From 7765cbd95f4b7beea61ad3b185062f2ed6f1de42 Mon Sep 17 00:00:00 2001
From: Oleg Kovalov <oleg@hey.com>
Date: Mon, 1 Jul 2024 15:05:49 +0200
Subject: [PATCH 1/2] feat(store): add Tail method

---
 headertest/store.go | 10 ++++++++++
 interface.go        |  3 +++
 p2p/server_test.go  |  6 ++++++
 store/store.go      | 16 ++++++++++++++++
 store/store_test.go | 14 ++++++++++++--
 5 files changed, 47 insertions(+), 2 deletions(-)

diff --git a/headertest/store.go b/headertest/store.go
index 8eae0e99..a6456d2a 100644
--- a/headertest/store.go
+++ b/headertest/store.go
@@ -51,6 +51,16 @@ 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) {
+	var err error
+	// TODO(cristaloleg): for now trying to return genesis. Fix for real oldest header.
+	h, ok := m.Headers[1]
+	if !ok {
+		err = header.ErrNotFound
+	}
+	return h, err
+}
+
 func (m *Store[H]) Get(ctx context.Context, hash header.Hash) (H, error) {
 	for _, header := range m.Headers {
 		if bytes.Equal(header.Hash(), hash) {
diff --git a/interface.go b/interface.go
index a2ab3e56..14ac4ca2 100644
--- a/interface.go
+++ b/interface.go
@@ -67,6 +67,9 @@ 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
 
diff --git a/p2p/server_test.go b/p2p/server_test.go
index 1e896b2e..7b6454be 100644
--- a/p2p/server_test.go
+++ b/p2p/server_test.go
@@ -135,6 +135,12 @@ 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
diff --git a/store/store.go b/store/store.go
index bd4cb3af..267fdcf3 100644
--- a/store/store.go
+++ b/store/store.go
@@ -191,6 +191,22 @@ 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
+	}
+
+	// TODO(cristaloleg): for now we return genesis header, return real tail.
+	tail, err := s.GetByHeight(ctx, 1)
+	if err != nil {
+		var zero H
+		return zero, nil
+	}
+	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 {
diff --git a/store/store_test.go b/store/store_test.go
index 9213f0b0..230539d4 100644
--- a/store/store_test.go
+++ b/store/store_test.go
@@ -19,8 +19,9 @@ 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())
 
@@ -86,6 +87,10 @@ func TestStore(t *testing.T) {
 	require.NoError(t, err)
 	assert.Equal(t, suite.Head().Hash(), head.Hash())
 
+	tail, err := store.Tail(ctx)
+	require.NoError(t, err)
+	assert.Equal(t, tail.Hash(), genesis.Hash())
+
 	out, err = store.getRangeByHeight(ctx, 1, 13)
 	require.NoError(t, err)
 	assert.Len(t, out, 12)
@@ -278,6 +283,11 @@ 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)
 }

From e84f4089407c6b22f8b1683e58fb3e5ee3136cab Mon Sep 17 00:00:00 2001
From: Oleg Kovalov <oleg@hey.com>
Date: Mon, 1 Jul 2024 17:59:54 +0200
Subject: [PATCH 2/2] store tailHeader

---
 store/store.go | 1 +
 1 file changed, 1 insertion(+)

diff --git a/store/store.go b/store/store.go
index 267fdcf3..a003bb3d 100644
--- a/store/store.go
+++ b/store/store.go
@@ -204,6 +204,7 @@ func (s *Store[H]) Tail(ctx context.Context) (H, error) {
 		var zero H
 		return zero, nil
 	}
+	s.tailHeader.Store(&tail)
 	return tail, nil
 }