From 584ea9076fc52d4e31e6b72dcc1f931cfc5bbd9e Mon Sep 17 00:00:00 2001
From: Marek Siarkowicz <siarkowicz@google.com>
Date: Fri, 11 Feb 2022 15:04:44 +0100
Subject: [PATCH] server: Snapshot after cluster version downgrade

---
 server/etcdserver/apply.go          |  8 +++++++-
 server/etcdserver/server.go         |  6 +++++-
 tests/e2e/cluster_downgrade_test.go | 13 -------------
 3 files changed, 12 insertions(+), 15 deletions(-)

diff --git a/server/etcdserver/apply.go b/server/etcdserver/apply.go
index f91f859f511d..67611fca45e0 100644
--- a/server/etcdserver/apply.go
+++ b/server/etcdserver/apply.go
@@ -936,7 +936,13 @@ func (a *applierV3backend) RoleList(r *pb.AuthRoleListRequest) (*pb.AuthRoleList
 }
 
 func (a *applierV3backend) ClusterVersionSet(r *membershippb.ClusterVersionSetRequest, shouldApplyV3 membership.ShouldApplyV3) {
-	a.s.cluster.SetVersion(semver.Must(semver.NewVersion(r.Ver)), api.UpdateCapability, shouldApplyV3)
+	prevVersion := a.s.Cluster().Version()
+	newVersion := semver.Must(semver.NewVersion(r.Ver))
+	a.s.cluster.SetVersion(newVersion, api.UpdateCapability, shouldApplyV3)
+	// Force snapshot after cluster version downgrade.
+	if prevVersion != nil && newVersion.LessThan(*prevVersion) {
+		a.s.forceSnapshot = true
+	}
 }
 
 func (a *applierV3backend) ClusterMemberAttrSet(r *membershippb.ClusterMemberAttrSetRequest, shouldApplyV3 membership.ShouldApplyV3) {
diff --git a/server/etcdserver/server.go b/server/etcdserver/server.go
index b93158a81093..cf69cd5af823 100644
--- a/server/etcdserver/server.go
+++ b/server/etcdserver/server.go
@@ -291,6 +291,9 @@ type EtcdServer struct {
 	clusterVersionChanged *notify.Notifier
 
 	*AccessController
+	// forceSnapshot can force snapshot be triggered after apply, independent of the snapshotCount.
+	// Should only be set within apply code path. Used to force snapshot after cluster version downgrade.
+	forceSnapshot bool
 }
 
 // NewServer creates a new EtcdServer from the supplied configuration. The
@@ -1081,9 +1084,10 @@ func (s *EtcdServer) applyEntries(ep *etcdProgress, apply *apply) {
 }
 
 func (s *EtcdServer) triggerSnapshot(ep *etcdProgress) {
-	if ep.appliedi-ep.snapi <= s.Cfg.SnapshotCount {
+	if !s.forceSnapshot && ep.appliedi-ep.snapi <= s.Cfg.SnapshotCount {
 		return
 	}
+	s.forceSnapshot = false
 
 	lg := s.Logger()
 	lg.Info(
diff --git a/tests/e2e/cluster_downgrade_test.go b/tests/e2e/cluster_downgrade_test.go
index 655067ceabd9..5e2eb9ff209a 100644
--- a/tests/e2e/cluster_downgrade_test.go
+++ b/tests/e2e/cluster_downgrade_test.go
@@ -17,7 +17,6 @@ package e2e
 import (
 	"context"
 	"fmt"
-	"strings"
 	"testing"
 	"time"
 
@@ -66,8 +65,6 @@ func startEtcd(t *testing.T, execPath, dataDirPath string) *e2e.EtcdProcessClust
 		ClusterSize:  1,
 		InitialToken: "new",
 		KeepDataDir:  true,
-		// TODO: REMOVE snapshot override when snapshotting is automated after lowering storage versiont l
-		SnapshotCount: 5,
 	})
 	if err != nil {
 		t.Fatalf("could not start etcd process cluster (%v)", err)
@@ -77,16 +74,6 @@ func startEtcd(t *testing.T, execPath, dataDirPath string) *e2e.EtcdProcessClust
 			t.Fatalf("error closing etcd processes (%v)", errC)
 		}
 	})
-
-	prefixArgs := []string{e2e.CtlBinPath, "--endpoints", strings.Join(epc.EndpointsV3(), ",")}
-	t.Log("Write keys to ensure wal snapshot is created so cluster version set is snapshotted")
-	e2e.ExecuteWithTimeout(t, 20*time.Second, func() {
-		for i := 0; i < 10; i++ {
-			if err := e2e.SpawnWithExpect(append(prefixArgs, "put", fmt.Sprintf("%d", i), "value"), "OK"); err != nil {
-				t.Fatal(err)
-			}
-		}
-	})
 	return epc
 }