"""Host port forwards for the QEMU user-mode network, read from a .forwards file.

Each line of the file is "LABEL HOST_PORT GUEST_PORT"; "#" starts a comment.
A QEMU config file section cannot hold more than one hostfwd entry, so the
launchers build the -netdev argument from these rules instead.
"""

import socket
import sys
from collections import namedtuple

Forward = namedtuple("Forward", "label host_port guest_port")


def _die(message):
    print(f"Error: {message}", file=sys.stderr)
    sys.exit(1)


def _port(text, where):
    if not (text.isascii() and text.isdigit() and 1 <= int(text) <= 65535):
        _die(f"{where}: not a port number from 1 to 65535: {text}")
    return int(text)


def port_in_use(port):
    """True if a host TCP port cannot be bound, as QEMU does for a hostfwd rule."""
    with socket.socket(socket.AF_INET, socket.SOCK_STREAM) as probe:
        probe.setsockopt(socket.SOL_SOCKET, socket.SO_REUSEADDR, 1)
        try:
            probe.bind(("", int(port)))
        except OSError:
            return True
    return False


def load_forwards(path, host_port_overrides=None):
    """Read the rules in a .forwards file.

    host_port_overrides maps a label to a host port that replaces the one in
    the file, e.g. {"SSH": "2200"}.
    """
    if not path.is_file():
        _die(f"port forward file was not found: {path}")
    forwards = []
    for number, line in enumerate(path.read_text().splitlines(), 1):
        fields = line.partition("#")[0].split()
        if not fields:
            continue
        where = f"{path}:{number}"
        if len(fields) != 3:
            _die(f"{where}: expected LABEL HOST_PORT GUEST_PORT: {line.strip()}")
        label, host_port, guest_port = fields
        if any(forward.label == label for forward in forwards):
            _die(f"{where}: duplicate label: {label}")
        forwards.append(Forward(label, _port(host_port, where), _port(guest_port, where)))
    for label, host_port in (host_port_overrides or {}).items():
        if not any(forward.label == label for forward in forwards):
            _die(f"{path} has no {label} rule to override the port of")
        forwards = [forward._replace(host_port=_port(str(host_port), f"{label} port"))
                    if forward.label == label else forward for forward in forwards]
    seen = set()
    for forward in forwards:
        if forward.host_port in seen:
            _die(f"{path}: host port {forward.host_port} is used by more than one rule")
        seen.add(forward.host_port)
    return forwards


def usable_forwards(forwards):
    """Drop (with a warning) the rules whose host port is taken.

    QEMU refuses to start when a forwarded host port is taken.
    """
    usable = []
    for forward in forwards:
        if port_in_use(forward.host_port):
            print(f"Warning: host port {forward.host_port} is already in use; "
                  f"{forward.label} will not be forwarded to the guest.", file=sys.stderr)
        else:
            usable.append(forward)
    return usable


def netdev_argument(forwards, netdev_id="net0"):
    """The value of -netdev for a user-mode network with the given rules."""
    return ",".join([f"user,id={netdev_id}", *(
        f"hostfwd=tcp::{forward.host_port}-:{forward.guest_port}" for forward in forwards)])


def print_forwards(forwards):
    for forward in forwards:
        print(f"{forward.label}: localhost:{forward.host_port} -> guest port "
              f"{forward.guest_port}", flush=True)
        if forward.guest_port == 22:
            print(f"  ssh -p {forward.host_port} guofang@localhost", flush=True)