etcdserver: a non-empty raft log snapshot should always be available

Signed-off-by: Clement <[email protected]>

server/etcdserver/server.go

@@ -1210,7 +1210,10 @@ func (s *EtcdServer) triggerSnapshot(ep *etcdProgress) {
 }
 
 func (s *EtcdServer) shouldSnapshot(ep *etcdProgress) bool {
-	return (s.forceSnapshot && ep.appliedi != ep.snapi) || (ep.appliedi-ep.snapi > s.Cfg.SnapshotCount)
+	return (s.forceSnapshot && ep.appliedi != ep.snapi) ||
+		(ep.appliedi-ep.snapi > s.Cfg.SnapshotCount) ||
+		// ensures a non-empty snapshot always exists
+		(ep.snapi == 0 && ep.appliedi > ep.snapi)
 }
 
 func (s *EtcdServer) hasMultipleVotingMembers() bool {

tests/integration/raft_log_snapshot_test.go

@@ -0,0 +1,78 @@
+// Copyright 2024 The etcd Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package integration
+
+import (
+	"context"
+	"errors"
+	"testing"
+	"time"
+
+	pb "go.etcd.io/etcd/api/v3/etcdserverpb"
+	"go.etcd.io/etcd/tests/v3/framework/integration"
+)
+
+// TestRaftLogSnapshotExistsPostStartUp ensures a non-empty raft log snapshot exists after startup
+func TestRaftLogSnapshotExistsPostStartUp(t *testing.T) {
+	integration.BeforeTest(t)
+
+	clus := integration.NewCluster(t, &integration.ClusterConfig{
+		Size:                   1,
+		SnapshotCount:          100,
+		SnapshotCatchUpEntries: 10,
+	})
+	defer clus.Terminate(t)
+
+	m := clus.Members[0]
+
+	ctx, cancel := context.WithTimeout(context.TODO(), 5*time.Second)
+	defer cancel()
+
+	_, err := m.LogObserver.Expect(ctx, "saved snapshot", 1)
+	if err != nil {
+		t.Fatalf("failed to expect (log:%s, count:%v): %v", "saved snapshot", 1, err)
+	}
+
+	kvc := integration.ToGRPC(clus.RandClient()).KV
+
+	// In order to trigger another snapshot, we should increase applied index from 1 to 102.
+	//
+	// NOTE: When starting a new cluster with 1 member, the member will
+	// apply 3 ConfChange directly at the beginning, meaning its applied
+	// index is 4.
+	for i := 0; i < 102-4; i++ {
+		_, err := kvc.Put(context.TODO(), &pb.PutRequest{Key: []byte("foo"), Value: []byte("bar")})
+		if err != nil {
+			t.Fatalf("#%d: couldn't put key (%v)", i, err)
+		}
+	}
+
+	ctx2, cancel2 := context.WithTimeout(context.TODO(), 5*time.Second)
+	defer cancel2()
+
+	_, err = m.LogObserver.Expect(ctx2, "saved snapshot", 2)
+	if err != nil {
+		t.Fatalf("failed to expect (log:%s, count:%v): %v", "saved snapshot", 1, err)
+	}
+
+	ctx3, cancel3 := context.WithTimeout(context.TODO(), 5*time.Second)
+	defer cancel3()
+
+	// Expect function should return a DeadlineExceeded error to ensure no more snapshots are present
+	_, err = m.LogObserver.Expect(ctx3, "saved snapshot", 3)
+	if !errors.Is(err, context.DeadlineExceeded) {
+		t.Fatalf("unexpected error, max snapshots allowed is %d: %v", 2, err)
+	}
+}