package mox

import (
	"context"
	"fmt"
	"net"
	"os"
	"os/signal"
	"runtime"
	"runtime/debug"
	"strings"
	"sync"
	"syscall"
	"time"

	"github.com/prometheus/client_golang/prometheus"
	"github.com/prometheus/client_golang/prometheus/promauto"

	"github.com/mjl-/mox/mlog"
)

// We start up as root, bind to sockets, and fork and exec as unprivileged user.
// During startup as root, we gather the fd's for the listen addresses in listens,
// and pass their addresses in an environment variable to the new process.
var listens = map[string]*os.File{}

// RestorePassedSockets reads addresses from $MOX_SOCKETS and prepares an os.File
// for each file descriptor, which are used by later calls of Listen.
func RestorePassedSockets() {
	s := os.Getenv("MOX_SOCKETS")
	if s == "" {
		var linuxhint string
		if runtime.GOOS == "linux" {
			linuxhint = " If you updated from v0.0.1, update the mox.service file to start as root (privileges are dropped): ./mox config printservice >mox.service && sudo systemctl daemon-reload && sudo systemctl restart mox."
		}
		xlog.Fatal("mox must be started as root, and will drop privileges after binding required sockets (missing environment variable MOX_SOCKETS)." + linuxhint)
	}
	addrs := strings.Split(s, ",")
	for i, addr := range addrs {
		// 0,1,2 are stdin,stdout,stderr, 3 is the network/address fd.
		f := os.NewFile(3+uintptr(i), addr)
		listens[addr] = f
	}
}

// Fork and exec as unprivileged user.
//
// We don't use just setuid because it is hard to guarantee that no other
// privileged go worker processes have been started before we get here. E.g. init
// functions in packages can start goroutines.
func ForkExecUnprivileged() {
	prog, err := os.Executable()
	if err != nil {
		xlog.Fatalx("finding executable for exec", err)
	}

	files := []*os.File{os.Stdin, os.Stdout, os.Stderr}
	var addrs []string
	for addr, f := range listens {
		files = append(files, f)
		addrs = append(addrs, addr)
	}
	env := os.Environ()
	env = append(env, "MOX_SOCKETS="+strings.Join(addrs, ","))

	p, err := os.StartProcess(prog, os.Args, &os.ProcAttr{
		Env:   env,
		Files: files,
		Sys: &syscall.SysProcAttr{
			Credential: &syscall.Credential{
				Uid: Conf.Static.UID,
				Gid: Conf.Static.GID,
			},
		},
	})
	if err != nil {
		xlog.Fatalx("fork and exec", err)
	}
	for _, f := range listens {
		err := f.Close()
		xlog.Check(err, "closing socket after passing to unprivileged child")
	}

	// If we get a interrupt/terminate signal, pass it on to the child. For interrupt,
	// the child probably already got it.
	// todo: see if we tie up child and root process so a kill -9 of the root process
	// kills the child process too.
	sigc := make(chan os.Signal, 1)
	signal.Notify(sigc, os.Interrupt, syscall.SIGTERM)
	go func() {
		sig := <-sigc
		p.Signal(sig)
	}()

	st, err := p.Wait()
	if err != nil {
		xlog.Fatalx("wait", err)
	}
	code := st.ExitCode()
	xlog.Print("stopping after child exit", mlog.Field("exitcode", code))
	os.Exit(code)
}

// CleanupPassedSockets closes the listening socket file descriptors passed in by
// the parent process. To be called after listeners have been recreated (they dup
// the file descriptor).
func CleanupPassedSockets() {
	for _, f := range listens {
		err := f.Close()
		xlog.Check(err, "closing listener socket file descriptor")
	}
}

// Make Listen listen immediately, regardless of running as root or other user, in
// case ForkExecUnprivileged is not used.
var ListenImmediate bool

// Listen returns a newly created network listener when starting as root, and
// otherwise (not root) returns a network listener from a file descriptor that was
// passed by the parent root process.
func Listen(network, addr string) (net.Listener, error) {
	if os.Getuid() != 0 && !ListenImmediate {
		f, ok := listens[addr]
		if !ok {
			return nil, fmt.Errorf("no file descriptor for listener %s", addr)
		}
		ln, err := net.FileListener(f)
		if err != nil {
			return nil, fmt.Errorf("making network listener from file descriptor for address %s: %v", addr, err)
		}
		return ln, nil
	}

	if _, ok := listens[addr]; ok {
		return nil, fmt.Errorf("duplicate listener: %s", addr)
	}

	ln, err := net.Listen(network, addr)
	if err != nil {
		return nil, err
	}
	tcpln, ok := ln.(*net.TCPListener)
	if !ok {
		return nil, fmt.Errorf("listener not a tcp listener, but %T, for network %s, address %s", ln, network, addr)
	}
	f, err := tcpln.File()
	if err != nil {
		return nil, fmt.Errorf("dup listener: %v", err)
	}
	listens[addr] = f
	return ln, err
}

// Shutdown is canceled when a graceful shutdown is initiated. SMTP, IMAP, periodic
// processes should check this before starting a new operation. If true, the
// operation should be aborted, and new connections should receive a message that
// the service is currently not available.
var Shutdown context.Context
var ShutdownCancel func()

// Context should be used as parent by all operations. It is canceled when mox is
// shutdown, aborting all pending operations.
//
// Operations typically have context timeouts, 30s for single i/o like DNS queries,
// and 1 minute for operations with more back and forth. These are set through a
// context.WithTimeout based on this context, so those contexts are still canceled
// when shutting down.
//
// HTTP servers don't get graceful shutdown, their connections are just aborted.
var Context context.Context
var ContextCancel func()

// Connections holds all active protocol sockets (smtp, imap). They will be given
// an immediate read/write deadline shortly after initiating mox shutdown, after
// which the connections get 1 more second for error handling before actual
// shutdown.
var Connections = &connections{
	conns:  map[net.Conn]connKind{},
	gauges: map[connKind]prometheus.GaugeFunc{},
	active: map[connKind]int64{},
}

type connKind struct {
	protocol string
	listener string
}

type connections struct {
	sync.Mutex
	conns  map[net.Conn]connKind
	dones  []chan struct{}
	gauges map[connKind]prometheus.GaugeFunc

	activeMutex sync.Mutex
	active      map[connKind]int64
}

// Register adds a connection for receiving an immediate i/o deadline on shutdown.
// When the connection is closed, Remove must be called to cancel the registration.
func (c *connections) Register(nc net.Conn, protocol, listener string) {
	// This can happen, when a connection was initiated before a shutdown, but it
	// doesn't hurt to log it.
	select {
	case <-Shutdown.Done():
		xlog.Error("new connection added while shutting down")
		debug.PrintStack()
	default:
	}

	ck := connKind{protocol, listener}

	c.activeMutex.Lock()
	c.active[ck]++
	c.activeMutex.Unlock()

	c.Lock()
	defer c.Unlock()
	c.conns[nc] = ck
	if _, ok := c.gauges[ck]; !ok {
		c.gauges[ck] = promauto.NewGaugeFunc(
			prometheus.GaugeOpts{
				Name: "mox_connections_count",
				Help: "Open connections, per protocol/listener.",
				ConstLabels: prometheus.Labels{
					"protocol": protocol,
					"listener": listener,
				},
			},
			func() float64 {
				c.activeMutex.Lock()
				defer c.activeMutex.Unlock()
				return float64(c.active[ck])
			},
		)
	}
}

// Unregister removes a connection for shutdown.
func (c *connections) Unregister(nc net.Conn) {
	c.Lock()
	defer c.Unlock()
	ck := c.conns[nc]

	defer func() {
		c.activeMutex.Lock()
		c.active[ck]--
		c.activeMutex.Unlock()
	}()

	delete(c.conns, nc)
	if len(c.conns) > 0 {
		return
	}
	for _, done := range c.dones {
		done <- struct{}{}
	}
	c.dones = nil
}

// Shutdown sets an immediate i/o deadline on all open registered sockets. Called
// some time after mox shutdown is initiated.
// The deadline will cause i/o's to be aborted, which should result in the
// connection being unregistered.
func (c *connections) Shutdown() {
	now := time.Now()
	c.Lock()
	defer c.Unlock()
	for nc := range c.conns {
		if err := nc.SetDeadline(now); err != nil {
			xlog.Errorx("setting immediate read/write deadline for shutdown", err)
		}
	}
}

// Done returns a new channel on which a value is sent when no more sockets are
// open, which could be immediate.
func (c *connections) Done() chan struct{} {
	c.Lock()
	defer c.Unlock()
	done := make(chan struct{}, 1)
	if len(c.conns) == 0 {
		done <- struct{}{}
		return done
	}
	c.dones = append(c.dones, done)
	return done
}