mvcc: fix rev inconsistency

Try:

./etcdctl put foo bar
./etcdctl del foo
./etcdctl compact 3

restart etcd

./etcdctl get foo
mvcc: required revision has been compacted

The error is unexpected when range over the head revision.

Internally, we incorrectly set current revision smaller than the
compacted revision when we remove all keys around compacted revision.

This commit fixes the issue by recovering the current revision at least
to compacted revision.

mvcc/kvstore.go

@@ -408,6 +408,13 @@ func (s *store) restore() error {
 		s.currentRev = rev
 	}
 
+	// keys in the range [compacted revision -N, compaction] might all be deleted due to compaction.
+	// the correct revision should be set to compaction revision in the case, not the largest revision
+	// we have seen.
+	if s.currentRev.main < s.compactMainRev {
+		s.currentRev.main = s.compactMainRev
+	}
+
 	for key, lid := range keyToLease {
 		if s.le == nil {
 			panic("no lessor to attach lease")

mvcc/kvstore_compaction_test.go

@@ -15,8 +15,10 @@
 package mvcc
 
 import (
+	"os"
 	"reflect"
 	"testing"
+	"time"
 
 	"github.com/coreos/etcd/lease"
 	"github.com/coreos/etcd/mvcc/backend"
@@ -93,3 +95,41 @@ func TestScheduleCompaction(t *testing.T) {
 		cleanup(s, b, tmpPath)
 	}
 }
+
+func TestCompactAllAndRestore(t *testing.T) {
+	b, tmpPath := backend.NewDefaultTmpBackend()
+	s0 := NewStore(b, &lease.FakeLessor{}, nil)
+	defer os.Remove(tmpPath)
+
+	s0.Put([]byte("foo"), []byte("bar"), lease.NoLease)
+	s0.Put([]byte("foo"), []byte("bar1"), lease.NoLease)
+	s0.Put([]byte("foo"), []byte("bar2"), lease.NoLease)
+	s0.DeleteRange([]byte("foo"), nil)
+
+	rev := s0.Rev()
+	// compact all keys
+	done, err := s0.Compact(rev)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	select {
+	case <-done:
+	case <-time.After(10 * time.Second):
+		t.Fatal("timeout waiting for compaction to finish")
+	}
+
+	err = s0.Close()
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	s1 := NewStore(b, &lease.FakeLessor{}, nil)
+	if s1.Rev() != rev {
+		t.Errorf("rev = %v, want %v", s1.Rev(), rev)
+	}
+	_, err = s1.Range([]byte("foo"), nil, RangeOptions{})
+	if err != nil {
+		t.Errorf("unexpect range error %v", err)
+	}
+}