build: remove expect as a dependency

I was packaging Lix 2.91 for nixpkgs and was annoyed at the expect
dependency. Turns out that you can replace unbuffer with a pretty-short
Python script.

It became less short after I found out that Linux was converting \n to
\r\n in the terminal subsystem, which was not very funny, but is at
least solved by twiddling termios bits.

Change-Id: I8a2700abcbbf6a9902e01b05b40fa9340c0ab90c

2 files changed, 83 insertions, 0 deletions

diff --git a/misc/meson.build b/misc/meson.build
index a8f09722c..bf3c157f7 100644
--- a/misc/meson.build
+++ b/misc/meson.build
@@ -4,3 +4,9 @@ subdir('zsh')
 
 subdir('systemd')
 subdir('flake-registry')
+
+runinpty = configure_file(
+  copy : true,
+  input : meson.current_source_dir() / 'runinpty.py',
+  output : 'runinpty.py',
+)
diff --git a/misc/runinpty.py b/misc/runinpty.py
new file mode 100755
index 000000000..a7649b735
--- /dev/null
+++ b/misc/runinpty.py
@@ -0,0 +1,77 @@
+#!/usr/bin/env python3
+# SPDX-FileCopyrightText: 2001, 2002, 2003, 2004, 2005, 2006 Python Software Foundation; All Rights Reserved
+# SPDX-FileCopyrightText: 2024 Jade Lovelace
+# SPDX-License-Identifier: LGPL-2.1-or-later
+"""
+This script exists to lose Lix a dependency on expect(1) for the ability to run
+something in a pty.
+
+Yes, it could be replaced by script(1) but macOS and Linux script(1) have
+diverged sufficiently badly that even specifying a subcommand to run is not the
+same.
+"""
+import pty
+import sys
+import os
+from termios import ONLCR, ONLRET, ONOCR, OPOST, TCSAFLUSH, tcgetattr, tcsetattr
+from tty import setraw
+import termios
+
+def setup_terminal():
+    # does not matter which fd we use because we are in a fresh pty
+    modi = tcgetattr(pty.STDOUT_FILENO)
+    [iflag, oflag, cflag, lflag, ispeed, ospeed, cc] = modi
+
+    # Turning \n into \r\n is not cool, Linux!
+    oflag &= ~ONLCR
+    # I don't know what "implementation dependent postprocessing means" but it
+    # sounds bad
+    oflag &= ~OPOST
+    # Assume that NL performs the role of CR; do not insert CRs at column 0
+    oflag |= ONLRET | ONOCR
+
+    modi = [iflag, oflag, cflag, lflag, ispeed, ospeed, cc]
+
+    tcsetattr(pty.STDOUT_FILENO, TCSAFLUSH, modi)
+
+
+def spawn(argv: list[str]):
+    """
+    As opposed to pty.spawn, this one more seriously controls the pty settings.
+    Necessary to turn off such fun functionality as onlcr (LF to CRLF).
+
+    This is essentially copy pasted from pty.spawn, since there is no way to
+    hook the child pre-execve
+    """
+    pid, master_fd = pty.fork()
+    if pid == pty.CHILD:
+        setup_terminal()
+        os.execlp(argv[0], *argv)
+
+    try:
+        mode = tcgetattr(pty.STDIN_FILENO)
+        setraw(pty.STDIN_FILENO)
+        restore = True
+    except termios.error:
+        restore = False
+
+    try:
+        pty._copy(master_fd, pty._read, pty._read) # type: ignore
+    finally:
+        if restore:
+            tcsetattr(pty.STDIN_FILENO, TCSAFLUSH, mode) # type: ignore
+
+    os.close(master_fd)
+    return os.waitpid(pid, 0)[1]
+
+
+def main():
+    if len(sys.argv) == 1:
+        print(f'Usage: {sys.argv[0]} [command args]', file=sys.stderr)
+        sys.exit(1)
+
+    sys.exit(os.waitstatus_to_exitcode(spawn(sys.argv[1:])))
+
+
+if __name__ == '__main__':
+    main()