/*
   Copyright The containerd Authors.

   Licensed under the Apache License, Version 2.0 (the "License");
   you may not use this file except in compliance with the License.
   You may obtain a copy of the License at

       http://www.apache.org/licenses/LICENSE-2.0

   Unless required by applicable law or agreed to in writing, software
   distributed under the License is distributed on an "AS IS" BASIS,
   WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
   See the License for the specific language governing permissions and
   limitations under the License.
*/

package ttrpc

import (
	"context"
	"io"
	"math/rand"
	"net"
	"sync"
	"sync/atomic"
	"time"

	"github.com/pkg/errors"
	"github.com/sirupsen/logrus"
	"google.golang.org/grpc/codes"
	"google.golang.org/grpc/status"
)

var (
	ErrServerClosed = errors.New("ttrpc: server closed")
)

type Server struct {
	config   *serverConfig
	services *serviceSet
	codec    codec

	mu          sync.Mutex
	listeners   map[net.Listener]struct{}
	connections map[*serverConn]struct{} // all connections to current state
	done        chan struct{}            // marks point at which we stop serving requests
}

func NewServer(opts ...ServerOpt) (*Server, error) {
	config := &serverConfig{}
	for _, opt := range opts {
		if err := opt(config); err != nil {
			return nil, err
		}
	}

	return &Server{
		config:      config,
		services:    newServiceSet(),
		done:        make(chan struct{}),
		listeners:   make(map[net.Listener]struct{}),
		connections: make(map[*serverConn]struct{}),
	}, nil
}

func (s *Server) Register(name string, methods map[string]Method) {
	s.services.register(name, methods)
}

func (s *Server) Serve(ctx context.Context, l net.Listener) error {
	s.addListener(l)
	defer s.closeListener(l)

	var (
		backoff    time.Duration
		handshaker = s.config.handshaker
	)

	if handshaker == nil {
		handshaker = handshakerFunc(noopHandshake)
	}

	for {
		conn, err := l.Accept()
		if err != nil {
			select {
			case <-s.done:
				return ErrServerClosed
			default:
			}

			if terr, ok := err.(interface {
				Temporary() bool
			}); ok && terr.Temporary() {
				if backoff == 0 {
					backoff = time.Millisecond
				} else {
					backoff *= 2
				}

				if max := time.Second; backoff > max {
					backoff = max
				}

				sleep := time.Duration(rand.Int63n(int64(backoff)))
				logrus.WithError(err).Errorf("ttrpc: failed accept; backoff %v", sleep)
				time.Sleep(sleep)
				continue
			}

			return err
		}

		backoff = 0

		approved, handshake, err := handshaker.Handshake(ctx, conn)
		if err != nil {
			logrus.WithError(err).Errorf("ttrpc: refusing connection after handshake")
			conn.Close()
			continue
		}

		sc := s.newConn(approved, handshake)
		go sc.run(ctx)
	}
}

func (s *Server) Shutdown(ctx context.Context) error {
	s.mu.Lock()
	select {
	case <-s.done:
	default:
		// protected by mutex
		close(s.done)
	}
	lnerr := s.closeListeners()
	s.mu.Unlock()

	ticker := time.NewTicker(200 * time.Millisecond)
	defer ticker.Stop()
	for {
		if s.closeIdleConns() {
			return lnerr
		}
		select {
		case <-ctx.Done():
			return ctx.Err()
		case <-ticker.C:
		}
	}
}

// Close the server without waiting for active connections.
func (s *Server) Close() error {
	s.mu.Lock()
	defer s.mu.Unlock()

	select {
	case <-s.done:
	default:
		// protected by mutex
		close(s.done)
	}

	err := s.closeListeners()
	for c := range s.connections {
		c.close()
		delete(s.connections, c)
	}

	return err
}

func (s *Server) addListener(l net.Listener) {
	s.mu.Lock()
	defer s.mu.Unlock()
	s.listeners[l] = struct{}{}
}

func (s *Server) closeListener(l net.Listener) error {
	s.mu.Lock()
	defer s.mu.Unlock()

	return s.closeListenerLocked(l)
}

func (s *Server) closeListenerLocked(l net.Listener) error {
	defer delete(s.listeners, l)
	return l.Close()
}

func (s *Server) closeListeners() error {
	var err error
	for l := range s.listeners {
		if cerr := s.closeListenerLocked(l); cerr != nil && err == nil {
			err = cerr
		}
	}
	return err
}

func (s *Server) addConnection(c *serverConn) {
	s.mu.Lock()
	defer s.mu.Unlock()

	s.connections[c] = struct{}{}
}

func (s *Server) closeIdleConns() bool {
	s.mu.Lock()
	defer s.mu.Unlock()
	quiescent := true
	for c := range s.connections {
		st, ok := c.getState()
		if !ok || st != connStateIdle {
			quiescent = false
			continue
		}
		c.close()
		delete(s.connections, c)
	}
	return quiescent
}

type connState int

const (
	connStateActive = iota + 1 // outstanding requests
	connStateIdle              // no requests
	connStateClosed            // closed connection
)

func (cs connState) String() string {
	switch cs {
	case connStateActive:
		return "active"
	case connStateIdle:
		return "idle"
	case connStateClosed:
		return "closed"
	default:
		return "unknown"
	}
}

func (s *Server) newConn(conn net.Conn, handshake interface{}) *serverConn {
	c := &serverConn{
		server:    s,
		conn:      conn,
		handshake: handshake,
		shutdown:  make(chan struct{}),
	}
	c.setState(connStateIdle)
	s.addConnection(c)
	return c
}

type serverConn struct {
	server    *Server
	conn      net.Conn
	handshake interface{} // data from handshake, not used for now
	state     atomic.Value

	shutdownOnce sync.Once
	shutdown     chan struct{} // forced shutdown, used by close
}

func (c *serverConn) getState() (connState, bool) {
	cs, ok := c.state.Load().(connState)
	return cs, ok
}

func (c *serverConn) setState(newstate connState) {
	c.state.Store(newstate)
}

func (c *serverConn) close() error {
	c.shutdownOnce.Do(func() {
		close(c.shutdown)
	})

	return nil
}

func (c *serverConn) run(sctx context.Context) {
	type (
		request struct {
			id  uint32
			req *Request
		}

		response struct {
			id   uint32
			resp *Response
		}
	)

	var (
		ch          = newChannel(c.conn)
		ctx, cancel = context.WithCancel(sctx)
		active      int
		state       connState = connStateIdle
		responses             = make(chan response)
		requests              = make(chan request)
		recvErr               = make(chan error, 1)
		shutdown              = c.shutdown
		done                  = make(chan struct{})
	)

	defer c.conn.Close()
	defer cancel()
	defer close(done)

	go func(recvErr chan error) {
		defer close(recvErr)
		sendImmediate := func(id uint32, st *status.Status) bool {
			select {
			case responses <- response{
				// even though we've had an invalid stream id, we send it
				// back on the same stream id so the client knows which
				// stream id was bad.
				id: id,
				resp: &Response{
					Status: st.Proto(),
				},
			}:
				return true
			case <-c.shutdown:
				return false
			case <-done:
				return false
			}
		}

		for {
			select {
			case <-c.shutdown:
				return
			case <-done:
				return
			default: // proceed
			}

			mh, p, err := ch.recv(ctx)
			if err != nil {
				status, ok := status.FromError(err)
				if !ok {
					recvErr <- err
					return
				}

				// in this case, we send an error for that particular message
				// when the status is defined.
				if !sendImmediate(mh.StreamID, status) {
					return
				}

				continue
			}

			if mh.Type != messageTypeRequest {
				// we must ignore this for future compat.
				continue
			}

			var req Request
			if err := c.server.codec.Unmarshal(p, &req); err != nil {
				ch.putmbuf(p)
				if !sendImmediate(mh.StreamID, status.Newf(codes.InvalidArgument, "unmarshal request error: %v", err)) {
					return
				}
				continue
			}
			ch.putmbuf(p)

			if mh.StreamID%2 != 1 {
				// enforce odd client initiated identifiers.
				if !sendImmediate(mh.StreamID, status.Newf(codes.InvalidArgument, "StreamID must be odd for client initiated streams")) {
					return
				}
				continue
			}

			// Forward the request to the main loop. We don't wait on s.done
			// because we have already accepted the client request.
			select {
			case requests <- request{
				id:  mh.StreamID,
				req: &req,
			}:
			case <-done:
				return
			}
		}
	}(recvErr)

	for {
		newstate := state
		switch {
		case active > 0:
			newstate = connStateActive
			shutdown = nil
		case active == 0:
			newstate = connStateIdle
			shutdown = c.shutdown // only enable this branch in idle mode
		}

		if newstate != state {
			c.setState(newstate)
			state = newstate
		}

		select {
		case request := <-requests:
			active++
			go func(id uint32) {
				p, status := c.server.services.call(ctx, request.req.Service, request.req.Method, request.req.Payload)
				resp := &Response{
					Status:  status.Proto(),
					Payload: p,
				}

				select {
				case responses <- response{
					id:   id,
					resp: resp,
				}:
				case <-done:
				}
			}(request.id)
		case response := <-responses:
			p, err := c.server.codec.Marshal(response.resp)
			if err != nil {
				logrus.WithError(err).Error("failed marshaling response")
				return
			}

			if err := ch.send(ctx, response.id, messageTypeResponse, p); err != nil {
				logrus.WithError(err).Error("failed sending message on channel")
				return
			}

			active--
		case err := <-recvErr:
			// TODO(stevvooe): Not wildly clear what we should do in this
			// branch. Basically, it means that we are no longer receiving
			// requests due to a terminal error.
			recvErr = nil // connection is now "closing"
			if err != nil && err != io.EOF {
				logrus.WithError(err).Error("error receiving message")
			}
		case <-shutdown:
			return
		}
	}
}
