kernel: Add functions to read block from disk to C header

diff --git a/src/kernel/bitcoinkernel.cpp b/src/kernel/bitcoinkernel.cpp
index 75348937..21193ce4 100644
--- a/src/kernel/bitcoinkernel.cpp
+++ b/src/kernel/bitcoinkernel.cpp
@@ -484,6 +484,7 @@ struct btck_Context : Handle<btck_Context, std::shared_ptr<const Context>> {};
 struct btck_ChainParameters : Handle<btck_ChainParameters, CChainParams> {};
 struct btck_ChainstateManagerOptions : Handle<btck_ChainstateManagerOptions, ChainstateManagerOptions> {};
 struct btck_ChainstateManager : Handle<btck_ChainstateManager, ChainMan> {};
+struct btck_Chain : Handle<btck_Chain, CChain> {};
 
 btck_Transaction* btck_transaction_create(const void* raw_transaction, size_t raw_transaction_len)
 {
@@ -769,6 +770,16 @@ void btck_context_destroy(btck_Context* context)
     delete context;
 }
 
+const btck_BlockTreeEntry* btck_block_tree_entry_get_previous(const btck_BlockTreeEntry* entry)
+{
+    if (!btck_BlockTreeEntry::get(entry).pprev) {
+        LogInfo("Genesis block has no previous.");
+        return nullptr;
+    }
+
+    return btck_BlockTreeEntry::ref(btck_BlockTreeEntry::get(entry).pprev);
+}
+
 btck_ValidationMode btck_block_validation_state_get_validation_mode(const btck_BlockValidationState* block_validation_state_)
 {
     auto& block_validation_state = btck_BlockValidationState::get(block_validation_state_);
@@ -983,6 +994,16 @@ void btck_block_destroy(btck_Block* block)
     delete block;
 }
 
+btck_Block* btck_block_read(const btck_ChainstateManager* chainman, const btck_BlockTreeEntry* entry)
+{
+    auto block{std::make_shared<CBlock>()};
+    if (!btck_ChainstateManager::get(chainman).m_chainman->m_blockman.ReadBlock(*block, btck_BlockTreeEntry::get(entry))) {
+        LogError("Failed to read block.");
+        return nullptr;
+    }
+    return btck_Block::create(block);
+}
+
 int btck_chainstate_manager_process_block(
     btck_ChainstateManager* chainman,
     const btck_Block* block,
@@ -995,3 +1016,20 @@ int btck_chainstate_manager_process_block(
     }
     return result ? 0 : -1;
 }
+
+const btck_Chain* btck_chainstate_manager_get_active_chain(const btck_ChainstateManager* chainman)
+{
+    return btck_Chain::ref(&WITH_LOCK(btck_ChainstateManager::get(chainman).m_chainman->GetMutex(), return btck_ChainstateManager::get(chainman).m_chainman->ActiveChain()));
+}
+
+const btck_BlockTreeEntry* btck_chain_get_tip(const btck_Chain* chain)
+{
+    LOCK(::cs_main);
+    return btck_BlockTreeEntry::ref(btck_Chain::get(chain).Tip());
+}
+
+int btck_chain_get_height(const btck_Chain* chain)
+{
+    LOCK(::cs_main);
+    return btck_Chain::get(chain).Height();
+}
diff --git a/src/kernel/bitcoinkernel.h b/src/kernel/bitcoinkernel.h
index 62db5daa..e8694514 100644
--- a/src/kernel/bitcoinkernel.h
+++ b/src/kernel/bitcoinkernel.h
@@ -206,6 +206,12 @@ typedef struct btck_Block btck_Block;
  */
 typedef struct btck_BlockValidationState btck_BlockValidationState;
 
+/**
+ * Opaque data structure for holding the currently known best-chain associated
+ * with a chainstate.
+ */
+typedef struct btck_Chain btck_Chain;
+
 /** Current sync state passed to tip changed callbacks. */
 typedef uint8_t btck_SynchronizationState;
 #define btck_SynchronizationState_INIT_REINDEX ((btck_SynchronizationState)(0))
@@ -800,6 +806,23 @@ BITCOINKERNEL_API void btck_context_destroy(btck_Context* context);
 
 ///@}
 
+/** @name BlockTreeEntry
+ * Functions for working with block tree entries.
+ */
+///@{
+
+/**
+ * @brief Returns the previous block tree entry in the chain, or null if the current
+ * block tree entry is the genesis block.
+ *
+ * @param[in] block_tree_entry Non-null.
+ * @return                     The previous block tree entry, or null on error or if the current block tree entry is the genesis block.
+ */
+BITCOINKERNEL_API const btck_BlockTreeEntry* BITCOINKERNEL_WARN_UNUSED_RESULT btck_block_tree_entry_get_previous(
+    const btck_BlockTreeEntry* block_tree_entry) BITCOINKERNEL_ARG_NONNULL(1);
+
+///@}
+
 /** @name ChainstateManagerOptions
  * Functions for working with chainstate manager options.
  */
@@ -931,6 +954,23 @@ BITCOINKERNEL_API int BITCOINKERNEL_WARN_UNUSED_RESULT btck_chainstate_manager_p
     const btck_Block* block,
     int* new_block) BITCOINKERNEL_ARG_NONNULL(1, 2, 3);
 
+/**
+ * @brief Returns the best known currently active chain. Its lifetime is
+ * dependent on the chainstate manager. It can be thought of as a view on a
+ * vector of block tree entries that form the best chain. The returned chain
+ * reference always points to the currently active best chain. However, state
+ * transitions within the chainstate manager (e.g., processing blocks) will
+ * update the chain's contents. Data retrieved from this chain is only
+ * consistent up to the point when new data is processed in the chainstate
+ * manager. It is the user's responsibility to guard against these
+ * inconsistencies.
+ *
+ * @param[in] chainstate_manager Non-null.
+ * @return                       The chain.
+ */
+BITCOINKERNEL_API const btck_Chain* BITCOINKERNEL_WARN_UNUSED_RESULT btck_chainstate_manager_get_active_chain(
+    const btck_ChainstateManager* chainstate_manager) BITCOINKERNEL_ARG_NONNULL(1);
+
 /**
  * Destroy the chainstate manager.
  */
@@ -943,6 +983,18 @@ BITCOINKERNEL_API void btck_chainstate_manager_destroy(btck_ChainstateManager* c
  */
 ///@{
 
+/**
+ * @brief Reads the block the passed in block tree entry points to from disk and
+ * returns it.
+ *
+ * @param[in] chainstate_manager Non-null.
+ * @param[in] block_tree_entry   Non-null.
+ * @return                       The read out block, or null on error.
+ */
+BITCOINKERNEL_API btck_Block* BITCOINKERNEL_WARN_UNUSED_RESULT btck_block_read(
+    const btck_ChainstateManager* chainstate_manager,
+    const btck_BlockTreeEntry* block_tree_entry) BITCOINKERNEL_ARG_NONNULL(1, 2);
+
 /**
  * @brief Parse a serialized raw block into a new block object.
  *
@@ -1024,6 +1076,32 @@ BITCOINKERNEL_API btck_BlockValidationResult btck_block_validation_state_get_blo
 
 ///@}
 
+/** @name Chain
+ * Functions for working with the chain
+ */
+///@{
+
+/**
+ * @brief Get the block tree entry of the current chain tip. Once returned,
+ * there is no guarantee that it remains in the active chain.
+ *
+ * @param[in] chain Non-null.
+ * @return          The block tree entry of the current tip, or null if the chain is empty.
+ */
+BITCOINKERNEL_API const btck_BlockTreeEntry* BITCOINKERNEL_WARN_UNUSED_RESULT btck_chain_get_tip(
+    const btck_Chain* chain) BITCOINKERNEL_ARG_NONNULL(1);
+
+/**
+ * @brief Return the height of the tip of the chain.
+ *
+ * @param[in] chain Non-null.
+ * @return          The current height.
+ */
+BITCOINKERNEL_API int32_t BITCOINKERNEL_WARN_UNUSED_RESULT btck_chain_get_height(
+    const btck_Chain* chain) BITCOINKERNEL_ARG_NONNULL(1);
+
+///@}
+
 #ifdef __cplusplus
 } // extern "C"
 #endif // __cplusplus
diff --git a/src/kernel/bitcoinkernel_wrapper.h b/src/kernel/bitcoinkernel_wrapper.h
index 53ecee3e..983a5e07 100644
--- a/src/kernel/bitcoinkernel_wrapper.h
+++ b/src/kernel/bitcoinkernel_wrapper.h
@@ -9,6 +9,7 @@
 
 #include <functional>
 #include <memory>
+#include <optional>
 #include <span>
 #include <stdexcept>
 #include <string>
@@ -598,13 +599,20 @@ public:
     }
 };
 
-class BlockTreeEntry : View<btck_BlockTreeEntry>
+class BlockTreeEntry : public View<btck_BlockTreeEntry>
 {
 public:
     BlockTreeEntry(const btck_BlockTreeEntry* entry)
         : View{entry}
     {
     }
+
+    std::optional<BlockTreeEntry> GetPrevious() const
+    {
+        auto entry{btck_block_tree_entry_get_previous(get())};
+        if (!entry) return std::nullopt;
+        return entry;
+    }
 };
 
 class KernelNotifications
@@ -766,6 +774,22 @@ public:
     }
 };
 
+class ChainView : public View<btck_Chain>
+{
+public:
+    explicit ChainView(const btck_Chain* ptr) : View{ptr} {}
+
+    BlockTreeEntry Tip() const
+    {
+        return btck_chain_get_tip(get());
+    }
+
+    int32_t Height() const
+    {
+        return btck_chain_get_height(get());
+    }
+};
+
 class ChainMan : UniqueHandle<btck_ChainstateManager, btck_chainstate_manager_destroy>
 {
 public:
@@ -795,6 +819,18 @@ public:
         if (new_block) *new_block = _new_block == 1;
         return res == 0;
     }
+
+    ChainView GetChain() const
+    {
+        return ChainView{btck_chainstate_manager_get_active_chain(get())};
+    }
+
+    std::optional<Block> ReadBlock(const BlockTreeEntry& entry) const
+    {
+        auto block{btck_block_read(get(), entry.get())};
+        if (!block) return std::nullopt;
+        return block;
+    }
 };
 
 } // namespace btck
diff --git a/src/test/kernel/test_kernel.cpp b/src/test/kernel/test_kernel.cpp
index fcb614c5..29765812 100644
--- a/src/test/kernel/test_kernel.cpp
+++ b/src/test/kernel/test_kernel.cpp
@@ -693,6 +693,20 @@ void chainman_mainnet_validation_test(TestDirectory& test_directory)
     BOOST_CHECK(!chainman->ProcessBlock(invalid_block, &new_block));
     BOOST_CHECK(!new_block);
 
+    auto chain{chainman->GetChain()};
+    BOOST_CHECK_EQUAL(chain.Height(), 1);
+    auto tip{chain.Tip()};
+    auto read_block{chainman->ReadBlock(tip)};
+    BOOST_REQUIRE(read_block);
+    check_equal(read_block.value().ToBytes(), raw_block);
+
+    // Check that we can read the previous block
+    auto tip_2{tip.GetPrevious()};
+    auto read_block_2{chainman->ReadBlock(tip_2.value())};
+
+    // It should be an error if we go another block back, since the genesis has no ancestor
+    BOOST_CHECK(!tip_2.value().GetPrevious());
+
     // If we try to validate it again, it should be a duplicate
     BOOST_CHECK(chainman->ProcessBlock(block, &new_block));
     BOOST_CHECK(!new_block);
@@ -758,4 +772,16 @@ BOOST_AUTO_TEST_CASE(btck_chainman_regtest_tests)
         BOOST_CHECK(chainman->ProcessBlock(block, &new_block));
         BOOST_CHECK(new_block);
     }
+
+    auto chain = chainman->GetChain();
+    auto tip = chain.Tip();
+    auto read_block = chainman->ReadBlock(tip).value();
+    check_equal(read_block.ToBytes(), hex_string_to_byte_vec(REGTEST_BLOCK_DATA[REGTEST_BLOCK_DATA.size() - 1]));
+
+    auto tip_2 = tip.GetPrevious().value();
+    auto read_block_2 = chainman->ReadBlock(tip_2).value();
+    check_equal(read_block_2.ToBytes(), hex_string_to_byte_vec(REGTEST_BLOCK_DATA[REGTEST_BLOCK_DATA.size() - 2]));
+
+    std::filesystem::remove_all(test_directory.m_directory / "blocks" / "blk00000.dat");
+    BOOST_CHECK(!chainman->ReadBlock(tip_2));
 }