Add an in-memory ring buffer

References: https://todo.sr.ht/~emersion/jounce/2

downstream.go

@@ -231,7 +231,17 @@ func (c *downstreamConn) register() error {
 				forwardChannel(c, ch)
 			}
 		}
+
+		consumer := uc.ring.Consumer()
+		for {
+			msg := consumer.Consume()
+			if msg == nil {
+				break
+			}
+			c.messages <- msg
+		}
 	})
+
 	return nil
 }
 

ring.go

@@ -0,0 +1,71 @@
+package jounce
+
+import (
+	"gopkg.in/irc.v3"
+)
+
+// Ring implements a single producer, multiple consumer ring buffer. The ring
+// buffer size is fixed. The ring buffer is stored in memory.
+type Ring struct {
+	buffer   []*irc.Message
+	cap, cur uint64
+
+	consumers []RingConsumer
+}
+
+func NewRing(capacity int) *Ring {
+	return &Ring{
+		buffer: make([]*irc.Message, capacity),
+		cap:    uint64(capacity),
+	}
+}
+
+func (r *Ring) Produce(msg *irc.Message) {
+	i := int(r.cur % r.cap)
+	r.buffer[i] = msg
+	r.cur++
+}
+
+func (r *Ring) Consumer() *RingConsumer {
+	return &RingConsumer{
+		ring: r,
+		cur:  0, // r.cur
+	}
+}
+
+type RingConsumer struct {
+	ring *Ring
+	cur  uint64
+}
+
+func (rc *RingConsumer) Diff() uint64 {
+	if rc.cur > rc.ring.cur {
+		panic("jounce: consumer cursor greater than producer cursor")
+	}
+	return rc.ring.cur - rc.cur
+}
+
+func (rc *RingConsumer) Peek() *irc.Message {
+	diff := rc.Diff()
+	if diff == 0 {
+		return nil
+	}
+	if diff > rc.ring.cap {
+		// Consumer drops diff - cap entries
+		rc.cur = rc.ring.cur - rc.ring.cap
+	}
+	i := int(rc.cur % rc.ring.cap)
+	msg := rc.ring.buffer[i]
+	if msg == nil {
+		panic("jounce: unexpected nil ring buffer entry")
+	}
+	return msg
+}
+
+func (rc *RingConsumer) Consume() *irc.Message {
+	msg := rc.Peek()
+	if msg != nil {
+		rc.cur++
+	}
+	return msg
+}

server.go

@@ -94,6 +94,7 @@ type Upstream struct {
 type Server struct {
 	Hostname  string
 	Logger    Logger
+	RingCap   int
 	Upstreams []Upstream // TODO: per-user
 
 	lock            sync.Mutex
@@ -104,6 +105,7 @@ type Server struct {
 func NewServer() *Server {
 	return &Server{
 		Logger:  log.New(log.Writer(), "", log.LstdFlags),
+		RingCap: 4096,
 		users:   make(map[string]*user),
 	}
 }

upstream.go

@@ -32,6 +32,7 @@ type upstreamConn struct {
 	srv      *Server
 	user     *user
 	messages chan<- *irc.Message
+	ring     *Ring
 
 	serverName            string
 	availableUserModes    string
@@ -63,6 +64,7 @@ func connectToUpstream(u *user, upstream *Upstream) (*upstreamConn, error) {
 		srv:      u.srv,
 		user:     u,
 		messages: msgs,
+		ring:     NewRing(u.srv.RingCap),
 		channels: make(map[string]*upstreamChannel),
 	}
 
@@ -302,6 +304,7 @@ func (c *upstreamConn) handleMessage(msg *irc.Message) error {
 			forwardChannel(dc, ch)
 		})
 	case "PRIVMSG":
+		c.ring.Produce(msg)
 		c.user.forEachDownstream(func(dc *downstreamConn) {
 			dc.messages <- msg
 		})