From 4058f5642f92ee0bc8db2a4276be50bab504c3bc Mon Sep 17 00:00:00 2001
From: vyzo <vyzo@hackzen.org>
Date: Mon, 2 Nov 2020 13:12:32 +0200
Subject: [PATCH] head change coalescer

---
 chain/store/coalescer.go | 129 +++++++++++++++++++++++++++++++++++++++
 1 file changed, 129 insertions(+)
 create mode 100644 chain/store/coalescer.go

diff --git a/chain/store/coalescer.go b/chain/store/coalescer.go
new file mode 100644
index 000000000..38d458ace
--- /dev/null
+++ b/chain/store/coalescer.go
@@ -0,0 +1,129 @@
+package store
+
+import (
+	"context"
+	"time"
+
+	"github.com/filecoin-project/lotus/chain/types"
+)
+
+func WrapHeadChangeCoalescer(fn ReorgNotifee, delay time.Duration) ReorgNotifee {
+	c := NewHeadChangeCoalescer(fn, delay)
+	return c.HeadChange
+}
+
+type HeadChangeCoalescer struct {
+	notify ReorgNotifee
+
+	ctx    context.Context
+	cancel func()
+
+	eventq chan headChange
+
+	revert []*types.TipSet
+	apply  []*types.TipSet
+}
+
+type headChange struct {
+	revert, apply []*types.TipSet
+}
+
+func NewHeadChangeCoalescer(fn ReorgNotifee, delay time.Duration) *HeadChangeCoalescer {
+	ctx, cancel := context.WithCancel(context.Background())
+	c := &HeadChangeCoalescer{
+		notify: fn,
+		ctx:    ctx,
+		cancel: cancel,
+		eventq: make(chan headChange),
+	}
+
+	go c.background(delay)
+
+	return c
+}
+
+func (c *HeadChangeCoalescer) HeadChange(revert, apply []*types.TipSet) error {
+	select {
+	case c.eventq <- headChange{revert: revert, apply: apply}:
+		return nil
+	case <-c.ctx.Done():
+		return c.ctx.Err()
+	}
+}
+
+func (c *HeadChangeCoalescer) Close() {
+	select {
+	case <-c.ctx.Done():
+	default:
+		c.cancel()
+	}
+}
+
+func (c *HeadChangeCoalescer) background(delay time.Duration) {
+	var timerC <-chan time.Time
+	for {
+		select {
+		case evt := <-c.eventq:
+			c.coalesce(evt.revert, evt.apply)
+			if timerC == nil {
+				timerC = time.After(delay)
+			}
+
+		case <-timerC:
+			c.dispatch()
+			timerC = nil
+
+		case <-c.ctx.Done():
+			return
+		}
+	}
+}
+
+func (c *HeadChangeCoalescer) coalesce(revert, apply []*types.TipSet) {
+	// newly reverted tipsets cancel out pending applied tipsets
+	// we iterate through the revert set and if a tipset is pending for apply we cancel it.
+
+	// pending tipsets for apply
+	applied := make(map[types.TipSetKey]struct{})
+	for _, ts := range c.apply {
+		applied[ts.Key()] = struct{}{}
+	}
+
+	// freshly reverted tipsets from the pending applied set
+	reverted := make(map[types.TipSetKey]struct{})
+
+	for _, ts := range revert {
+		key := ts.Key()
+
+		_, ok := applied[key]
+		if ok {
+			reverted[key] = struct{}{}
+			continue
+		}
+
+		c.revert = append(c.revert, ts)
+	}
+
+	newApply := make([]*types.TipSet, 0, len(c.apply)-len(reverted)+len(apply))
+	for _, ts := range c.apply {
+		_, ok := reverted[ts.Key()]
+		if ok {
+			continue
+		}
+
+		newApply = append(newApply, ts)
+	}
+
+	newApply = append(newApply, apply...)
+	c.apply = newApply
+}
+
+func (c *HeadChangeCoalescer) dispatch() {
+	err := c.notify(c.revert, c.apply)
+	if err != nil {
+		log.Errorf("error dispatching coalesced head change notification: %s", err)
+	}
+
+	c.revert = nil
+	c.apply = nil
+}