child_process: fix custom stdio pipe writable property

lib/internal/child_process.js

@@ -332,8 +332,21 @@ function flushStdio(subprocess) {
 }
 
 
-function createSocket(pipe, readable) {
-  return net.Socket({ handle: pipe, readable });
+function createSocket(pipe, readable, index) {
+  // Determine socket readability based on stdio index:
+  // - stdin (0): parent writes → readable: false
+  // - stdout/stderr (1,2): parent reads → readable: true
+  // - custom pipes (3+): parent writes by default → readable: false
+  // See: https://github.com/nodejs/node/issues/[fork-stdio-bug]
+  let actualReadable;
+  if (index !== undefined) {
+    // Use index-based logic for correctness
+    actualReadable = (index === 1 || index === 2);
+  } else {
+    // Fallback for when index is not provided (for compatibility)
+    actualReadable = readable;
+  }
+  return net.Socket({ handle: pipe, readable: actualReadable, writable: !actualReadable });
 }
 
 
@@ -489,7 +502,7 @@ ChildProcess.prototype.spawn = function spawn(options) {
 
     if (stream.handle) {
       stream.socket = createSocket(this.pid !== 0 ?
-        stream.handle : null, i > 0);
+        stream.handle : null, i > 0, i);
 
       if (i > 0 && this.pid !== 0) {
         this._closesNeeded++;

test/parallel/test-child-process-custom-pipe.js

@@ -0,0 +1,39 @@
+'use strict';
+
+const common = require('../common');
+const assert = require('assert');
+const cp = require('child_process');
+
+// Test that custom stdio pipes have correct readable/writable properties
+// Regression test for pre-existing fork-stdio bug where custom pipes (stdio[3+])
+// were incorrectly marked as readable instead of writable.
+
+if (process.argv[2] === 'child') {
+  // Child process just exits
+  process.exit(0);
+} else {
+  // Parent process: verify socket properties
+  const child = cp.fork(__filename, ['child'], {
+    stdio: [0, 'ignore', 'pipe', 'ipc', 'pipe']
+  });
+
+  // stdio[0] = inherit (stdin)
+  // stdio[1] = ignore 
+  // stdio[2] = pipe (stderr)
+  // stdio[3] = ipc
+  // stdio[4] = pipe (custom pipe)
+
+  const customPipe = child.stdio[4];
+
+  // Custom pipe should be writable (parent writes data to child)
+  assert.strictEqual(customPipe.writable, true,
+    'Custom stdio pipe should be writable');
+
+  // Custom pipe should not be readable by default
+  assert.strictEqual(customPipe.readable, false,
+    'Custom stdio pipe should not be readable by default');
+
+  child.on('exit', common.mustCall((code) => {
+    assert.strictEqual(code, 0);
+  }));
+}