path: root/rpc/subscription.go

// Copyright 2016 The go-ethereum Authors
// This file is part of the go-ethereum library.
//
// The go-ethereum library is free software: you can redistribute it and/or modify
// it under the terms of the GNU Lesser General Public License as published by
// the Free Software Foundation, either version 3 of the License, or
// (at your option) any later version.
//
// The go-ethereum library is distributed in the hope that it will be useful,
// but WITHOUT ANY WARRANTY; without even the implied warranty of
// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
// GNU Lesser General Public License for more details.
//
// You should have received a copy of the GNU Lesser General Public License
// along with the go-ethereum library. If not, see <http://www.gnu.org/licenses/>.

package rpc

import (
    "context"
    "errors"
    "sync"
)

var (
    // ErrNotificationsUnsupported is returned when the connection doesn't support notifications
    ErrNotificationsUnsupported = errors.New("notifications not supported")
    // ErrNotificationNotFound is returned when the notification for the given id is not found
    ErrSubscriptionNotFound = errors.New("subscription not found")
)

// ID defines a pseudo random number that is used to identify RPC subscriptions.
type ID string

// a Subscription is created by a notifier and tight to that notifier. The client can use
// this subscription to wait for an unsubscribe request for the client, see Err().
type Subscription struct {
    ID  ID
    err chan error // closed on unsubscribe
}

// Err returns a channel that is closed when the client send an unsubscribe request.
func (s *Subscription) Err() <-chan error {
    return s.err
}

// notifierKey is used to store a notifier within the connection context.
type notifierKey struct{}

// Notifier is tight to a RPC connection that supports subscriptions.
// Server callbacks use the notifier to send notifications.
type Notifier struct {
    codec    ServerCodec
    subMu    sync.RWMutex // guards active and inactive maps
    stopped  bool
    active   map[ID]*Subscription
    inactive map[ID]*Subscription
}

// newNotifier creates a new notifier that can be used to send subscription
// notifications to the client.
func newNotifier(codec ServerCodec) *Notifier {
    return &Notifier{
        codec:    codec,
        active:   make(map[ID]*Subscription),
        inactive: make(map[ID]*Subscription),
    }
}

// NotifierFromContext returns the Notifier value stored in ctx, if any.
func NotifierFromContext(ctx context.Context) (*Notifier, bool) {
    n, ok := ctx.Value(notifierKey{}).(*Notifier)
    return n, ok
}

// CreateSubscription returns a new subscription that is coupled to the
// RPC connection. By default subscriptions are inactive and notifications
// are dropped until the subscription is marked as active. This is done
// by the RPC server after the subscription ID is send to the client.
func (n *Notifier) CreateSubscription() *Subscription {
    s := &Subscription{NewID(), make(chan error)}
    n.subMu.Lock()
    n.inactive[s.ID] = s
    n.subMu.Unlock()
    return s
}

// Notify sends a notification to the client with the given data as payload.
// If an error occurs the RPC connection is closed and the error is returned.
func (n *Notifier) Notify(id ID, data interface{}) error {
    n.subMu.RLock()
    defer n.subMu.RUnlock()

    _, active := n.active[id]
    if active {
        notification := n.codec.CreateNotification(string(id), data)
        if err := n.codec.Write(notification); err != nil {
            n.codec.Close()
            return err
        }
    }
    return nil
}

// Closed returns a channel that is closed when the RPC connection is closed.
func (n *Notifier) Closed() <-chan interface{} {
    return n.codec.Closed()
}

// unsubscribe a subscription.
// If the subscription could not be found ErrSubscriptionNotFound is returned.
func (n *Notifier) unsubscribe(id ID) error {
    n.subMu.Lock()
    defer n.subMu.Unlock()
    if s, found := n.active[id]; found {
        close(s.err)
        delete(n.active, id)
        return nil
    }
    return ErrSubscriptionNotFound
}

// activate enables a subscription. Until a subscription is enabled all
// notifications are dropped. This method is called by the RPC server after
// the subscription ID was sent to client. This prevents notifications being
// send to the client before the subscription ID is send to the client.
func (n *Notifier) activate(id ID) {
    n.subMu.Lock()
    defer n.subMu.Unlock()
    if sub, found := n.inactive[id]; found {
        n.active[id] = sub
        delete(n.inactive, id)
    }
}