// Package remotelog sends the lines smallwebwaf writes on stdout to the
// remote log endpoint, SWWAF_LOG_REMOTE_URL, as the "Request log" section
// of SPEC.md describes: each line as the message of an RFC 5424 syslog
// record, over UDP, TCP or TLS. Lines wait in a bounded buffer, so a slow
// or unreachable endpoint never holds up a request or stdout.
package remotelog

import (
	"bytes"
	"context"
	"crypto/tls"
	"crypto/x509"
	"errors"
	"fmt"
	"log/slog"
	"net"
	"net/url"
	"os"
	"strconv"
	"sync/atomic"
	"syscall"
	"time"

	"sneak.berlin/go/smallwebwaf/internal/requestlog"
)

// The forms of SWWAF_LOG_REMOTE_URL, by its scheme.
const (
	SchemeUDP = "syslog+udp"
	SchemeTCP = "syslog+tcp"
	SchemeTLS = "syslog+tls"
)

// A record's priority is the number of its facility times the number of
// severities there are, plus the number of its severity. Every record's
// severity is informational.
const (
	severities    = 8
	informational = 6
)

const (
	// dialTimeout bounds connecting to the endpoint, the TLS handshake
	// included.
	dialTimeout = 10 * time.Second
	// After a failed attempt to connect, or a connection on which a record
	// fails, the next attempt to connect is made a second later, and
	// retryDelayFactor times as long after each further failure in a row,
	// up to a minute. A connection that fails after it has stayed up for
	// resetRetryDelayAfter ends the row.
	firstRetryDelay      = time.Second
	retryDelayFactor     = 2
	maxRetryDelay        = time.Minute
	resetRetryDelayAfter = time.Minute
)

// Params are what New needs.
type Params struct {
	// URL is the endpoint (SWWAF_LOG_REMOTE_URL): SchemeUDP, SchemeTCP or
	// SchemeTLS, a host and a port.
	URL *url.URL
	// RootCAs are the certificates a SchemeTLS endpoint's certificate
	// must chain to (SWWAF_LOG_REMOTE_TLS_CA_FILE), nil for the host's.
	RootCAs *x509.CertPool
	// Buffer is the most lines held while they wait to be sent
	// (SWWAF_LOG_REMOTE_BUFFER).
	Buffer int
	// Facility is the number of the records' syslog facility
	// (SWWAF_LOG_REMOTE_FACILITY), and AppName their APP-NAME
	// (SWWAF_LOG_REMOTE_APP_NAME).
	Facility int
	AppName  string
}

// Sender sends lines to the endpoint. Write puts them in its buffer, and
// Run sends them from there.
type Sender struct {
	url       *url.URL
	tlsConfig *tls.Config
	// beforeTime and afterTime are the parts of every record's header
	// before and after its time, as RFC 5424 lays the header out.
	beforeTime string
	afterTime  string
	// records is the buffer: each line's record, framed to be sent.
	records chan []byte
	sent    atomic.Int64
	dropped atomic.Int64
}

// New returns a Sender for the endpoint params.URL.
func New(params Params) *Sender {
	hostname, err := os.Hostname()
	if err != nil || hostname == "" {
		hostname = "-" // RFC 5424's value for a field that has none
	}

	priority := params.Facility*severities + informational

	return &Sender{
		url: params.URL,
		tlsConfig: &tls.Config{
			RootCAs:    params.RootCAs,
			MinVersion: tls.VersionTLS12,
		},
		// The 1 is the version of the format. The process id, the message
		// id and the structured data have no value.
		beforeTime: "<" + strconv.Itoa(priority) + ">1 ",
		afterTime:  " " + hostname + " " + params.AppName + " - - - ",
		records:    make(chan []byte, params.Buffer),
	}
}

// Write puts each line in p in the buffer, as the message of a record of
// its own, and never waits: when the buffer is full, the oldest record in
// it is dropped to make room. It is safe for concurrent use.
func (s *Sender) Write(p []byte) (int, error) {
	at := requestlog.FormatTime(time.Now())

	for line := range bytes.Lines(p) {
		line = bytes.TrimSuffix(line, []byte("\n"))
		if len(line) > 0 {
			s.put(s.record(at, line))
		}
	}

	return len(p), nil
}

// Sent is how many records have been sent.
func (s *Sender) Sent() int64 {
	return s.sent.Load()
}

// Dropped is how many records were dropped: the oldest in a full buffer,
// and those whose sending failed.
func (s *Sender) Dropped() int64 {
	return s.dropped.Load()
}

// Depth is how many records are in the buffer.
func (s *Sender) Depth() int {
	return len(s.records)
}

// Run connects to the endpoint and sends each record as it comes into the
// buffer, until ctx is done. Then it sends the records still in the buffer,
// on the connection open at that time or, if there is none, on a new one,
// until none is left or one fails, and returns. How long it may take over
// that is for the caller to bound.
//
// A connection on which a record fails is closed and the record dropped.
// That failure, like a failed attempt to connect, is logged to processLog
// and followed by the next attempt after firstRetryDelay, retryDelayFactor
// times as long after each further failure in a row up to maxRetryDelay,
// and firstRetryDelay again after a connection that stayed up for
// resetRetryDelayAfter. Meanwhile the records wait in the buffer.
func (s *Sender) Run(ctx context.Context, processLog *slog.Logger) {
	conn := s.send(ctx, processLog)
	if conn == nil && len(s.records) > 0 {
		conn, _ = s.dial(context.WithoutCancel(ctx))
	}

	if conn == nil {
		return
	}

	defer func() {
		_ = conn.Close()
	}()

	for {
		select {
		case record := <-s.records:
			if s.write(conn, record) != nil {
				return
			}
		default:
			return
		}
	}
}

// record returns line as an RFC 5424 record made at the time at, framed
// for the endpoint: on its own over UDP, since each datagram holds one,
// and over TCP and TLS after its length in bytes and a space, the
// octet-counted framing of RFC 6587 and RFC 5425.
func (s *Sender) record(at string, line []byte) []byte {
	record := make([]byte, 0, len(s.beforeTime)+len(at)+len(s.afterTime)+len(line))
	record = append(record, s.beforeTime...)
	record = append(record, at...)
	record = append(record, s.afterTime...)
	record = append(record, line...)

	if s.url.Scheme == SchemeUDP {
		return record
	}

	return append([]byte(strconv.Itoa(len(record))+" "), record...)
}

// put adds record to the buffer, first dropping the oldest record in it
// while it is full.
func (s *Sender) put(record []byte) {
	for {
		select {
		case s.records <- record:
			return
		default:
		}

		select {
		case <-s.records:
			s.dropped.Add(1)
		default:
		}
	}
}

// send connects to the endpoint and sends each record as it comes into
// the buffer, until ctx is done, and returns the connection then open, or
// nil.
func (s *Sender) send(ctx context.Context, processLog *slog.Logger) net.Conn {
	delay := firstRetryDelay

	for {
		conn, err := s.dial(ctx)
		if ctx.Err() != nil {
			return conn
		}

		if err == nil {
			connected := time.Now()

			err = s.sendOn(ctx, conn)
			if err == nil {
				return conn
			}

			_ = conn.Close()

			if time.Since(connected) >= resetRetryDelayAfter {
				delay = firstRetryDelay
			}
		}

		processLog.Warn("sending to SWWAF_LOG_REMOTE_URL failed",
			"error", err.Error(), "connecting_again_in", delay.String())

		select {
		case <-time.After(delay):
		case <-ctx.Done():
			return nil
		}

		delay = min(retryDelayFactor*delay, maxRetryDelay)
	}
}

// sendOn sends each record on conn as it comes into the buffer, until one
// fails, whose error it returns, or ctx is done.
func (s *Sender) sendOn(ctx context.Context, conn net.Conn) error {
	for {
		select {
		case record := <-s.records:
			err := s.write(conn, record)
			if err != nil {
				return err
			}
		case <-ctx.Done():
			return nil
		}
	}
}

// write sends record on conn, and counts it as sent or, if that fails,
// as dropped. A record too long for one UDP datagram is dropped without
// an error, since the connection has not failed: a long request must not
// hold up the lines after it.
func (s *Sender) write(conn net.Conn, record []byte) error {
	_, err := conn.Write(record)
	if err != nil {
		s.dropped.Add(1)

		if errors.Is(err, syscall.EMSGSIZE) {
			return nil
		}

		return fmt.Errorf("send a record: %w", err)
	}

	s.sent.Add(1)

	return nil
}

// dial connects to the endpoint.
func (s *Sender) dial(ctx context.Context) (net.Conn, error) {
	dialer := &net.Dialer{Timeout: dialTimeout}

	switch s.url.Scheme {
	case SchemeUDP:
		return dialer.DialContext(ctx, "udp", s.url.Host)
	case SchemeTLS:
		tlsDialer := &tls.Dialer{NetDialer: dialer, Config: s.tlsConfig}

		return tlsDialer.DialContext(ctx, "tcp", s.url.Host)
	default:
		return dialer.DialContext(ctx, "tcp", s.url.Host)
	}
}