// Package uevent provides userspace client for consuming events from a
// NETLINK_KOBJECT_UEVENT socket, as well as helpers for supplementing
// events received from the kernel.
package uevent

import (
	"context"
	"errors"
	"strconv"
	"sync/atomic"
	"syscall"

	"hakurei.app/internal/netlink"
)

type (
	// Recoverable is satisfied by errors that are safe to recover from.
	Recoverable interface{ recoverable() }
	// Nontrivial is satisfied by errors preferring a JSON encoding.
	Nontrivial interface{ nontrivial() }

	// NeedsColdboot is satisfied by errors indicating divergence of local state
	// from the kernel, usually from lost uevent data.
	NeedsColdboot interface {
		Recoverable
		coldboot()
	}
)

const (
	exclConsume = iota

	_exclLen
)

// Conn represents a NETLINK_KOBJECT_UEVENT socket.
type Conn struct {
	conn *netlink.Conn

	// Whether currently between a call to enterExcl and exitExcl.
	excl [_exclLen]atomic.Bool
}

// enterExcl must be called entering a critical section that interacts with conn.
func (c *Conn) enterExcl(k int) error {
	if !c.excl[k].CompareAndSwap(false, true) {
		return syscall.EAGAIN
	}
	return nil
}

// exitExcl must be called exiting a critical section that interacts with conn.
func (c *Conn) exitExcl(k int) { c.excl[k].Store(false) }

// Close closes the underlying socket.
func (c *Conn) Close() error { return c.conn.Close() }

// Dial returns the address of a newly connected [Conn].
func Dial(rcvbuf int64) (*Conn, error) {
	// kernel group is hard coded in lib/kobject_uevent.c, undocumented
	c, err := netlink.Dial(syscall.NETLINK_KOBJECT_UEVENT, 1, rcvbuf)
	if err != nil {
		return nil, err
	}
	return &Conn{conn: c}, err
}

var (
	// ErrBadSocket is returned by [Conn.Consume] for a reply from a
	// syscall.Sockaddr with unexpected concrete type.
	ErrBadSocket = errors.New("unexpected socket address")
)

// ReceiveBufferError indicates one or more [Message] being lost due to the
// socket receive buffer filling up. This is usually caused by epoll waking the
// receiving program up too late.
type ReceiveBufferError struct{ _ [0]*ReceiveBufferError }

var _ NeedsColdboot = ReceiveBufferError{}

func (ReceiveBufferError) recoverable()    {}
func (ReceiveBufferError) coldboot()       {}
func (ReceiveBufferError) Unwrap() error   { return syscall.ENOBUFS }
func (e ReceiveBufferError) Error() string { return syscall.ENOBUFS.Error() }

// BadPortError is returned by [Conn.Consume] upon receiving a message that did
// not come from the kernel.
type BadPortError syscall.SockaddrNetlink

var _ Recoverable = new(BadPortError)

func (*BadPortError) recoverable() {}
func (e *BadPortError) Error() string {
	return "unexpected message from port id " + strconv.Itoa(int(e.Pid)) +
		" on NETLINK_KOBJECT_UEVENT"
}

// receiveEvent receives a single event and returns the address of its [Message].
func (c *Conn) receiveEvent(ctx context.Context) (*Message, error) {
	data, _, from, err := c.conn.Recvmsg(ctx, 0)
	if err != nil {
		if errors.Is(err, syscall.ENOBUFS) {
			return nil, ReceiveBufferError{}
		}
		return nil, err
	}

	// lib/kobject_uevent.c:
	// set portid 0 to inform userspace message comes from kernel
	if v, ok := from.(*syscall.SockaddrNetlink); !ok {
		return nil, ErrBadSocket
	} else if v.Pid != 0 {
		return nil, (*BadPortError)(v)

	}

	var msg Message
	if err = msg.UnmarshalBinary(data); err != nil {
		return nil, err
	}
	return &msg, err
}

// Consume continuously receives and parses events from the kernel and handles
// [Recoverable] and [NeedsColdboot] errors via caller-supplied functions,
// entering coldboot when required.
//
// For each uevent file visited by [Coldboot], handleColdbootVisited is called
// with its pathname. This function must never block.
//
// When consuming events, a non-nil error not satisfying [Recoverable] is
// returned immediately. Otherwise, handleConsumeErr is called with the error
// value. If the error satisfies [NeedsColdboot], a [Coldboot] is arranged
// before event processing resumes. If handleConsumeErr returns false, the error
// value is immediately returned as is.
//
// Callers are expected to reject excessively frequent [NeedsColdboot] errors
// in handleConsumeErr to avoid being stuck in a [Coldboot] loop. Event
// processing is allowed to restart without initial coldboot after recovering
// from such a condition, provided the caller adequately reports the degraded,
// diverging state to the user.
//
// Callers must not restart event processing after a non-nil error that does not
// satisfy [Recoverable] is returned.
func (c *Conn) Consume(
	ctx context.Context,
	sysfs string,
	events chan<- *Message,
	coldboot bool,

	handleColdbootVisited func(string),
	handleConsumeErr func(error) bool,
	handleWalkErr func(error) error,
) error {
	if err := c.enterExcl(exclConsume); err != nil {
		return err
	}
	defer c.exitExcl(exclConsume)

	filterErr := func(err error) (error, bool) {
		if _, ok := err.(Recoverable); !ok {
			return err, true
		}

		// avoids dropping pending coldboot
		if _, ok := err.(NeedsColdboot); ok {
			coldboot = true
		}

		return err, !handleConsumeErr(err)
	}

retry:
	if coldboot {
		goto coldboot
	}
	for {
		msg, err := c.receiveEvent(ctx)
		if err == nil {
			events <- msg
			continue
		}

		if _, ok := filterErr(err); ok {
			return err
		}
	}

coldboot:
	coldboot = false

	visited := make(chan string)
	ctxColdboot, cancelColdboot := context.WithCancel(ctx)
	var coldbootErr error
	go func() {
		coldbootErr = Coldboot(ctxColdboot, sysfs, visited, handleWalkErr)
		close(visited)
	}()
	for pathname := range visited {
		handleColdbootVisited(pathname)

		for {
			msg, err := c.receiveEvent(nil)
			if err == nil {
				events <- msg
				continue
			}
			if errors.Is(err, syscall.EWOULDBLOCK) {
				break
			}
			if filteredErr, ok := filterErr(err); ok {
				cancelColdboot()
				return filteredErr
			}
		}
	}
	cancelColdboot()
	if coldbootErr != nil {
		return coldbootErr
	}
	goto retry
}