kernel: Add function for copying block data to C header

This adds a function for streaming bytes into a user-owned data
structure.

Use it in the tests for verifying the implementation of the validation
interface's `BlockChecked` method.

src/kernel/bitcoinkernel.cpp

@@ -967,6 +967,17 @@ const btck_Transaction* btck_block_get_transaction_at(const btck_Block* block, s
     return btck_Transaction::ref(&btck_Block::get(block)->vtx[index]);
 }
 
+int btck_block_to_bytes(const btck_Block* block, btck_WriteBytes writer, void* user_data)
+{
+    try {
+        WriterStream ws{writer, user_data};
+        ws << TX_WITH_WITNESS(*btck_Block::get(block));
+        return 0;
+    } catch (...) {
+        return -1;
+    }
+}
+
 void btck_block_destroy(btck_Block* block)
 {
     delete block;

src/kernel/bitcoinkernel.h

@@ -249,6 +249,11 @@ typedef void (*btck_ValidationInterfacePoWValidBlock)(void* user_data, btck_Bloc
 typedef void (*btck_ValidationInterfaceBlockConnected)(void* user_data, btck_Block* block, const btck_BlockTreeEntry* entry);
 typedef void (*btck_ValidationInterfaceBlockDisconnected)(void* user_data, btck_Block* block, const btck_BlockTreeEntry* entry);
 
+/**
+ * Function signature for serializing data.
+ */
+typedef int (*btck_WriteBytes)(const void* bytes, size_t size, void* userdata);
+
 /**
  * Whether a validated data structure is valid, invalid, or an error was
  * encountered during processing.
@@ -389,11 +394,6 @@ typedef uint8_t btck_ChainType;
 #define btck_ChainType_SIGNET ((btck_ChainType)(3))
 #define btck_ChainType_REGTEST ((btck_ChainType)(4))
 
-/**
- * Function signature for serializing data.
- */
-typedef int (*btck_WriteBytes)(const void* bytes, size_t size, void* userdata);
-
 /** @name Transaction
  * Functions for working with transactions.
  */
@@ -983,6 +983,21 @@ BITCOINKERNEL_API size_t BITCOINKERNEL_WARN_UNUSED_RESULT btck_block_count_trans
 BITCOINKERNEL_API const btck_Transaction* BITCOINKERNEL_WARN_UNUSED_RESULT btck_block_get_transaction_at(
     const btck_Block* block, size_t transaction_index) BITCOINKERNEL_ARG_NONNULL(1);
 
+/**
+ * @brief Serializes the block through the passed in callback to bytes.
+ * This is consensus serialization that is also used for the P2P network.
+ *
+ * @param[in] block     Non-null.
+ * @param[in] writer    Non-null, callback to a write bytes function.
+ * @param[in] user_data Holds a user-defined opaque structure that will be
+ *                      passed back through the writer callback.
+ * @return              0 on success.
+ */
+BITCOINKERNEL_API int btck_block_to_bytes(
+    const btck_Block* block,
+    btck_WriteBytes writer,
+    void* user_data) BITCOINKERNEL_ARG_NONNULL(1, 2);
+
 /**
  * Destroy the block.
  */

src/kernel/bitcoinkernel_wrapper.h

@@ -548,6 +548,11 @@ public:
     }
 
     MAKE_RANGE_METHOD(Transactions, Block, &Block::CountTransactions, &Block::GetTransaction, *this)
+
+    std::vector<std::byte> ToBytes() const
+    {
+        return write_bytes(get(), btck_block_to_bytes);
+    }
 };
 
 inline void logging_disable()

src/test/kernel/test_kernel.cpp

@@ -16,6 +16,7 @@
 #include <filesystem>
 #include <iostream>
 #include <memory>
+#include <optional>
 #include <random>
 #include <ranges>
 #include <span>
@@ -129,9 +130,14 @@ public:
 class TestValidationInterface : public ValidationInterface
 {
 public:
+    std::optional<std::vector<std::byte>> m_expected_valid_block = std::nullopt;
+
     void BlockChecked(Block block, const BlockValidationState state) override
     {
-        std::cout << "Block checked: ";
+        if (m_expected_valid_block.has_value()) {
+            auto ser_block{block.ToBytes()};
+            check_equal(m_expected_valid_block.value(), ser_block);
+        }
 
         auto mode{state.GetValidationMode()};
         switch (mode) {
@@ -545,8 +551,8 @@ BOOST_AUTO_TEST_CASE(btck_context_tests)
 BOOST_AUTO_TEST_CASE(btck_block)
 {
     Block block{hex_string_to_byte_vec(REGTEST_BLOCK_DATA[0])};
-    Block block_1{hex_string_to_byte_vec(REGTEST_BLOCK_DATA[1])};
-    CheckHandle(block, block_1);
+    Block block_100{hex_string_to_byte_vec(REGTEST_BLOCK_DATA[100])};
+    CheckHandle(block, block_100);
     Block block_tx{hex_string_to_byte_vec(REGTEST_BLOCK_DATA[205])};
     CheckRange(block_tx.Transactions(), block_tx.CountTransactions());
 }
@@ -673,10 +679,20 @@ void chainman_mainnet_validation_test(TestDirectory& test_directory)
                                return tx.CountOutputs();
                            })).begin();
     BOOST_CHECK_EQUAL(output_counts, 1);
+
+    validation_interface->m_expected_valid_block.emplace(raw_block);
+    auto ser_block{block.ToBytes()};
+    check_equal(ser_block, raw_block);
     bool new_block = false;
     BOOST_CHECK(chainman->ProcessBlock(block, &new_block));
     BOOST_CHECK(new_block);
 
+    validation_interface->m_expected_valid_block = std::nullopt;
+    new_block = false;
+    Block invalid_block{hex_string_to_byte_vec(REGTEST_BLOCK_DATA[REGTEST_BLOCK_DATA.size() - 1])};
+    BOOST_CHECK(!chainman->ProcessBlock(invalid_block, &new_block));
+    BOOST_CHECK(!new_block);
+
     // If we try to validate it again, it should be a duplicate
     BOOST_CHECK(chainman->ProcessBlock(block, &new_block));
     BOOST_CHECK(!new_block);