// Package pubsub implements an event dispatching server with a single publisher
// and multiple subscriber clients. Multiple goroutines can safely publish to a
// single Server instance.
//
// Clients register subscriptions with a query to select which messages they
// wish to receive. When messages are published, they are broadcast to all
// clients whose subscription query matches that message. Queries are
// constructed using the github.com/tendermint/tendermint/libs/pubsub/query
// package.
//
// Example:
//
//     q, err := query.New("account.name='John'")
//     if err != nil {
//         return err
//     }
//     ctx, cancel := context.WithTimeout(context.Background(), 1*time.Second)
//     defer cancel()
//     subscription, err := pubsub.Subscribe(ctx, "johns-transactions", q)
//     if err != nil {
//         return err
//     }
//
//     for {
//         select {
//         case msg <- subscription.Out():
//             // handle msg.Data() and msg.Events()
//         case <-subscription.Canceled():
//             return subscription.Err()
//         }
//     }
//
package pubsub

import (
	"context"
	"errors"
	"fmt"
	"sync"

	"github.com/tendermint/tendermint/abci/types"
	"github.com/tendermint/tendermint/libs/service"
)

var (
	// ErrSubscriptionNotFound is returned when a client tries to unsubscribe
	// from not existing subscription.
	ErrSubscriptionNotFound = errors.New("subscription not found")

	// ErrAlreadySubscribed is returned when a client tries to subscribe twice or
	// more using the same query.
	ErrAlreadySubscribed = errors.New("already subscribed")

	// ErrServerStopped is returned when attempting to publish or subscribe to a
	// server that has been stopped.
	ErrServerStopped = errors.New("pubsub server is stopped")
)

// Query defines an interface for a query to be used for subscribing. A query
// matches against a map of events. Each key in this map is a composite of the
// even type and an attribute key (e.g. "{eventType}.{eventAttrKey}") and the
// values are the event values that are contained under that relationship. This
// allows event types to repeat themselves with the same set of keys and
// different values.
type Query interface {
	Matches(events []types.Event) (bool, error)
	String() string
}

// UnsubscribeArgs are the parameters to remove a subscription.
// The subscriber ID must be populated, and at least one of the client ID or
// the registered query.
type UnsubscribeArgs struct {
	Subscriber string // subscriber ID chosen by the client (required)
	ID         string // subscription ID (assigned by the server)
	Query      Query  // the query registered with the subscription
}

// Validate returns nil if args are valid to identify a subscription to remove.
// Otherwise, it reports an error.
func (args UnsubscribeArgs) Validate() error {
	if args.Subscriber == "" {
		return errors.New("must specify a subscriber")
	}
	if args.ID == "" && args.Query == nil {
		return fmt.Errorf("subscription is not fully defined [subscriber=%q]", args.Subscriber)
	}

	return nil
}

// Server allows clients to subscribe/unsubscribe for messages, publishing
// messages with or without events, and manages internal state.
type Server struct {
	service.BaseService

	queue  chan item
	done   <-chan struct{} // closed when server should exit
	stop   func()          // signal the server to exit
	pubs   sync.RWMutex    // excl: shutdown; shared: active publisher
	exited chan struct{}   // server exited

	// All subscriptions currently known.
	// Lock exclusive to add, remove, or cancel subscriptions.
	// Lock shared to look up or publish to subscriptions.
	subs struct {
		sync.RWMutex
		index *subIndex
	}

	// TODO(creachadair): Rework the options so that this does not need to live
	// as a field. It is not otherwise needed.
	queueCap int
}

// Option sets a parameter for the server.
type Option func(*Server)

// NewServer returns a new server. See the commentary on the Option functions
// for a detailed description of how to configure buffering. If no options are
// provided, the resulting server's queue is unbuffered.
func NewServer(options ...Option) *Server {
	s := new(Server)
	for _, opt := range options {
		opt(s)
	}
	s.BaseService = *service.NewBaseService(nil, "PubSub", s)

	// The queue receives items to be published.
	s.queue = make(chan item, s.queueCap)

	// The index tracks subscriptions by ID and query terms.
	s.subs.index = newSubIndex()

	return s
}

// BufferCapacity allows you to specify capacity for publisher's queue.  This
// is the number of messages that can be published without blocking.  If no
// buffer is specified, publishing is synchronous with delivery.  This function
// will panic if cap < 0.
func BufferCapacity(cap int) Option {
	if cap < 0 {
		panic("negative buffer capacity")
	}
	return func(s *Server) { s.queueCap = cap }
}

// BufferCapacity returns capacity of the publication queue.
func (s *Server) BufferCapacity() int { return cap(s.queue) }

// Subscribe creates a subscription for the given client.
//
// An error will be returned to the caller if the context is canceled or if
// subscription already exist for pair clientID and query.
//
// outCapacity can be used to set a capacity for Subscription#Out channel (1 by
// default). Panics if outCapacity is less than or equal to zero. If you want
// an unbuffered channel, use SubscribeUnbuffered.
func (s *Server) Subscribe(
	ctx context.Context,
	clientID string,
	query Query,
	outCapacity ...int) (*Subscription, error) {
	outCap := 1
	if len(outCapacity) > 0 {
		if outCapacity[0] <= 0 {
			panic("Negative or zero capacity. Use SubscribeUnbuffered if you want an unbuffered channel")
		}
		outCap = outCapacity[0]
	}

	return s.subscribe(ctx, clientID, query, outCap)
}

// SubscribeUnbuffered does the same as Subscribe, except it returns a
// subscription with unbuffered channel. Use with caution as it can freeze the
// server.
func (s *Server) SubscribeUnbuffered(ctx context.Context, clientID string, query Query) (*Subscription, error) {
	return s.subscribe(ctx, clientID, query, 0)
}

func (s *Server) subscribe(ctx context.Context, clientID string, query Query, outCapacity int) (*Subscription, error) {
	s.subs.Lock()
	defer s.subs.Unlock()

	if s.subs.index == nil {
		return nil, ErrServerStopped
	} else if s.subs.index.contains(clientID, query.String()) {
		return nil, ErrAlreadySubscribed
	}

	sub, err := newSubscription(outCapacity)
	if err != nil {
		return nil, err
	}
	s.subs.index.add(&subInfo{
		clientID: clientID,
		query:    query,
		subID:    sub.id,
		sub:      sub,
	})
	return sub, nil
}

// Unsubscribe removes the subscription for the given client and/or query.  It
// returns ErrSubscriptionNotFound if no such subscription exists.
func (s *Server) Unsubscribe(ctx context.Context, args UnsubscribeArgs) error {
	if err := args.Validate(); err != nil {
		return err
	}
	s.subs.Lock()
	defer s.subs.Unlock()
	if s.subs.index == nil {
		return ErrServerStopped
	}

	// TODO(creachadair): Do we need to support unsubscription for an "empty"
	// query?  I believe that case is not possible by the Query grammar, but we
	// should make sure.
	//
	// Revisit this logic once we are able to remove indexing by query.

	var evict subInfoSet
	if args.Subscriber != "" {
		evict = s.subs.index.findClientID(args.Subscriber)
		if args.Query != nil {
			evict = evict.withQuery(args.Query.String())
		}
	} else {
		evict = s.subs.index.findQuery(args.Query.String())
	}

	if len(evict) == 0 {
		return ErrSubscriptionNotFound
	}
	s.removeSubs(evict, ErrUnsubscribed)
	return nil
}

// UnsubscribeAll removes all subscriptions for the given client ID.
// It returns ErrSubscriptionNotFound if no subscriptions exist for that client.
func (s *Server) UnsubscribeAll(ctx context.Context, clientID string) error {
	s.subs.Lock()
	defer s.subs.Unlock()

	evict := s.subs.index.findClientID(clientID)
	if len(evict) == 0 {
		return ErrSubscriptionNotFound
	}
	s.removeSubs(evict, ErrUnsubscribed)
	return nil
}

// NumClients returns the number of clients.
func (s *Server) NumClients() int {
	s.subs.RLock()
	defer s.subs.RUnlock()
	return len(s.subs.index.byClient)
}

// NumClientSubscriptions returns the number of subscriptions the client has.
func (s *Server) NumClientSubscriptions(clientID string) int {
	s.subs.RLock()
	defer s.subs.RUnlock()
	return len(s.subs.index.findClientID(clientID))
}

// Publish publishes the given message. An error will be returned to the caller
// if the context is canceled.
func (s *Server) Publish(ctx context.Context, msg interface{}) error {
	return s.publish(ctx, msg, []types.Event{})
}

// PublishWithEvents publishes the given message with the set of events. The set
// is matched with clients queries. If there is a match, the message is sent to
// the client.
func (s *Server) PublishWithEvents(ctx context.Context, msg interface{}, events []types.Event) error {
	return s.publish(ctx, msg, events)
}

// OnStop implements Service.OnStop by shutting down the server.
func (s *Server) OnStop() { s.stop() }

// Wait implements Service.Wait by blocking until the server has exited, then
// yielding to the base service wait.
func (s *Server) Wait() { <-s.exited; s.BaseService.Wait() }

// OnStart implements Service.OnStart by starting the server.
func (s *Server) OnStart() error { s.run(); return nil }

// OnReset implements Service.OnReset. It has no effect for this service.
func (s *Server) OnReset() error { return nil }

func (s *Server) publish(ctx context.Context, data interface{}, events []types.Event) error {
	s.pubs.RLock()
	defer s.pubs.RUnlock()

	select {
	case <-s.done:
		return ErrServerStopped
	case <-ctx.Done():
		return ctx.Err()
	case s.queue <- item{
		Data:   data,
		Events: events,
	}:
		return nil
	}
}

func (s *Server) run() {
	// The server runs until ctx is canceled.
	ctx, cancel := context.WithCancel(context.Background())
	s.done = ctx.Done()
	s.stop = cancel

	// Shutdown monitor: When the context ends, wait for any active publish
	// calls to exit, then close the queue to signal the sender to exit.
	go func() {
		<-ctx.Done()
		s.pubs.Lock()
		defer s.pubs.Unlock()
		close(s.queue)
	}()

	s.exited = make(chan struct{})
	go func() {
		defer close(s.exited)

		// Sender: Service the queue and forward messages to subscribers.
		for it := range s.queue {
			if err := s.send(it.Data, it.Events); err != nil {
				s.Logger.Error("Error sending event", "err", err)
			}
		}
		// Terminate all subscribers without error before exit.
		s.subs.Lock()
		defer s.subs.Unlock()
		for si := range s.subs.index.all {
			si.sub.cancel(nil)
		}
		s.subs.index = nil
	}()
}

// removeSubs cancels and removes all the subscriptions in evict with the given
// error. The caller must hold the s.subs lock.
func (s *Server) removeSubs(evict subInfoSet, reason error) {
	for si := range evict {
		si.sub.cancel(reason)
	}
	s.subs.index.removeAll(evict)
}

// send delivers the given message to all matching subscribers.  An error in
// query matching stops transmission and is returned.
func (s *Server) send(data interface{}, events []types.Event) error {
	// At exit, evict any subscriptions that were too slow.
	evict := make(subInfoSet)
	defer func() {
		if len(evict) != 0 {
			s.subs.Lock()
			defer s.subs.Unlock()
			s.removeSubs(evict, ErrOutOfCapacity)
		}
	}()

	// N.B. Order is important here. We must acquire and defer the lock release
	// AFTER deferring the eviction cleanup: The cleanup must happen after the
	// reader lock has released, or it will deadlock.
	s.subs.RLock()
	defer s.subs.RUnlock()

	for si := range s.subs.index.all {
		match, err := si.query.Matches(events)
		if err != nil {
			return fmt.Errorf("match failed against query: %w", err)
			// TODO(creachadair): Should we evict this subscription?
		} else if !match {
			continue
		}

		// Subscriptions may be buffered or unbuffered. Unbuffered subscriptions
		// are intended for internal use such as indexing, where we don't want to
		// penalize a slow reader. Buffered subscribers must keep up with their
		// queue, or they will be terminated.
		//
		// TODO(creachadair): Unbuffered subscriptions used by the event indexer
		// to avoid losing events if it happens to be slow. Rework this so that
		// use case doesn't require this affordance, and then remove unbuffered
		// subscriptions.
		msg := NewMessage(si.sub.id, data, events)
		if err := si.sub.putMessage(msg); err != nil {
			// The subscriber was too slow, cancel them.
			evict.add(si)
		}
	}

	return nil
}