Interactive processes do not flush process.stdout until process terminates #5607
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Set owner to @sgjesse. |
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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. |
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Removed Area-IO label. |
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This comment was originally written by mikelewis256...@gmail.com This still seems to be an issue for other types of executables. For example, this bash script: === Code === read -p "name: " name will fail to display the "name: " prompt. Passing the script through stdbuf as above doesn't help. |
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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 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'; void main() { |
DartBot
added
Type-Defect
area-core-library
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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: 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! |
This provides the guts of an implementation of zulip#60. I'd have liked to write this in Dart, rather than in shell. But when running this script interactively (vs. in CI), a key ingredient for a good developer experience is that the child processes like `flutter test` should have their stdout and stderr connected directly to those of the parent script, i.e. to the developer's terminal. Crucially, that lets those processes know to print their output in a form that takes advantage of terminal features to get a good interactive experience. The gold standard for a CLI tool is to have a way to control that choice directly, but `flutter test` and some other Dart-based tools lack that capability. And in Dart, as far as I can tell, there simply is no way to start a child process that inherits any of the parent's file handles; instead the child's output will always be wired up to pipes for the parent to consume. There's advice for how the parent can copy the output, chunk by chunk, to its own output: dart-lang/sdk#44573 dart-lang/sdk#5607 https://stackoverflow.com/questions/72183086/dart-dont-buffer-process-stdout-tell-process-that-it-is-running-from-a-termina but that doesn't solve the problem of the child's behavior changing when it sees a pipe instead of a terminal. The nastiest consequence of this behavior is that the output of `flutter test` is hundreds of lines long, even for our small codebase, even when only a single test case fails or none at all. That's fine in CI, but pretty much intolerable for a development workflow. So, shell it is. (Python, or Javascript with Node, or many other languages would also handle this just fine, but would mean an extra system of dependencies to manage.) Specifically, Bash. --- Fortunately, using Bash does mean we get to reuse the work that's already gone into the `tools/test` script in zulip-mobile. This commit introduces a bare-bones version, with most of the features (notably --diff and its friends) stripped out, and the test suites replaced with a first two for our codebase. This version also uses `set -u` and `set -o pipefail`, unlike the one in zulip-mobile, in addition to `set -e`.
This issue was originally filed by @butlermatt
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.
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