fix(service/header): lazily load Store head

CHANGELOG-PENDING.md

@@ -39,6 +39,7 @@ Month, DD, YYYY
 
 ### BUG FIXES
 
+- [fix(service/header): lazily load Store head #458](https://github.com/celestiaorg/celestia-node/pull/458) [@Wondertan](https://github.com/Wondertan)
 - [fix(service/header): allow some clock drift during verification #435](https://github.com/celestiaorg/celestia-node/pull/435) [@Wondertan](https://github.com/Wondertan)
 - [service/header: fix ExtendedHeader message duplicates on the network #409](https://github.com/celestiaorg/celestia-node/pull/409) [@Wondertan](https://github.com/Wondertan)
 - [fix(header/service): #339 race](https://github.com/celestiaorg/celestia-node/pull/343) [@Wondertan](https://github.com/Wondertan)

service/header/store.go

@@ -99,21 +99,11 @@ func (s *store) Init(_ context.Context, initial *ExtendedHeader) error {
 		return err
 	}
 
-	log.Infow("initial head", "height", initial.Height, "hash", initial.Hash())
+	log.Infow("initialized head", "height", initial.Height, "hash", initial.Hash())
 	return nil
 }
 
-func (s *store) Start(ctx context.Context) error {
-	head, err := s.readHead(ctx)
-	switch err {
-	default:
-		return err
-	case datastore.ErrNotFound:
-		log.Warnf("starting uninitialized store")
-	case nil:
-		s.heightSub.SetHeight(uint64(head.Height))
-	}
-
+func (s *store) Start(context.Context) error {
 	go s.flushLoop()
 	return nil
 }
@@ -140,10 +130,21 @@ func (s *store) Stop(ctx context.Context) error {
 
 func (s *store) Head(ctx context.Context) (*ExtendedHeader, error) {
 	head, err := s.GetByHeight(ctx, s.heightSub.Height())
-	if err == ErrNotFound {
+	if err == nil {
+		return head, err
+	}
+
+	head, err = s.readHead(ctx)
+	switch err {
+	default:
+		return nil, err
+	case datastore.ErrNotFound, ErrNotFound:
 		return nil, ErrNoHead
+	case nil:
+		s.heightSub.SetHeight(uint64(head.Height))
+		log.Infow("loaded head", "height", head.Height, "hash", head.Hash())
+		return head, nil
 	}
-	return head, err
 }
 
 func (s *store) Get(_ context.Context, hash tmbytes.HexBytes) (*ExtendedHeader, error) {

service/header/store_init_test.go

@@ -0,0 +1,56 @@
+package header
+
+import (
+	"context"
+	"testing"
+	"time"
+
+	"github.com/ipfs/go-datastore"
+	"github.com/ipfs/go-datastore/sync"
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+func TestInitStore_NoReinit(t *testing.T) {
+	ctx, cancel := context.WithTimeout(context.Background(), time.Second*5)
+	t.Cleanup(cancel)
+
+	suite := NewTestSuite(t, 3)
+	head := suite.Head()
+	exchange := NewLocalExchange(NewTestStore(ctx, t, head))
+
+	ds := sync.MutexWrap(datastore.NewMapDatastore())
+	store, err := NewStore(ds)
+	require.NoError(t, err)
+
+	err = InitStore(ctx, store, exchange, head.Hash())
+	assert.NoError(t, err)
+
+	err = store.Start(ctx)
+	require.NoError(t, err)
+
+	_, err = store.Append(ctx, suite.GenExtendedHeaders(10)...)
+	require.NoError(t, err)
+
+	err = store.Stop(ctx)
+	require.NoError(t, err)
+
+	reopenedStore, err := NewStore(ds)
+	require.NoError(t, err)
+
+	err = InitStore(ctx, reopenedStore, exchange, head.Hash())
+	assert.NoError(t, err)
+
+	err = reopenedStore.Start(ctx)
+	require.NoError(t, err)
+
+	reopenedHead, err := reopenedStore.Head(ctx)
+	require.NoError(t, err)
+
+	// check that reopened head changed and the store wasn't reinitialized
+	assert.Equal(t, suite.Head().Height, reopenedHead.Height)
+	assert.NotEqual(t, head.Height, reopenedHead.Height)
+
+	err = reopenedStore.Stop(ctx)
+	require.NoError(t, err)
+}