feat!(store): add Tail (#261)

Fixes #203

Author: cristaloleg

headertest/store.go

@@ -51,6 +51,19 @@ func (m *Store[H]) Head(context.Context, ...header.HeadOption[H]) (H, error) {
 	return m.Headers[m.HeadHeight], nil
 }
 
+func (m *Store[H]) Tail(context.Context) (H, error) {
+	err := header.ErrNotFound
+
+	var tail H
+	for _, h := range m.Headers {
+		if tail.IsZero() || h.Height() < tail.Height() {
+			tail = h
+		}
+	}
+
+	return tail, err
+}
+
 func (m *Store[H]) Get(_ context.Context, hash header.Hash) (H, error) {
 	for _, header := range m.Headers {
 		if bytes.Equal(header.Hash(), hash) {

interface.go

@@ -70,6 +70,9 @@ type Store[H Header[H]] interface {
 	// Init initializes Store with the given head, meaning it is initialized with the genesis header.
 	Init(context.Context, H) error
 
+	// Tail returns the oldest known header.
+	Tail(context.Context) (H, error)
+
 	// Height reports current height of the chain head.
 	Height() uint64
 

p2p/server_test.go

@@ -139,6 +139,12 @@ func (timeoutStore[H]) Head(ctx context.Context, _ ...header.HeadOption[H]) (H,
 	return zero, ctx.Err()
 }
 
+func (timeoutStore[H]) Tail(ctx context.Context) (H, error) {
+	<-ctx.Done()
+	var zero H
+	return zero, ctx.Err()
+}
+
 func (timeoutStore[H]) Get(ctx context.Context, _ header.Hash) (H, error) {
 	<-ctx.Done()
 	var zero H

store/keys.go

@@ -8,7 +8,10 @@ import (
 	"github.com/celestiaorg/go-header"
 )
 
-var headKey = datastore.NewKey("head")
+var (
+	headKey = datastore.NewKey("head")
+	tailKey = datastore.NewKey("tail")
+)
 
 func heightKey(h uint64) datastore.Key {
 	return datastore.NewKey(strconv.FormatUint(h, 10))

store/store.go

@@ -206,6 +206,23 @@ func (s *Store[H]) Head(ctx context.Context, _ ...header.HeadOption[H]) (H, erro
 	}
 }
 
+// Tail implements [header.Store] interface.
+func (s *Store[H]) Tail(ctx context.Context) (H, error) {
+	tailPtr := s.tailHeader.Load()
+	if tailPtr != nil {
+		return *tailPtr, nil
+	}
+
+	tail, err := s.readTail(ctx)
+	if err != nil {
+		var zero H
+		return zero, err
+	}
+
+	s.tailHeader.Store(&tail)
+	return tail, nil
+}
+
 func (s *Store[H]) Get(ctx context.Context, hash header.Hash) (H, error) {
 	var zero H
 	if v, ok := s.cache.Get(hash.String()); ok {
@@ -476,6 +493,22 @@ func (s *Store[H]) readHead(ctx context.Context) (H, error) {
 	return s.Get(ctx, head)
 }
 
+// readTail loads the tail from the datastore.
+func (s *Store[H]) readTail(ctx context.Context) (H, error) {
+	var zero H
+	b, err := s.ds.Get(ctx, tailKey)
+	if err != nil {
+		return zero, err
+	}
+
+	var tail header.Hash
+	if err := tail.UnmarshalJSON(b); err != nil {
+		return zero, err
+	}
+
+	return s.Get(ctx, tail)
+}
+
 func (s *Store[H]) get(ctx context.Context, hash header.Hash) ([]byte, error) {
 	startTime := time.Now()
 	data, err := s.ds.Get(ctx, datastore.NewKey(hash.String()))

store/store_test.go

@@ -23,15 +23,22 @@ func TestStore(t *testing.T) {
 
 	suite := headertest.NewTestSuite(t)
 
+	genesis := suite.Head()
 	ds := sync.MutexWrap(datastore.NewMapDatastore())
-	store := NewTestStore(t, ctx, ds, suite.Head())
+	store := NewTestStore(t, ctx, ds, genesis)
+
+	assert.Equal(t, *store.tailHeader.Load(), suite.Head())
 
 	assert.Equal(t, *store.tailHeader.Load(), suite.Head())
 
 	head, err := store.Head(ctx)
 	require.NoError(t, err)
 	assert.EqualValues(t, suite.Head().Hash(), head.Hash())
 
+	tail, err := store.Tail(ctx)
+	require.NoError(t, err)
+	assert.Equal(t, tail.Hash(), genesis.Hash())
+
 	in := suite.GenDummyHeaders(10)
 	err = store.Append(ctx, in...)
 	require.NoError(t, err)
@@ -521,6 +528,11 @@ func TestStoreInit(t *testing.T) {
 	require.NoError(t, err)
 
 	headers := suite.GenDummyHeaders(10)
-	err = store.Init(ctx, headers[len(headers)-1]) // init should work with any height, not only 1
+	h := headers[len(headers)-1]
+	err = store.Init(ctx, h) // init should work with any height, not only 1
+	require.NoError(t, err)
+
+	tail, err := store.Tail(ctx)
+	assert.Equal(t, tail.Hash(), h.Hash())
 	require.NoError(t, err)
 }