commit 83850ac5fa8cfd58a0dcbfe7103b6455a2a1d523
author Jonas Termansen <sortie@google.com> Tue May 31 15:36:33 2022 +0000
committer Commit Bot <commit-bot@chromium.org> Tue May 31 15:36:33 2022 +0000
tree b44d3b9ea3ddedd4b4f37e57108e108a07d99851
parent 69b9af223014f9ab9d2b22261e88b05ee547434f

[io] Don't restore terminal state on exit.

This is breaking change #45630.

The Dart VM has until now restored the terminal settings upon exit to
their initial values for stdin, stdout, and stderr. This change removes
that automatic behavior in favor of having the program do the
restoration. Previously the intention was that dart programs can
enable/disable echoing and line buffering and not worry about restoring
the original settings.

However, the VM doing so unconditionally leads to undesirable behavior
e.g. when the program does not care about terminal settings and is
sharing a process group with a program that does care. E.g. if dart's
output is piped into less(1), then there is a race condition where dart
might see the raw terminal settings set by less(1), and if the dart VM
exits after less(1) has exited, then it will restore the raw terminal
settings, leaving the user with a seemingly defective shell with echo
disabled. This race condition can be reproduced using:

    cat > yes.dart << EOF
    main() {
      for (int i = 0; i < 1000000; i++) {
        print("yes");
      }
    }
    EOF
    stty; (sleep 1 && dart yes.dart) | less; stty; stty sane

The user will end up with a shell with echo behavior disabled. The stty
command shows the current terminal settings, where the difference can be
seen, and 'stty sane' fixes the settings before returning to the shell
prompt. The 'stty sane' call can be omitted to see the defective shell
prompt.

This change removes the terminal restoring behavior (added in Dart
2.0.0) and instead asks applications to do the restoration themselves.
The new design matches how programs in other programming languages
implement interactive input that changes terminal settings.

Furthermore the `echoMode` setting now only controls the `echo` local
mode and no longer sets the `echonl` local mode on POSIX systems (which
controls whether newline are echoed even if the regular echo mode is
disabled). The `echonl` local mode is usually turned off in common shell
environments. Programs that wish to control the `echonl` local mode can
use the new `echoNewlineMode` setting. This change is required to
prevent the reoccurence of #30318 when programs manually restore
`echoMode`.

Windows has further considerations: It also saves the console code pages
and restore them if they were not UTF-8. This behavior is retained as it
is useful and needed for Dart's output to function properly. ANSI output
sequences are also turned on via ENABLE_VIRTUAL_TERMINAL_PROCESSING,
which is slightly changed in this change to only rsetore that setting if
it wasn't already on for consistency.

Closes https://github.com/dart-lang/sdk/issues/36453
Closes https://github.com/dart-lang/sdk/issues/45630

TEST=Reproduced with less as above

Change-Id: I2991f9c7f47b97fe475c1ad6edeb769024f8d0db
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/190484
Reviewed-by: Lasse Nielsen <lrn@google.com>
Commit-Queue: Alexander Aprelev <aam@google.com>
Reviewed-by: Jonas Termansen <sortie@google.com>
Reviewed-by: Alexander Aprelev <aam@google.com>