kernel: Add functions for the block validation state to C header

These allow for the interpretation of the data in a `BlockChecked`
validation interface callback. The validation state passed through
`BlockChecked` is the source of truth for the validity of a block (the
mode). It is
also useful to get richer information in case a block failed to
validate (the result).

src/kernel/bitcoinkernel.cpp

@@ -769,6 +769,40 @@ void btck_context_destroy(btck_Context* context)
     delete context;
 }
 
+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_);
+    if (block_validation_state.IsValid()) return btck_ValidationMode_VALID;
+    if (block_validation_state.IsInvalid()) return btck_ValidationMode_INVALID;
+    return btck_ValidationMode_INTERNAL_ERROR;
+}
+
+btck_BlockValidationResult btck_block_validation_state_get_block_validation_result(const btck_BlockValidationState* block_validation_state_)
+{
+    auto& block_validation_state = btck_BlockValidationState::get(block_validation_state_);
+    switch (block_validation_state.GetResult()) {
+    case BlockValidationResult::BLOCK_RESULT_UNSET:
+        return btck_BlockValidationResult_UNSET;
+    case BlockValidationResult::BLOCK_CONSENSUS:
+        return btck_BlockValidationResult_CONSENSUS;
+    case BlockValidationResult::BLOCK_CACHED_INVALID:
+        return btck_BlockValidationResult_CACHED_INVALID;
+    case BlockValidationResult::BLOCK_INVALID_HEADER:
+        return btck_BlockValidationResult_INVALID_HEADER;
+    case BlockValidationResult::BLOCK_MUTATED:
+        return btck_BlockValidationResult_MUTATED;
+    case BlockValidationResult::BLOCK_MISSING_PREV:
+        return btck_BlockValidationResult_MISSING_PREV;
+    case BlockValidationResult::BLOCK_INVALID_PREV:
+        return btck_BlockValidationResult_INVALID_PREV;
+    case BlockValidationResult::BLOCK_TIME_FUTURE:
+        return btck_BlockValidationResult_TIME_FUTURE;
+    case BlockValidationResult::BLOCK_HEADER_LOW_WORK:
+        return btck_BlockValidationResult_HEADER_LOW_WORK;
+    } // no default case, so the compiler can warn about missing cases
+    assert(false);
+}
+
 btck_ChainstateManagerOptions* btck_chainstate_manager_options_create(const btck_Context* context, const char* data_dir, size_t data_dir_len, const char* blocks_dir, size_t blocks_dir_len)
 {
     try {

src/kernel/bitcoinkernel.h

@@ -249,6 +249,29 @@ 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);
 
+/**
+ * Whether a validated data structure is valid, invalid, or an error was
+ * encountered during processing.
+ */
+typedef uint8_t btck_ValidationMode;
+#define btck_ValidationMode_VALID ((btck_ValidationMode)(0))
+#define btck_ValidationMode_INVALID ((btck_ValidationMode)(1))
+#define btck_ValidationMode_INTERNAL_ERROR ((btck_ValidationMode)(2))
+
+/**
+ * A granular "reason" why a block was invalid.
+ */
+typedef uint32_t btck_BlockValidationResult;
+#define btck_BlockValidationResult_UNSET ((btck_BlockValidationResult)(0))           //!< initial value. Block has not yet been rejected
+#define btck_BlockValidationResult_CONSENSUS ((btck_BlockValidationResult)(1))       //!< invalid by consensus rules (excluding any below reasons)
+#define btck_BlockValidationResult_CACHED_INVALID ((btck_BlockValidationResult)(2))  //!< this block was cached as being invalid and we didn't store the reason why
+#define btck_BlockValidationResult_INVALID_HEADER ((btck_BlockValidationResult)(3))  //!< invalid proof of work or time too old
+#define btck_BlockValidationResult_MUTATED ((btck_BlockValidationResult)(4))         //!< the block's data didn't match the data committed to by the PoW
+#define btck_BlockValidationResult_MISSING_PREV ((btck_BlockValidationResult)(5))    //!< We don't have the previous block the checked one is built on
+#define btck_BlockValidationResult_INVALID_PREV ((btck_BlockValidationResult)(6))    //!< A block this one builds on is invalid
+#define btck_BlockValidationResult_TIME_FUTURE ((btck_BlockValidationResult)(7))     //!< block timestamp was > 2 hours in the future (or our clock is bad)
+#define btck_BlockValidationResult_HEADER_LOW_WORK ((btck_BlockValidationResult)(8)) //!< the block header may be on a too-little-work chain
+
 /**
  * Holds the validation interface callbacks. The user data pointer may be used
  * to point to user-defined structures to make processing the validation
@@ -967,6 +990,25 @@ BITCOINKERNEL_API void btck_block_destroy(btck_Block* block);
 
 ///@}
 
+/** @name BlockValidationState
+ * Functions for working with block validation states.
+ */
+///@{
+
+/**
+ * Returns the validation mode from an opaque block validation state pointer.
+ */
+BITCOINKERNEL_API btck_ValidationMode btck_block_validation_state_get_validation_mode(
+    const btck_BlockValidationState* block_validation_state) BITCOINKERNEL_ARG_NONNULL(1);
+
+/**
+ * Returns the validation result from an opaque block validation state pointer.
+ */
+BITCOINKERNEL_API btck_BlockValidationResult btck_block_validation_state_get_block_validation_result(
+    const btck_BlockValidationState* block_validation_state) BITCOINKERNEL_ARG_NONNULL(1);
+
+///@}
+
 #ifdef __cplusplus
 } // extern "C"
 #endif // __cplusplus

src/kernel/bitcoinkernel_wrapper.h

@@ -58,6 +58,24 @@ enum class Warning : btck_Warning {
     LARGE_WORK_INVALID_CHAIN = btck_Warning_LARGE_WORK_INVALID_CHAIN
 };
 
+enum class ValidationMode : btck_ValidationMode {
+    VALID = btck_ValidationMode_VALID,
+    INVALID = btck_ValidationMode_INVALID,
+    INTERNAL_ERROR = btck_ValidationMode_INTERNAL_ERROR
+};
+
+enum class BlockValidationResult : btck_BlockValidationResult {
+    UNSET = btck_BlockValidationResult_UNSET,
+    CONSENSUS = btck_BlockValidationResult_CONSENSUS,
+    CACHED_INVALID = btck_BlockValidationResult_CACHED_INVALID,
+    INVALID_HEADER = btck_BlockValidationResult_INVALID_HEADER,
+    MUTATED = btck_BlockValidationResult_MUTATED,
+    MISSING_PREV = btck_BlockValidationResult_MISSING_PREV,
+    INVALID_PREV = btck_BlockValidationResult_INVALID_PREV,
+    TIME_FUTURE = btck_BlockValidationResult_TIME_FUTURE,
+    HEADER_LOW_WORK = btck_BlockValidationResult_HEADER_LOW_WORK
+};
+
 enum class ScriptVerifyStatus : btck_ScriptVerifyStatus {
     OK = btck_ScriptVerifyStatus_OK,
     ERROR_INVALID_FLAGS_COMBINATION = btck_ScriptVerifyStatus_ERROR_INVALID_FLAGS_COMBINATION,
@@ -616,6 +634,16 @@ public:
     BlockValidationState& operator=(const BlockValidationState&) = delete;
     BlockValidationState(BlockValidationState&&) = delete;
     BlockValidationState& operator=(BlockValidationState&&) = delete;
+
+    ValidationMode GetValidationMode() const
+    {
+        return static_cast<ValidationMode>(btck_block_validation_state_get_validation_mode(m_state));
+    }
+
+    BlockValidationResult GetBlockValidationResult() const
+    {
+        return static_cast<BlockValidationResult>(btck_block_validation_state_get_block_validation_result(m_state));
+    }
 };
 
 class ValidationInterface

src/test/kernel/test_kernel.cpp

@@ -131,7 +131,53 @@ class TestValidationInterface : public ValidationInterface
 public:
     void BlockChecked(Block block, const BlockValidationState state) override
     {
-        std::cout << "Block checked." << std::endl;
+        std::cout << "Block checked: ";
+
+        auto mode{state.GetValidationMode()};
+        switch (mode) {
+        case ValidationMode::VALID: {
+            std::cout << "Valid block" << std::endl;
+            return;
+        }
+        case ValidationMode::INVALID: {
+            std::cout << "Invalid block: ";
+            auto result{state.GetBlockValidationResult()};
+            switch (result) {
+            case BlockValidationResult::UNSET:
+                std::cout << "initial value. Block has not yet been rejected" << std::endl;
+                break;
+            case BlockValidationResult::HEADER_LOW_WORK:
+                std::cout << "the block header may be on a too-little-work chain" << std::endl;
+                break;
+            case BlockValidationResult::CONSENSUS:
+                std::cout << "invalid by consensus rules (excluding any below reasons)" << std::endl;
+                break;
+            case BlockValidationResult::CACHED_INVALID:
+                std::cout << "this block was cached as being invalid and we didn't store the reason why" << std::endl;
+                break;
+            case BlockValidationResult::INVALID_HEADER:
+                std::cout << "invalid proof of work or time too old" << std::endl;
+                break;
+            case BlockValidationResult::MUTATED:
+                std::cout << "the block's data didn't match the data committed to by the PoW" << std::endl;
+                break;
+            case BlockValidationResult::MISSING_PREV:
+                std::cout << "We don't have the previous block the checked one is built on" << std::endl;
+                break;
+            case BlockValidationResult::INVALID_PREV:
+                std::cout << "A block this one builds on is invalid" << std::endl;
+                break;
+            case BlockValidationResult::TIME_FUTURE:
+                std::cout << "block timestamp was > 2 hours in the future (or our clock is bad)" << std::endl;
+                break;
+            }
+            return;
+        }
+        case ValidationMode::INTERNAL_ERROR: {
+            std::cout << "Internal error" << std::endl;
+            return;
+        }
+        }
     }
 
     void BlockConnected(Block block, BlockTreeEntry entry) override