child_process: only stop readable side of stream passed to proc

Closing the underlying resource completely has the unwanted side effect
that the stream can no longer be used at all, including passing it
to other child processes.

What we want to avoid is accidentally reading from the stream;
accordingly, it should be sufficient to stop its readable side
manually, and otherwise leave the underlying resource intact.

Fixes: nodejs#27097
Refs: nodejs#21209

Author: addaleax

lib/internal/child_process.js

@@ -384,12 +384,12 @@ ChildProcess.prototype.spawn = function(options) {
       continue;
     }
 
-    // The stream is already cloned and piped, thus close it.
+    // The stream is already cloned and piped, thus stop its readable side,
+    // otherwise we might attempt to read from the stream when at the same time
+    // the child process does.
     if (stream.type === 'wrap') {
-      stream.handle.close();
-      if (stream._stdio && stream._stdio instanceof EventEmitter) {
-        stream._stdio.emit('close');
-      }
+      stream.handle.readStop();
+      stream._stdio.pause();
       continue;
     }
 

test/parallel/test-child-process-server-close.js

@@ -8,11 +8,11 @@ const tmpdir = require('../common/tmpdir');
 tmpdir.refresh();
 
 const server = net.createServer((conn) => {
-  conn.on('close', common.mustCall());
-
   spawn(process.execPath, ['-v'], {
     stdio: ['ignore', conn, 'ignore']
-  }).on('close', common.mustCall());
+  }).on('close', common.mustCall(() => {
+    conn.end();
+  }));
 }).listen(common.PIPE, () => {
   const client = net.connect(common.PIPE, common.mustCall());
   client.on('data', () => {

test/parallel/test-child-process-stdio-merge-stdouts-into-cat.js

@@ -0,0 +1,30 @@
+'use strict';
+const common = require('../common');
+const assert = require('assert');
+
+// Regression test for https://github.com/nodejs/node/issues/27097.
+// Check that (cat [p1] ; cat [p2]) | cat [p3] works.
+
+const { spawn } = require('child_process');
+const p3 = spawn('cat', { stdio: ['pipe', 'pipe', process.stderr] });
+const p1 = spawn('cat', { stdio: ['pipe', p3.stdin, process.stderr] });
+const p2 = spawn('cat', { stdio: ['pipe', p3.stdin, process.stderr] });
+p3.stdout.setEncoding('utf8');
+
+// Write three different chunks:
+// - 'hello' from this process to p1 to p3 back to us
+// - 'world' from this process to p2 to p3 back to us
+// - 'foobar' from this process  to p3 back to us
+// Do so sequentially in order to avoid race conditions.
+p1.stdin.end('hello\n');
+p3.stdout.once('data', common.mustCall((chunk) => {
+  assert.strictEqual(chunk, 'hello\n');
+  p2.stdin.end('world\n');
+  p3.stdout.once('data', common.mustCall((chunk) => {
+    assert.strictEqual(chunk, 'world\n');
+    p3.stdin.end('foobar\n');
+    p3.stdout.once('data', common.mustCall((chunk) => {
+      assert.strictEqual(chunk, 'foobar\n');
+    }));
+  }));
+}));