print and writeln are not thread-safe

[ntkme]

Here is the minimum reproduction:

import 'dart:isolate';

void main() {
  Future.wait([
    for (var i = 0; i < 100; i += 1)
      Isolate.run(() {
        print('test');
      })
  ]);
}

The expected output is 100 lines that each contains a single test, but the actual result is that some of the lines are empty and some of the lines will have testtest.

The underneath issue is that print uses writeln internally, and writeln would output the message and a newline character in two separate calls, causing a race condition between threads.

---

As a workaround, directly calling write with newline character added to message is thread-safe.

import 'dart:isolate';
import 'dart:io';

void main() {
  Future.wait([
    for (var i = 0; i < 100; i += 1)
      Isolate.run(() {
        stdout.write('test\n');
      })
  ]);
}

[lrhn]

Adding the \n on the string will copy the entire string one extra time. That's definitely unnecessary overhead.
(And on Windows, we'd still have to convert the \n to \r\n afterwards.)

One thing we could perhaps do, is to let the internal implementation of print include the line terminator in the same native memory buffer that we convert the string to UTF-8 into anyway. Then we can emit the line terminator along with the string contents, in the same native write call.

If we give Dart_StringToUTF8 an extra integer argument, the number of extra bytes to make room for in the allocation, after the bytes of the string, then we can insert the newline into that, instead writing it as a second call.

[a-siva]

https://dart-review.googlesource.com/c/sdk/+/330999