_process.js

@@ -217,7 +217,7 @@ 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)
+    if (!stream || !stream.readable)
       return;
     stream.resume();
   });

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

@@ -0,0 +1,17 @@
+'use strict';
+const cp = require('child_process');
+const common = require('../common');
+const assert = require('assert');
+
+const p = cp.spawn('echo');
+
+p.on('close', common.mustCall(function(code, signal) {
+  assert.strictEqual(code, 0);
+  assert.strictEqual(signal, null);
+}));
+
+p.stdout.read();
+
+setTimeout(function() {
+  p.kill();
+}, 100);