[io] Fix a bug where Process.stdin.add exceptions could not be caught

Change-Id: I2383a74bfa6950ab8f8934087fb68218f06dd681
Bug:https://github.com/dart-lang/sdk/issues/48501
Tested: Unit test
CoreLibraryReviewExempt: dart:io only
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/351380
Reviewed-by: Alexander Aprelev <aam@google.com>
Commit-Queue: Brian Quinlan <bquinlan@google.com>

sdk/lib/_internal/vm/bin/process_patch.dart

@@ -282,6 +282,11 @@ base class _ProcessImpl extends _ProcessImplNativeWrapper implements _Process {
     if (_modeHasStdio(_mode)) {
       // stdin going to process.
       _stdin = new _StdSink(new _Socket._writePipe().._owner = this);
+      // Ignore errors if the `Process.stdin.done` future is not consumed.
+      // Developers catch errors writing to `Process.stdin` by consuming
+      // `Process.stdin.done` or calling `Process.stdin.flush()`.
+      _stdin!.done.ignore();
+
       // stdout coming from process.
       _stdout = new _StdStream(new _Socket._readPipe().._owner = this);
       // stderr coming from process.

sdk/lib/io/process.dart

@@ -492,6 +492,29 @@ abstract interface class Process {
   Stream<List<int>> get stderr;
 
   /// The standard input stream of the process as an [IOSink].
+  ///
+  /// `stdin` is implemented as a pipe between the parent process and the
+  /// spawned subprocess.
+  ///
+  /// Data added to the [IOSink] (E.g. `Process.stdin.writeln('Hello!')`) is
+  /// written to to the pipe asynchronously.
+  ///
+  /// Errors writing the data (e.g. due to the subprocess exiting) can be
+  /// caught by awaiting `Process.stdin.flush()`. For example:
+  ///
+  /// ```dart
+  /// import 'dart:io';
+  ///
+  /// main() async {
+  ///   final process = await Process.start('false', const <String>[]);
+  ///   process.stdin.writeln('Hello World\n');  // May already have exited.
+  ///   try {
+  ///     await process.stdin.flush();
+  ///   } catch (e) {
+  ///     print(e);
+  ///   }
+  /// }
+  /// ```
   IOSink get stdin;
 
   /// The process id of the process.

tests/standalone/io/process_stdin_broken_pipe_test.dart

@@ -0,0 +1,54 @@
+// Copyright (c) 2024, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+//
+// Verify that failing to write to `Process.stdin` results in an exception
+// being thrown by `process.stdin.flush()` and `process.stdin.done`.
+//
+// See https://github.com/dart-lang/sdk/issues/48501
+//
+// VMOptions=
+// VMOptions=--short_socket_read
+// VMOptions=--short_socket_write
+// VMOptions=--short_socket_read --short_socket_write
+
+import "package:expect/expect.dart";
+import 'dart:async';
+import 'dart:io';
+import 'dart:math';
+
+import "process_test_util.dart";
+
+Future test(Process process) async {}
+
+void main() async {
+  if (!Platform.isLinux && !Platform.isMacOS) {
+    print('test not supported on ${Platform.operatingSystem}');
+    return;
+  }
+
+  final process = await Process.start('false', const <String>[]);
+  try {
+    for (var i = 0; i < 20; ++i) {
+      // Ensure that the pipe is broken while we are writing.
+      process.stdin.add([1, 2, 3]);
+      await Future.delayed(const Duration(milliseconds: 50));
+    }
+
+    try {
+      await process.stdin.flush();
+      Expect.fail('await process.stdin.flush(): expected exception');
+    } on SocketException catch (e) {
+      Expect.equals(32, e.osError!.errorCode); // Broken pipe
+    }
+
+    try {
+      await process.stdin.done;
+      Expect.fail('await process.stdin.done: expected exception');
+    } on SocketException catch (e) {
+      Expect.equals(32, e.osError!.errorCode); // Broken pipe
+    }
+  } finally {
+    process.kill();
+  }
+}