status-im/status-go

package node

import (
    "context"
    "errors"

    "github.com/ethereum/go-ethereum/log"
    "github.com/ethereum/go-ethereum/node"
    "github.com/ethereum/go-ethereum/p2p"
    "github.com/status-im/status-go/common"
)

// All general log messages in this package should be routed through this logger.
var logger = log.New("package", "status-go/metrics/node")

// SubscribeServerEvents subscribes to server and listens to
// PeerEventTypeAdd and PeerEventTypeDrop events.
func SubscribeServerEvents(ctx context.Context, node *node.Node) error {
    server := node.Server()

    if server == nil {
        return errors.New("server is unavailable")
    }

    ch := make(chan *p2p.PeerEvent, server.MaxPeers)
    subscription := server.SubscribeEvents(ch)
    defer subscription.Unsubscribe()

    logger.Debug("Subscribed to server events")

    for {
        select {
        case event := <-ch:
            if isAddDropPeerEvent(event.Type) {
                // We start a goroutine here because updateNodeMetrics
                // is calling a method on the p2p server, which
                // blocks until the server is available:
                // https://github.com/status-im/status-go/blob/e60f425b45d00d3880b42fdd77b460ec465a9f55/vendor/github.com/ethereum/go-ethereum/p2p/server.go#L301
                // https://github.com/status-im/status-go/blob/e60f425b45d00d3880b42fdd77b460ec465a9f55/vendor/github.com/ethereum/go-ethereum/p2p/server.go#L746
                // If there's back-pressure on the peer event feed
                // https://github.com/status-im/status-go/blob/e60f425b45d00d3880b42fdd77b460ec465a9f55/vendor/github.com/ethereum/go-ethereum/p2p/server.go#L783
                // The event channel above might become full while updateNodeMetrics
                // is called, which means is never consumed, the server blocks on publishing on
                // it, and the two will deadlock (server waits for the channel above to be consumed,
                // this code waits for the server to respond to peerCount, which is in the same
                // event loop).
                // Calling it in a different go-routine will allow this code to keep
                // processing peer added events, therefore the server will not lock and
                // keep processing requests.
                go func() {
                    defer common.LogOnPanic()
                    if err := updateNodeMetrics(node, event.Type); err != nil {
                        logger.Error("failed to update node metrics", "err", err)
                    }
                }()
            }
        case err := <-subscription.Err():
            if err != nil {
                logger.Error("Subscription failed", "err", err)
            }
            return err
        case <-ctx.Done():
            return nil
        }
    }
}

func isAddDropPeerEvent(eventType p2p.PeerEventType) bool {
    return eventType == p2p.PeerEventTypeAdd || eventType == p2p.PeerEventTypeDrop
}