// Package namespace provides primitives for managing Linux user and network
// namespaces, including bootstrapping and pinning.
//
// Rootless Bootstrap Loop & Host-Socket Preservation:
// To achieve rootless network isolation without interfering with the Go runtime's multi-threaded
// scheduler, and to maintain encrypted UDP socket connectivity over the host's network,
// wg-wrap employs an advanced bootstrap loop:
//
//  1. Host-Bound Socket Creation: During the initial host-level start, a UDP socket is opened
//     on 0.0.0.0:0 on the host, and its FD is stored in the environment (WG_WRAP_HOST_SOCKET_FD).
//  2. Helper Deployment: An embedded single-threaded C launcher is used to bridge the transition.
//  3. Namespace Transition: The process replaces itself with the C launcher via syscall.Exec.
//  4. Isolation: The launcher performs the unshare(CLONE_NEWUSER | CLONE_NEWNS | CLONE_NEWNET)
//     sequence to isolate Mount, User, and Network environments.
//  5. Re-entry: The launcher then execvp's the original wg-wrap binary.
//  6. FDBind Tunnel Initialization: The second instance of wg-wrap wraps the host socket FD
//     inside a custom FDBind struct to initialize wireguard-go.
//
// User Namespace Sequence:
// To create a network namespace without root, wg-wrap follows the sequence:
// CLONE_NEWUSER -> CLONE_NEWNET -> Setuid/Setgid -> Configure Interfaces.
package namespace

import (
	_ "embed"
	"fmt"
	"net"
	"os"
	"syscall"

	"git.theodohertyfamily.com/wg-wrap/internal/network"
	"golang.org/x/sys/unix"
)

// MountOps abstracts the filesystem mount operations.
type MountOps interface {
	Mount(source, target, fstype string, flags uintptr, data string) error
	Unmount(target string, flags int) error
}

// realMountOps is the production implementation using unix.Mount.
type realMountOps struct{}

func (r *realMountOps) Mount(source, target, fstype string, flags uintptr, data string) error {
	return unix.Mount(source, target, fstype, flags, data)
}
func (r *realMountOps) Unmount(target string, flags int) error {
	return unix.Unmount(target, flags)
}

// DefaultMountOps is the global instance used by the package functions.
var DefaultMountOps MountOps = &realMountOps{}

// FileSystem abstracts the basic filesystem operations used for isolation.
type FileSystem interface {
	Stat(name string) (os.FileInfo, error)
	MkdirAll(path string, perm os.FileMode) error
	CreateTemp(dir, pattern string) (*os.File, error)
	MkdirTemp(dir, pattern string) (string, error)
	Remove(name string) error
}

// realFS is the production implementation using the os package.
type realFS struct{}

func (r *realFS) Stat(name string) (os.FileInfo, error) { return os.Stat(name) }
func (r *realFS) MkdirAll(path string, perm os.FileMode) error {
	return os.MkdirAll(path, perm)
}
func (r *realFS) CreateTemp(dir, pattern string) (*os.File, error) {
	return os.CreateTemp(dir, pattern)
}
func (r *realFS) MkdirTemp(dir, pattern string) (string, error) {
	return os.MkdirTemp(dir, pattern)
}
func (r *realFS) Remove(name string) error { return os.Remove(name) }

// DefaultFS is the global instance used by the package functions.
var DefaultFS FileSystem = &realFS{}

// ResetDefaults restores the default implementations of MountOps and FileSystem.
func ResetDefaults() {
	DefaultMountOps = &realMountOps{}
	DefaultFS = &realFS{}
}

// BootstrapConfig contains the environment and arguments needed to execute the bootstrap launcher.
type BootstrapConfig struct {
	Args     []string
	Env      []string
	Fds      []int
	ExecPath string
}

//go:embed launcher.bin
var launcherBytes []byte

// IsIsolated checks if the current process is running as root in a new network namespace.
func IsIsolated() bool {
	return os.Getuid() == 0
}

// VerifyIsolation performs a set of sanity checks to ensure the process is
// actually isolated in a new network namespace and has the correct identity.
func VerifyIsolation() (bool, string) {
	// 1. Check UID
	if os.Getuid() != 0 {
		return false, fmt.Sprintf("Expected UID 0, got %d", os.Getuid())
	}

	// 2. Check Network Isolation
	// We expect a fresh network namespace to have only the loopback interface.
	interfaces, err := network.ListInterfaces()
	if err != nil {
		return false, fmt.Sprintf("failed to list interfaces: %v", err)
	}

	// In a fresh netns, we typically only see 'lo'.
	// If we see more than just loopback, or loopback is missing, it might not be isolated.
	if len(interfaces) == 0 {
		return false, "no network interfaces found"
	}

	hasLo := false
	for _, iface := range interfaces {
		if iface.Name == "lo" {
			hasLo = true
		} else {
			// If we find any other interface (eth0, wlan0, etc.), we aren't isolated.
			return false, fmt.Sprintf("detected non-isolated interface: %s", iface.Name)
		}
	}

	if !hasLo {
		return false, "loopback interface missing"
	}

	// 3. Check Filesystem Transparency
	home := os.Getenv("HOME")
	if home != "" {
		if _, err := os.ReadDir(home); err != nil {
			return false, fmt.Sprintf("cannot read home directory: %v", err)
		}
	}

	return true, "Isolated and root"
}

// VerifyArguments prints the current process arguments as hex-encoded strings.
// This is used for E2E testing to verify that the data path is 8-bit clean
// and that no bytes are mutated during the bootstrap loop.
func VerifyArguments(args []string) error {
	for i, arg := range args {
		fmt.Printf("%d:%x\n", i, arg)
	}
	return nil
}

// Bootstrap ensures the process is running in an isolated user and network namespace.
// It uses memfd_create to run the embedded C launcher from memory, bypassing
// disk-based noexec restrictions.
func Bootstrap() (err error) {
	if IsIsolated() {
		return nil
	}

	config, err := PrepareBootstrap()
	if err != nil {
		return err
	}

	defer func() {
		for _, fd := range config.Fds {
			_ = syscall.Close(fd)
		}
	}()

	err = syscall.Exec(config.ExecPath, config.Args, config.Env)
	if err != nil {
		return fmt.Errorf("launcher exec failed: %w", err)
	}

	return nil
}

// PrepareBootstrap calculates the environment and arguments needed for the bootstrap launcher.
func PrepareBootstrap() (*BootstrapConfig, error) {
	// 0. Validate current arguments for null bytes before proceeding.
	for i, arg := range os.Args {
		for j := 0; j < len(arg); j++ {
			if arg[j] == 0 {
				return nil, fmt.Errorf("argument %d contains null byte at position %d", i, j)
			}
		}
	}

	self, err := os.Executable()
	if err != nil {
		return nil, fmt.Errorf("failed to get executable path: %w", err)
	}

	execFd, err := prepareLauncher()
	if err != nil {
		return nil, err
	}

	// Clear close-on-exec
	if flags, err := unix.FcntlInt(uintptr(execFd), unix.F_GETFD, 0); err == nil {
		_, _ = unix.FcntlInt(uintptr(execFd), unix.F_SETFD, flags&^unix.FD_CLOEXEC)
	}

	// Prepare arguments for the launcher.
	args := []string{self}
	args = append(args, os.Args[1:]...)

	for i, arg := range args {
		for j := 0; j < len(arg); j++ {
			if arg[j] == 0 {
				return nil, fmt.Errorf("launcher argument %d contains null byte at position %d", i, j)
			}
		}
	}

	// Open the host network namespace file descriptor before unsharing.
	hostNetFd, err := syscall.Open("/proc/self/ns/net", syscall.O_RDONLY, 0)
	if err != nil {
		return nil, fmt.Errorf("failed to open host netns: %w", err)
	}

	// Clear close-on-exec
	if flags, err := unix.FcntlInt(uintptr(hostNetFd), unix.F_GETFD, 0); err == nil {
		_, _ = unix.FcntlInt(uintptr(hostNetFd), unix.F_SETFD, flags&^unix.FD_CLOEXEC)
	}

	env := append(os.Environ(), fmt.Sprintf("WG_WRAP_HOST_NETNS_FD=%d", hostNetFd))

	// Open a host UDP socket on 0.0.0.0:0 before unsharing network namespace.
	laddr, errAddr := net.ResolveUDPAddr("udp", "0.0.0.0:0")
	if errAddr == nil {
		if conn, errConn := net.ListenUDP("udp", laddr); errConn == nil {
			if file, errFile := conn.File(); errFile == nil {
				hostSocketFd := file.Fd()
				if flags, fcntlErr := unix.FcntlInt(hostSocketFd, unix.F_GETFD, 0); fcntlErr == nil {
					_, _ = unix.FcntlInt(hostSocketFd, unix.F_SETFD, flags&^unix.FD_CLOEXEC)
				}
				env = append(env, fmt.Sprintf("WG_WRAP_HOST_SOCKET_FD=%d", hostSocketFd))
				_ = conn.Close()
			}
		}
	}

	return &BootstrapConfig{
		Args:     args,
		Env:      env,
		Fds:      []int{execFd, hostNetFd},
		ExecPath: fmt.Sprintf("/proc/self/fd/%d", execFd),
	}, nil
}

// BootstrapJoin joins the namespaces of the target PID and replaces the current process.
func BootstrapJoin(targetPid int) (err error) {
	if IsIsolated() {
		return nil
	}

	config, err := PrepareBootstrapJoin(targetPid)
	if err != nil {
		return err
	}

	defer func() {
		for _, fd := range config.Fds {
			_ = syscall.Close(fd)
		}
	}()

	err = syscall.Exec(config.ExecPath, config.Args, config.Env)
	if err != nil {
		return fmt.Errorf("launcher exec failed: %w", err)
	}

	return nil
}

// PrepareBootstrapJoin calculates the environment and arguments needed to join a namespace.
func PrepareBootstrapJoin(targetPid int) (*BootstrapConfig, error) {
	for i, arg := range os.Args {
		for j := 0; j < len(arg); j++ {
			if arg[j] == 0 {
				return nil, fmt.Errorf("argument %d contains null byte at position %d", i, j)
			}
		}
	}

	self, err := os.Executable()
	if err != nil {
		return nil, fmt.Errorf("failed to get executable path: %w", err)
	}

	execFd, err := prepareLauncher()
	if err != nil {
		return nil, err
	}

	if flags, err := unix.FcntlInt(uintptr(execFd), unix.F_GETFD, 0); err == nil {
		_, _ = unix.FcntlInt(uintptr(execFd), unix.F_SETFD, flags&^unix.FD_CLOEXEC)
	}

	args := []string{self}
	args = append(args, os.Args[1:]...)

	for i, arg := range args {
		for j := 0; j < len(arg); j++ {
			if arg[j] == 0 {
				return nil, fmt.Errorf("launcher argument %d contains null byte at position %d", i, j)
			}
		}
	}

	env := append(os.Environ(),
		fmt.Sprintf("WG_WRAP_JOIN_PID=%d", targetPid),
		"WG_WRAP_JOINED=1",
	)

	return &BootstrapConfig{
		Args:     args,
		Env:      env,
		Fds:      []int{execFd},
		ExecPath: fmt.Sprintf("/proc/self/fd/%d", execFd),
	}, nil
}

func prepareLauncher() (int, error) {
	// Use memfd_create to create an anonymous file in memory.
	// This bypasses the need for a temporary disk file and avoids noexec restrictions.
	fd, err := unix.MemfdCreate("wg-wrap-launcher", 0)
	if err != nil {
		return 0, fmt.Errorf("failed to create memfd: %w", err)
	}

	if _, err = unix.Write(fd, launcherBytes); err != nil {
		_ = unix.Close(fd)
		return 0, fmt.Errorf("failed to write launcher binary to memfd: %w", err)
	}

	return fd, nil
}