# Copyright Spack Project Developers. See COPYRIGHT file for details.
#
# SPDX-License-Identifier: (Apache-2.0 OR MIT)

"""Utility classes for logging the output of blocks of code."""
import atexit
import ctypes
import errno
import io
import multiprocessing
import os
import re
import select
import signal
import sys
import threading
import traceback
from contextlib import contextmanager
from multiprocessing.connection import Connection
from threading import Thread
from typing import IO, Callable, Optional, Tuple

import spack.llnl.util.tty as tty

try:
    import termios
except ImportError:
    termios = None  # type: ignore[assignment]


esc, bell, lbracket, bslash, newline = r"\x1b", r"\x07", r"\[", r"\\", r"\n"
# Ansi Control Sequence Introducers (CSI) are a well-defined format
# Standard ECMA-48: Control Functions for Character-Imaging I/O Devices, section 5.4
# https://www.ecma-international.org/wp-content/uploads/ECMA-48_5th_edition_june_1991.pdf
csi_pre = f"{esc}{lbracket}"
csi_param, csi_inter, csi_post = r"[0-?]", r"[ -/]", r"[@-~]"
ansi_csi = f"{csi_pre}{csi_param}*{csi_inter}*{csi_post}"
# General ansi escape sequences have well-defined prefixes,
#  but content and suffixes are less reliable.
# Conservatively assume they end with either "<ESC>\" or "<BELL>",
#  with no intervening "<ESC>"/"<BELL>" keys or newlines
esc_pre = f"{esc}[@-_]"
esc_content = f"[^{esc}{bell}{newline}]"
esc_post = f"(?:{esc}{bslash}|{bell})"
ansi_esc = f"{esc_pre}{esc_content}*{esc_post}"
# Use this to strip escape sequences
_escape = re.compile(f"{ansi_csi}|{ansi_esc}")

# control characters for enabling/disabling echo
#
# We use control characters to ensure that echo enable/disable are inline
# with the other output.  We always follow these with a newline to ensure
# one per line the following newline is ignored in output.
xon, xoff = "\x11\n", "\x13\n"
control = re.compile("(\x11\n|\x13\n)")


@contextmanager
def ignore_signal(signum):
    """Context manager to temporarily ignore a signal."""
    old_handler = signal.signal(signum, signal.SIG_IGN)
    try:
        yield
    finally:
        signal.signal(signum, old_handler)


def _is_background_tty(stdin: IO[str]) -> bool:
    """True if the stream is a tty and calling process is in the background."""
    return stdin.isatty() and os.getpgrp() != os.tcgetpgrp(stdin.fileno())


def _strip(line: str) -> str:
    """Strip color and control characters from a line."""
    return _escape.sub("", line)


class preserve_terminal_settings:
    """Context manager to preserve terminal settings on a stream.

    Stores terminal settings before the context and ensures they are restored after.
    Ensures that things like echo and canonical line mode are not left disabled if
    terminal settings in the context are not properly restored.
    """

    def __init__(self, stdin: Optional[IO[str]]) -> None:
        """Create a context manager that preserves terminal settings on a stream.

        Args:
            stream: keyboard input stream, typically sys.stdin
        """
        self.stdin = stdin

    def _restore_default_terminal_settings(self) -> None:
        """Restore the original input configuration on ``self.stdin``."""
        # Can be called in foreground or background. When called in the background, tcsetattr
        # triggers SIGTTOU, which we must ignore, or the process will be stopped.
        assert self.stdin is not None and self.old_cfg is not None and termios is not None
        with ignore_signal(signal.SIGTTOU):
            termios.tcsetattr(self.stdin, termios.TCSANOW, self.old_cfg)

    def __enter__(self) -> "preserve_terminal_settings":
        """Store terminal settings."""
        self.old_cfg = None

        # Ignore all this if the input stream is not a tty.
        if not self.stdin or not self.stdin.isatty() or not termios:
            return self

        # save old termios settings to restore later
        self.old_cfg = termios.tcgetattr(self.stdin)

        # add an atexit handler to ensure the terminal is restored
        atexit.register(self._restore_default_terminal_settings)

        return self

    def __exit__(self, exc_type, exception, traceback):
        """If termios was available, restore old settings."""
        if self.old_cfg:
            self._restore_default_terminal_settings()
            atexit.unregister(self._restore_default_terminal_settings)


class keyboard_input(preserve_terminal_settings):
    """Context manager to disable line editing and echoing.

    Use this with ``sys.stdin`` for keyboard input, e.g.::

        with keyboard_input(sys.stdin) as kb:
            while True:
                kb.check_fg_bg()
                r, w, x = select.select([sys.stdin], [], [])
                # ... do something with keypresses ...

    The ``keyboard_input`` context manager disables canonical
    (line-based) input and echoing, so that keypresses are available on
    the stream immediately, and they are not printed to the
    terminal. Typically, standard input is line-buffered, which means
    keypresses won't be sent until the user hits return. In this mode, a
    user can hit, e.g., ``v``, and it will be read on the other end of the
    pipe immediately but not printed.

    The handler takes care to ensure that terminal changes only take
    effect when the calling process is in the foreground. If the process
    is backgrounded, canonical mode and echo are re-enabled. They are
    disabled again when the calling process comes back to the foreground.

    This context manager works through a single signal handler for
    ``SIGTSTP``, along with a poolling routine called ``check_fg_bg()``.
    Here are the relevant states, transitions, and POSIX signals::

        [Running] -------- Ctrl-Z sends SIGTSTP ------------.
        [ in FG ] <------- fg sends SIGCONT --------------. |
           ^                                              | |
           | fg (no signal)                               | |
           |                                              | v
        [Running] <------- bg sends SIGCONT ---------- [Stopped]
        [ in BG ]                                      [ in BG ]

    We handle all transitions exept for ``SIGTSTP`` generated by Ctrl-Z
    by periodically calling ``check_fg_bg()``.  This routine notices if
    we are in the background with canonical mode or echo disabled, or if
    we are in the foreground without canonical disabled and echo enabled,
    and it fixes the terminal settings in response.

    ``check_fg_bg()`` works *except* for when the process is stopped with
    ``SIGTSTP``.  We cannot rely on a periodic timer in this case, as it
    may not rrun before the process stops.  We therefore restore terminal
    settings in the ``SIGTSTP`` handler.

    Additional notes:

    * We mostly use polling here instead of a SIGARLM timer or a
      thread. This is to avoid the complexities of many interrupts, which
      seem to make system calls (like I/O) unreliable in older Python
      versions (2.6 and 2.7).  See these issues for details:

      1. https://www.python.org/dev/peps/pep-0475/
      2. https://bugs.python.org/issue8354

      There are essentially too many ways for asynchronous signals to go
      wrong if we also have to support older Python versions, so we opt
      not to use them.

    * ``SIGSTOP`` can stop a process (in the foreground or background),
      but it can't be caught. Because of this, we can't fix any terminal
      settings on ``SIGSTOP``, and the terminal will be left with
      ``ICANON`` and ``ECHO`` disabled until it is resumes execution.

    * Technically, a process *could* be sent ``SIGTSTP`` while running in
      the foreground, without the shell backgrounding that process. This
      doesn't happen in practice, and we assume that ``SIGTSTP`` always
      means that defaults should be restored.

    * We rely on ``termios`` support.  Without it, or if the stream isn't
      a TTY, ``keyboard_input`` has no effect.

    """

    def __init__(self, stdin: Optional[IO[str]]) -> None:
        """Create a context manager that will enable keyboard input on stream.

        Args:
            stdin: text io wrapper of stdin (keyboard input)

        Note that stdin can be None, in which case ``keyboard_input`` will do nothing.
        """
        super().__init__(stdin)

    def _is_background(self) -> bool:
        """True iff calling process is in the background."""
        assert self.stdin is not None, "stdin should be available"
        return _is_background_tty(self.stdin)

    def _get_canon_echo_flags(self) -> Tuple[bool, bool]:
        """Get current termios canonical and echo settings."""
        assert termios is not None and self.stdin is not None
        cfg = termios.tcgetattr(self.stdin)
        return (bool(cfg[3] & termios.ICANON), bool(cfg[3] & termios.ECHO))

    def _enable_keyboard_input(self) -> None:
        """Disable canonical input and echoing on ``self.stdin``."""
        # "enable" input by disabling canonical mode and echo
        assert termios is not None and self.stdin is not None
        new_cfg = termios.tcgetattr(self.stdin)
        new_cfg[3] &= ~termios.ICANON
        new_cfg[3] &= ~termios.ECHO

        # Apply new settings for terminal
        with ignore_signal(signal.SIGTTOU):
            termios.tcsetattr(self.stdin, termios.TCSANOW, new_cfg)

    def _tstp_handler(self, signum, frame):
        self._restore_default_terminal_settings()
        os.kill(os.getpid(), signal.SIGSTOP)

    def check_fg_bg(self) -> None:
        # old_cfg is set up in __enter__ and indicates that we have
        # termios and a valid stream.
        if not self.old_cfg:
            return

        # query terminal flags and fg/bg status
        flags = self._get_canon_echo_flags()
        bg = self._is_background()

        # restore sanity if flags are amiss -- see diagram in class docs
        if not bg and any(flags):  # fg, but input not enabled
            self._enable_keyboard_input()
        elif bg and not all(flags):  # bg, but input enabled
            self._restore_default_terminal_settings()

    def __enter__(self) -> "keyboard_input":
        """Enable immediate keypress input, while this process is foreground.

        If the stream is not a TTY or the system doesn't support termios,
        do nothing.
        """
        super().__enter__()
        self.old_handlers = {}

        # Ignore all this if the input stream is not a tty.
        if not self.stdin or not self.stdin.isatty():
            return self

        if termios:
            # Install a signal handler to disable/enable keyboard input
            # when the process moves between foreground and background.
            self.old_handlers[signal.SIGTSTP] = signal.signal(signal.SIGTSTP, self._tstp_handler)

            # enable keyboard input initially (if foreground)
            if not self._is_background():
                self._enable_keyboard_input()

        return self

    def __exit__(self, exc_type, exception, traceback):
        """If termios was available, restore old settings."""
        super().__exit__(exc_type, exception, traceback)

        # restore SIGSTP and SIGCONT handlers
        if self.old_handlers:
            for signum, old_handler in self.old_handlers.items():
                signal.signal(signum, old_handler)


class Unbuffered:
    """Wrapper for Python streams that forces them to be unbuffered.

    This is implemented by forcing a flush after each write.
    """

    def __init__(self, stream):
        self.stream = stream

    def write(self, data):
        self.stream.write(data)
        self.stream.flush()

    def writelines(self, datas):
        self.stream.writelines(datas)
        self.stream.flush()

    def __getattr__(self, attr):
        return getattr(self.stream, attr)


def log_output(*args, **kwargs):
    """Context manager that logs its output to a file.

    In the simplest case, the usage looks like this::

        with log_output('logfile.txt'):
            # do things ... output will be logged

    Any output from the with block will be redirected to ``logfile.txt``.
    If you also want the output to be echoed to ``stdout``, use the
    ``echo`` parameter::

        with log_output('logfile.txt', echo=True):
            # do things ... output will be logged and printed out

    The following is available on Unix only. No-op on Windows.
    And, if you just want to echo *some* stuff from the parent, use
    ``force_echo``::

        with log_output('logfile.txt', echo=False) as logger:
            # do things ... output will be logged

            with logger.force_echo():
                # things here will be echoed *and* logged

    See individual log classes for more information.


    This method is actually a factory serving a per platform
    (unix vs windows) log_output class
    """
    if sys.platform == "win32":
        return winlog(*args, **kwargs)
    else:
        return nixlog(*args, **kwargs)


class nixlog:
    """
    Under the hood, we spawn a daemon and set up a pipe between this
    process and the daemon.  The daemon writes our output to both the
    file and to stdout (if echoing).  The parent process can communicate
    with the daemon to tell it when and when not to echo; this is what
    force_echo does.  You can also enable/disable echoing by typing ``v``.

    We use OS-level file descriptors to do the redirection, which
    redirects output for subprocesses and system calls.
    """

    def __init__(
        self,
        filename: str,
        echo=False,
        debug=0,
        buffer=False,
        env=None,
        filter_fn=None,
        append=False,
    ):
        """Create a new output log context manager.

        Args:
            filename (str): path to file where output should be logged
            echo (bool): whether to echo output in addition to logging it
            debug (int): positive to enable tty debug mode during logging
            buffer (bool): pass buffer=True to skip unbuffering output; note
                this doesn't set up any *new* buffering
            filter_fn (callable, optional): Callable[str] -> str to filter each
                line of output
            append (bool): whether to append to file ('a' mode)

        The filename will be opened and closed entirely within ``__enter__``
        and ``__exit__``.

        By default, we unbuffer sys.stdout and sys.stderr because the
        logger will include output from executed programs and from python
        calls.  If stdout and stderr are buffered, their output won't be
        printed in the right place w.r.t. output from commands.

        Logger daemon is not started until ``__enter__()``.

        """
        self.filename = filename
        self.echo = echo
        self.debug = debug
        self.buffer = buffer
        self.filter_fn = filter_fn
        self.append = append

        self._active = False  # used to prevent re-entry

    def __enter__(self):
        if self._active:
            raise RuntimeError("Can't re-enter the same log_output!")

        # record parent color settings before redirecting.  We do this
        # because color output depends on whether the *original* stdout
        # is a TTY.  New stdout won't be a TTY so we force colorization.
        self._saved_color = tty.color._force_color
        forced_color = tty.color.get_color_when()

        # also record parent debug settings -- in case the logger is
        # forcing debug output.
        self._saved_debug = tty._debug

        # Pipe for redirecting output to logger
        read_fd, self.write_fd = multiprocessing.Pipe(duplex=False)

        # Pipe for communication back from the daemon
        # Currently only used to save echo value between uses
        self.parent_pipe, child_pipe = multiprocessing.Pipe(duplex=False)

        stdin_fd = None
        stdout_fd = None
        try:
            # need to pass this b/c multiprocessing closes stdin in child.
            try:
                if sys.stdin.isatty():
                    stdin_fd = Connection(os.dup(sys.stdin.fileno()))
            except BaseException:
                # just don't forward input if this fails
                pass

            # If our process has redirected stdout after the forkserver was started, we need to
            # make the forked processes use the new file descriptors.
            if multiprocessing.get_start_method() == "forkserver":
                stdout_fd = Connection(os.dup(sys.stdout.fileno()))

            self.process = multiprocessing.Process(
                target=_writer_daemon,
                args=(
                    stdin_fd,
                    stdout_fd,
                    read_fd,
                    self.write_fd,
                    self.echo,
                    self.filename,
                    self.append,
                    child_pipe,
                    self.filter_fn,
                ),
            )
            self.process.daemon = True  # must set before start()
            self.process.start()

        finally:
            if stdin_fd:
                stdin_fd.close()
            if stdout_fd:
                stdout_fd.close()
            read_fd.close()

        # Flush immediately before redirecting so that anything buffered
        # goes to the original stream
        sys.stdout.flush()
        sys.stderr.flush()

        # Now do the actual output redirection.
        # We use OS-level file descriptors, as this
        # redirects output for subprocesses and system calls.
        self._redirected_fds = {}

        # sys.stdout and sys.stderr may have been replaced with file objects under pytest, so
        # redirect their file descriptors in addition to the original fds 1 and 2.
        fds = {sys.stdout.fileno(), sys.stderr.fileno(), 1, 2}
        for fd in fds:
            self._redirected_fds[fd] = os.dup(fd)
            os.dup2(self.write_fd.fileno(), fd)

        self.write_fd.close()

        # Unbuffer stdout and stderr at the Python level
        if not self.buffer:
            sys.stdout = Unbuffered(sys.stdout)
            sys.stderr = Unbuffered(sys.stderr)

        # Force color and debug settings now that we have redirected.
        tty.color.set_color_when(forced_color)
        tty._debug = self.debug

        # track whether we're currently inside this log_output
        self._active = True

        # return this log_output object so that the user can do things
        # like temporarily echo some output.
        return self

    def __exit__(self, exc_type, exc_val, exc_tb):
        # Flush any buffered output to the logger daemon.
        sys.stdout.flush()
        sys.stderr.flush()

        # restore previous output settings using the OS-level way
        for fd, saved_fd in self._redirected_fds.items():
            os.dup2(saved_fd, fd)
            os.close(saved_fd)

        # recover and store echo settings from the child before it dies
        try:
            self.echo = self.parent_pipe.recv()
        except EOFError:
            # This may occur if some exception prematurely terminates the
            # _writer_daemon. An exception will have already been generated.
            pass

        # now that the write pipe is closed (in this __exit__, when we restore
        # stdout with dup2), the logger daemon process loop will terminate. We
        # wait for that here.
        self.process.join()

        # restore old color and debug settings
        tty.color._force_color = self._saved_color
        tty._debug = self._saved_debug

        self._active = False  # safe to enter again

    @contextmanager
    def force_echo(self):
        """Context manager to force local echo, even if echo is off."""
        if not self._active:
            raise RuntimeError("Can't call force_echo() outside log_output region!")

        # This uses the xon/xoff to highlight regions to be echoed in the
        # output. We us these control characters rather than, say, a
        # separate pipe, because they're in-band and assured to appear
        # exactly before and after the text we want to echo.
        sys.stdout.write(xon)
        sys.stdout.flush()
        try:
            yield
        finally:
            sys.stdout.write(xoff)
            sys.stdout.flush()


class StreamWrapper:
    """Wrapper class to handle redirection of io streams"""

    def __init__(self, sys_attr):
        self.sys_attr = sys_attr
        self.saved_stream = None
        if sys.platform.startswith("win32"):
            if hasattr(sys, "gettotalrefcount"):  # debug build
                libc = ctypes.CDLL("ucrtbased")
            else:
                libc = ctypes.CDLL("api-ms-win-crt-stdio-l1-1-0")

            kernel32 = ctypes.WinDLL("kernel32")

            # https://docs.microsoft.com/en-us/windows/console/getstdhandle
            if self.sys_attr == "stdout":
                STD_HANDLE = -11
            elif self.sys_attr == "stderr":
                STD_HANDLE = -12
            else:
                raise KeyError(self.sys_attr)

            c_stdout = kernel32.GetStdHandle(STD_HANDLE)
            self.libc = libc
            self.c_stream = c_stdout
        else:
            self.libc = ctypes.CDLL(None)
            self.c_stream = ctypes.c_void_p.in_dll(self.libc, self.sys_attr)
        self.sys_stream = getattr(sys, self.sys_attr)
        self.orig_stream_fd = self.sys_stream.fileno()
        # Save a copy of the original stdout fd in saved_stream
        self.saved_stream = os.dup(self.orig_stream_fd)

    def redirect_stream(self, to_fd):
        """Redirect stdout to the given file descriptor."""
        # Flush the C-level buffer stream
        if sys.platform.startswith("win32"):
            self.libc.fflush(None)
        else:
            self.libc.fflush(self.c_stream)
        # Flush and close sys_stream - also closes the file descriptor (fd)
        sys_stream = getattr(sys, self.sys_attr)
        sys_stream.flush()
        sys_stream.close()
        # Make orig_stream_fd point to the same file as to_fd
        os.dup2(to_fd, self.orig_stream_fd)
        # Set sys_stream to a new stream that points to the redirected fd
        new_buffer = open(self.orig_stream_fd, "wb")
        new_stream = io.TextIOWrapper(new_buffer)
        setattr(sys, self.sys_attr, new_stream)
        self.sys_stream = getattr(sys, self.sys_attr)

    def flush(self):
        if sys.platform.startswith("win32"):
            self.libc.fflush(None)
        else:
            self.libc.fflush(self.c_stream)
        self.sys_stream.flush()

    def close(self):
        """Redirect back to the original system stream, and close stream"""
        try:
            if self.saved_stream is not None:
                self.redirect_stream(self.saved_stream)
        finally:
            if self.saved_stream is not None:
                os.close(self.saved_stream)


class winlog:
    """
    Similar to nixlog, with underlying
    functionality ported to support Windows.

    Does not support the use of ``v`` toggling as nixlog does.
    """

    def __init__(
        self, filename: str, echo=False, debug=0, buffer=False, filter_fn=None, append=False
    ):
        self.debug = debug
        self.echo = echo
        self.logfile = filename
        self.stdout = StreamWrapper("stdout")
        self.stderr = StreamWrapper("stderr")
        self._active = False
        self.old_stdout = sys.stdout
        self.old_stderr = sys.stderr
        self.append = append

    def __enter__(self):
        if self._active:
            raise RuntimeError("Can't re-enter the same log_output!")

        # Open both write and reading on logfile
        write_mode = "ab+" if self.append else "wb+"
        self.writer = open(self.logfile, mode=write_mode)
        self.reader = open(self.logfile, mode="rb+")

        # Dup stdout so we can still write to it after redirection
        self.echo_writer = open(os.dup(sys.stdout.fileno()), "w", encoding=sys.stdout.encoding)
        # Redirect stdout and stderr to write to logfile
        self.stderr.redirect_stream(self.writer.fileno())
        self.stdout.redirect_stream(self.writer.fileno())
        self._kill = threading.Event()

        def background_reader(reader, echo_writer, _kill):
            # for each line printed to logfile, read it
            # if echo: write line to user
            try:
                while True:
                    is_killed = _kill.wait(0.1)
                    # Flush buffered build output to file
                    # stdout/err fds refer to log file
                    self.stderr.flush()
                    self.stdout.flush()

                    line = reader.readline()
                    if self.echo and line:
                        echo_writer.write("{0}".format(line.decode()))
                        echo_writer.flush()

                    if is_killed:
                        break
            finally:
                reader.close()

        self._active = True
        self._thread = Thread(
            target=background_reader, args=(self.reader, self.echo_writer, self._kill)
        )
        self._thread.start()
        return self

    def __exit__(self, exc_type, exc_val, exc_tb):
        self.writer.close()
        self.echo_writer.flush()
        self.stdout.flush()
        self.stderr.flush()
        self._kill.set()
        self._thread.join()
        self.stdout.close()
        self.stderr.close()
        self._active = False

    @contextmanager
    def force_echo(self):
        """Context manager to force local echo, even if echo is off."""
        if not self._active:
            raise RuntimeError("Can't call force_echo() outside log_output region!")
        yield


def _writer_daemon(
    stdin_fd: Optional[Connection],
    stdout_fd: Optional[Connection],
    read_fd: Connection,
    write_fd: Connection,
    echo: bool,
    log_filename: str,
    append: bool,
    control_fd: Connection,
    filter_fn: Optional[Callable[[str], str]],
) -> None:
    """Daemon used by ``log_output`` to write to a log file and to ``stdout``.

    The daemon receives output from the parent process and writes it both
    to a log and, optionally, to ``stdout``.  The relationship looks like
    this::

        Terminal
           |
           |          +-------------------------+
           |          | Parent Process          |
           +--------> |   with log_output():    |
           | stdin    |     ...                 |
           |          +-------------------------+
           |            ^             | write_fd (parent's redirected stdout)
           |            | control     |
           |            | pipe        |
           |            |             v read_fd
           |          +-------------------------+   stdout
           |          | Writer daemon           |------------>
           +--------> |   read from read_fd     |   log_file
             stdin    |   write to out and log  |------------>
                      +-------------------------+

    Within the ``log_output`` handler, the parent's output is redirected
    to a pipe from which the daemon reads.  The daemon writes each line
    from the pipe to a log file and (optionally) to ``stdout``.  The user
    can hit ``v`` to toggle output on ``stdout``.

    In addition to the input and output file descriptors, the daemon
    interacts with the parent via ``control_pipe``.  It reports whether
    ``stdout`` was enabled or disabled when it finished.

    Arguments:
        stdin_fd: optional input from the terminal
        read_fd: pipe for reading from parent's redirected stdout
        echo: initial echo setting -- controlled by user and preserved across multiple writer
            daemons
        log_filename: filename where output should be logged
        append: whether to append to the file or overwrite it
        control_pipe: multiprocessing pipe on which to send control information to the parent
        filter_fn: optional function to filter each line of output

    """
    # This process depends on closing all instances of write_pipe to terminate the reading loop
    write_fd.close()

    # 1. Use line buffering (3rd param = 1) since Python 3 has a bug
    #    that prevents unbuffered text I/O. [needs citation]
    # 2. Enforce a UTF-8 interpretation of build process output with errors replaced by '?'.
    #    The downside is that the log file will not contain the exact output of the build process.
    # 3. closefd=False because Connection has "ownership"
    read_file = os.fdopen(
        read_fd.fileno(), "r", 1, encoding="utf-8", errors="replace", closefd=False
    )

    if stdin_fd:
        stdin_file = os.fdopen(stdin_fd.fileno(), closefd=False)
    else:
        stdin_file = None

    if stdout_fd:
        os.dup2(stdout_fd.fileno(), sys.stdout.fileno())
        stdout_fd.close()

    # list of streams to select from
    istreams = [read_file, stdin_file] if stdin_file else [read_file]
    force_echo = False  # parent can force echo for certain output
    log_file = open(log_filename, mode="a" if append else "w", encoding="utf-8")

    try:
        with keyboard_input(stdin_file) as kb:
            while True:
                # fix the terminal settings if we recently came to
                # the foreground
                kb.check_fg_bg()

                # wait for input from any stream. use a coarse timeout to
                # allow other checks while we wait for input
                rlist, _, _ = select.select(istreams, [], [], 0.1)

                # Allow user to toggle echo with 'v' key.
                # Currently ignores other chars.
                # only read stdin if we're in the foreground
                if stdin_file and stdin_file in rlist and not _is_background_tty(stdin_file):
                    # it's possible to be backgrounded between the above
                    # check and the read, so we ignore SIGTTIN here.
                    with ignore_signal(signal.SIGTTIN):
                        try:
                            if stdin_file.read(1) == "v":
                                echo = not echo
                        except OSError as e:
                            # If SIGTTIN is ignored, the system gives EIO
                            # to let the caller know the read failed b/c it
                            # was in the bg. Ignore that too.
                            if e.errno != errno.EIO:
                                raise

                if read_file in rlist:
                    line_count = 0
                    try:
                        while line_count < 100:
                            # Handle output from the calling process.
                            line = read_file.readline()

                            if not line:
                                return
                            line_count += 1

                            # find control characters and strip them.
                            clean_line, num_controls = control.subn("", line)

                            # Echo to stdout if requested or forced.
                            if echo or force_echo:
                                output_line = clean_line
                                if filter_fn:
                                    output_line = filter_fn(clean_line)
                                enc = sys.stdout.encoding
                                if enc != "utf-8":
                                    # On Python 3.6 and 3.7-3.14 with non-{utf-8,C} locale stdout
                                    # may not be able to handle utf-8 output. We do an inefficient
                                    # dance of re-encoding with errors replaced, so stdout.write
                                    # does not raise.
                                    output_line = output_line.encode(enc, "replace").decode(enc)
                                sys.stdout.write(output_line)

                            # Stripped output to log file.
                            log_file.write(_strip(clean_line))

                            if num_controls > 0:
                                controls = control.findall(line)
                                if xon in controls:
                                    force_echo = True
                                if xoff in controls:
                                    force_echo = False

                            if not _input_available(read_file):
                                break
                    finally:
                        if line_count > 0:
                            if echo or force_echo:
                                sys.stdout.flush()
                            log_file.flush()

    except BaseException:
        tty.error("Exception occurred in writer daemon!")
        traceback.print_exc()

    finally:
        log_file.close()
        read_fd.close()
        if stdin_fd:
            stdin_fd.close()

        # send echo value back to the parent so it can be preserved.
        control_fd.send(echo)


def _input_available(f):
    return f in select.select([f], [], [], 0)[0]
