Interactive processes do not flush process.stdout until process terminates

[DartBot]

This issue was originally filed by @butlermatt

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If using Process.start() to launch an interactive process, you cannot retrieve any of the data from Process.stdout until after the process has terminated in some way. As an example see the following code:
(Assumes linux with BSD games installed)
=== Code ===
import 'dart:io';

void main() {
  var userIn = new StringInputStream(stdin);
  var p = Process.start('/usr/games/adventure', []);
  p.onError = (e) {
    print('Error: $e');
    exit(1);
  };
  
  p.onStart = () {
    p.stdout.onData = () {
      var str = new String.fromCharCodes(p.stdout.read());
      print(str);
    };
    
    var errFromProcess = new StringInputStream(p.stderr);
    errFromProcess.onLine = () {
      var str = errFromProcess.readLine();
      print(str);
    };
    
    userIn.onLine = () {
      var str = userIn.readLine().trim();
      print('Sending: $str');
      p.stdin.writeString('$str\n');

    };
  };
  
// p.onExit = (code) {
// p.close();
// exit(code);
// };
}
=== end Code ===

Now if you run this application, nothing will appear. In command however type in:
no
quit
yes

Each on their own line. no will respond to "do you wish instructions?"
quit will request the process to exit
and yes will confirm the request. Once you type 'yes' then the actual output of the process is THEN sent to the p.stdout.onData() function. However if I uncomment out the p.onExit(); then that data is never received. It only appears to flush the stream if it is closed by the process not by the application.

[kasperl]

Added Area-IO, Triaged labels.

[madsager]

Set owner to @sgjesse.

[sgjesse]

This is caused by libc which by default puts stdout int buffered mode when it is not connected to a terminal. On Linux stdbuf from GNU cureutils can be used to change the libc default buffering. So changing

  var p = Process.start('/usr/games/adventure', []);

to

  var p = Process.start('/usr/bin/stdbuf', ["--output=L", "/usr/games/adventure"]);

makes the game playable.

For more background see http://www.pixelbeat.org/programming/stdio_buffering/ and the stdbuf man page.

---

Added AsDesigned label.

[kevmoo]

Removed Area-IO label.
Added Area-Library, Library-IO labels.

[DartBot]

This comment was originally written by mikelewis256...@gmail.com

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This still seems to be an issue for other types of executables. For example, this bash script:

=== Code ===
#!/usr/bin/env bash

read -p "name: " name
echo "$name"
=== end Code ===

will fail to display the "name: " prompt. Passing the script through stdbuf as above doesn't help.

[sgjesse]

The "name:" prompt should not be displayed, as read only displays it if the input is coming from a terminal. This is from the documentation for the bash read builtin:

              -p prompt
                     Display prompt on standard error, without a trailing new‐
                     line, before attempting to read any input. The prompt is
                     displayed only if input is coming from a terminal.

PS.: The code in the bug no longer runs, as it was pre Dart 1.0 dart:io library. The following works:

import 'dart:convert';
import 'dart:io';

void main() {
  Process.start('./test.sh', []).then((p) {
    p.stdout.transform(new Utf8Decoder()).listen(print);
    p.stderr.transform(new Utf8Decoder()).listen(print);
    stdin.pipe(p.stdin);
  });
}

[Sidewinder1138]

I'm still having this problem, on Windows platform, and I don't seem to be able to find any way to alleviate it.

I'm doing this:
Process.start(exePath, [], workingDirectory: workingDir, mode: ProcessStartMode.detachedWithStdio) .then((process) { process.stdout.pipe(stdout); process.stderr.pipe(stderr); });

It works, and I start to see some of the stdout from the exe I'm launching, but then it stops. When I kill the exe, the rest of the stdout dumps out. Any hints on how to fix this would be greatly appreciated!