Add option to join by common parents

Signed-off-by: Jakub Sztandera <kubuxu@protocol.ai>

chain/sync_manager.go

@@ -2,6 +2,7 @@ package chain
 
 import (
 	"context"
+	"os"
 	"sort"
 	"strings"
 	"sync"
@@ -12,6 +13,14 @@ import (
 
 const BootstrapPeerThreshold = 2
 
+var coalesceForksParents = false
+
+func init() {
+	if os.Getenv("LOTUS_SYNC_REL_PARENT") == "yes" {
+		coalesceForksParents = true
+	}
+}
+
 const (
 	BSStateInit      = 0
 	BSStateSelected  = 1
@@ -256,6 +265,9 @@ func (stb *syncTargetBucket) sameChainAs(ts *types.TipSet) bool {
 		if ts.Parents() == t.Key() {
 			return true
 		}
+		if coalesceForksParents && ts.Parents() == t.Parents() {
+			return true
+		}
 	}
 	return false
 }