package ident

import (
	"bufio"
	"context"
	"fmt"
	"net"
	"sync"
	"time"

	"github.com/golang/glog"
)

// NewServer returns an ident Server.
func NewServer() *Server {
	return &Server{
		handler: unsetHandler,
	}
}

// Server is an ident protocol server (per RFC1413). It must be configured with
// a HandlerFunc before Serve is called.
// Multiple goroutines may invoke methods on Server simultaneously, but the
// Server can only Serve one listener at a time.
type Server struct {
	handler HandlerFunc
	// mu guards stopC and stop.
	mu sync.Mutex
	// stopC is set if Serve() is already running. If it gets closed, Serve()
	// will quit and set stopC to nil.
	stopC chan struct{}
	// stop can be set to true if Serve() is not yet running but has already
	// been requested to Stop() (eg. if Serve() is ran in a goroutine which
	// hasn't yet scheduled). It will be set back to false when Serve() sees it
	// set and exits.
	stop bool
}

// ResponseWriter is passed to HandlerFuncs and is used to signal to the Server
// that the HandlerFunc wants to respond to the incoming Request in a certain
// way.
// Only the goroutine that the HandlerFunc has been started in may invoke
// methods on the ResponseWriter.
type ResponseWriter interface {
	// SendError returns an ident ErrorResponse to the ident client. This can
	// only be called once, and cannot be called after SendIdent.
	SendError(ErrorResponse) error
	// SendIdent returns an ident IdentResponse to the ident client. This can
	// only be called once, and cannot be called after SendError.
	SendIdent(*IdentResponse) error
}

// HandlerFunc is a function that will be called to serve a given ident
// Request. Users of the Server must implement this and configure a Server to
// use it by invoking Server.HandleFunc.
// Each HandlerFunc will be started in its own goroutine. When HandlerFunc
// returns, the Server will attempt to serve more incoming requests from the
// ident client.
// The Server does not limit the amount of concurrent ident connections that it
// serves. If the Server user wishes to limit concurrency, she must do it
// herself, eg. by using a semaphore. The Server will continue accepting new
// connections and starting new HandlerFuncs, if the user code needs to push
// back it should return as early as possible. There currently is no way to
// make the Server refuse connections above some concurrncy limit.
// The Server does not impose any execution timeout on handlers. If the Server
// user wishes to impose an execution timeout, she must do it herself, eg.
// using context.WithTimeout or time.After.
// The passed Context will be canceled when the ident client disconnects or the
// Server shuts down. The HandlerFunc must return as early as it can detect
// that the context is done.
type HandlerFunc func(ctx context.Context, w ResponseWriter, r *Request)

// responseWriter implements ResponseWriter for a Server.
type responseWriter struct {
	conn      net.Conn
	req       *Request
	responded bool
}

// sendResponse sends a Response to the ident client. The Response must already
// be fully populated.
func (w *responseWriter) sendResponse(r *Response) error {
	if w.responded {
		return fmt.Errorf("handler already sent a response")
	}
	w.responded = true
	data := r.encode()
	if data == nil {
		return fmt.Errorf("failed to encode response")
	}
	glog.V(3).Infof(" -> %q", data)
	_, err := w.conn.Write(data)
	if err != nil {
		return fmt.Errorf("writing response failed: %w", err)
	}
	return nil
}

func (w *responseWriter) SendError(e ErrorResponse) error {
	if !e.IsError() {
		return fmt.Errorf("error response must contain a valid error")
	}
	return w.sendResponse(&Response{
		ClientPort: w.req.ClientPort,
		ServerPort: w.req.ServerPort,
		Error:      e,
	})
}

func (w *responseWriter) SendIdent(i *IdentResponse) error {
	ir := *i
	// TODO(q3k): enforce RFC1413 limits.
	if ir.OperatingSystem == "" {
		ir.OperatingSystem = "UNIX"
	}
	if ir.UserID == "" {
		return fmt.Errorf("ident response must have UserID set")
	}
	return w.sendResponse(&Response{
		ClientPort: w.req.ClientPort,
		ServerPort: w.req.ServerPort,
		Ident:      &ir,
	})
}

var (
	unsetHandlerErrorOnce sync.Once
)

// unsetHandler is the default handler that is configured for a Server. It
// returns UNKNOWN-ERROR to the ident client and logs an error once if it's
// called (telling the user about a misconfiguration / programming error).
func unsetHandler(ctx context.Context, w ResponseWriter, r *Request) {
	unsetHandlerErrorOnce.Do(func() {
		glog.Errorf("Server with no handler configured - will always return UNKNOWN-ERROR")
	})
	w.SendError(UnknownError)
}

// HandleFunc sets the HandlerFunc that the server will call for every incoming
// ident request. If a HandlerFunc is already set, it will be overwritten by
// the given new function.
func (s *Server) HandleFunc(fn HandlerFunc) {
	s.handler = fn
}

// Serve runs the ident server, blocking until a transport-level error occurs
// or Stop() is invoked. The returned error will be nil on Stop(), and will
// wrap the underlying transport-level error otherwise.
//
// Only one invokation of Serve() can be run at a time, but Serve can be called
// again after Stop() is called, and can be ran on a different Listener - no
// state is kept in between subsequent Serve() runs.
func (s *Server) Serve(lis net.Listener) error {
	s.mu.Lock()
	if s.stopC != nil {
		s.mu.Unlock()
		return fmt.Errorf("cannot Serve() an already serving server")
	}
	// Stop() has been invoked before Serve() started.
	if s.stop == true {
		s.stop = false
		s.mu.Unlock()
		return nil
	}
	// Set stopC to allow Stop() calls to stop this running Serve(). It will be
	// set to nil on exit.
	stopC := make(chan struct{})
	s.stopC = stopC
	s.mu.Unlock()

	defer func() {
		s.mu.Lock()
		s.stopC = nil
		s.mu.Unlock()
	}()

	ctx, ctxC := context.WithCancel(context.Background())
	for {
		lisConnC := make(chan net.Conn)
		lisErrC := make(chan error)
		go func() {
			conn, err := lis.Accept()
			select {
			case <-stopC:
				// Server stopped, drop the accepted connection (if any)
				// and return.
				glog.V(2).Infof("Accept goroutine stopping...")
				if err == nil {
					conn.Close()
				}
				return
			default:
			}
			if err == nil {
				glog.V(5).Infof("Accept ok: %v", conn.RemoteAddr())
				lisConnC <- conn
			} else {
				glog.V(5).Infof("Accept err: %v", err)
				lisErrC <- err
			}
		}()

		select {
		case <-stopC:
			// Server stopped, return.
			ctxC()
			return nil
		case err := <-lisErrC:
			ctxC()
			// Accept() failed, return error.
			return err
		case conn := <-lisConnC:
			// Accept() succeeded, serve request.
			go s.serve(ctx, conn)
		}
	}
}

func (s *Server) Stop() {
	s.mu.Lock()
	defer s.mu.Unlock()
	if s.stopC != nil {
		close(s.stopC)
	} else {
		s.stop = true
	}
}

func (s *Server) serve(ctx context.Context, conn net.Conn) {
	glog.V(2).Infof("Serving connection %v", conn.RemoteAddr())
	scanner := bufio.NewScanner(conn)
	// The RFC does not place a limit on the request line length, only on
	// response length. We set an arbitrary limit to 1024 bytes.
	scanner.Buffer(nil, 1024)

	for {
		// Implement an arbitrary timeout for receiving data from client.
		// TODO(q3k): make this configurable
		go func() {
			timer := time.NewTimer(10 * time.Second)
			defer timer.Stop()
			select {
			case <-ctx.Done():
				return
			case <-timer.C:
				glog.V(1).Infof("Connection %v: terminating on receive timeout", conn.RemoteAddr())
				conn.Close()
			}
		}()
		if !scanner.Scan() {
			err := scanner.Err()
			if err == nil {
				// EOF, just return.
				return
			}
			// Some other transport level error occured, or the request line
			// was too long. We can only log this and be done.
			glog.V(1).Infof("Connection %v: scan failed: %v", conn.RemoteAddr(), err)
			conn.Close()
			return
		}
		data := scanner.Bytes()
		glog.V(3).Infof(" <- %q", data)
		req, err := decodeRequest(data)
		if err != nil {
			glog.V(1).Infof("Connection %v: could not decode request: %v", conn.RemoteAddr(), err)
			conn.Close()
			return
		}
		req.ClientAddress = conn.RemoteAddr()
		rw := responseWriter{
			req:  req,
			conn: conn,
		}
		s.handler(ctx, &rw, req)
		if !rw.responded {
			glog.Warningf("Connection %v: handler did not send response, sending UNKNOWN-ERROR", conn.RemoteAddr())
			rw.SendError(UnknownError)
		}
	}
}