kernel: Add notifications context option to C header

diff --git a/src/kernel/bitcoinkernel.cpp b/src/kernel/bitcoinkernel.cpp
index 35e03372..2670287a 100644
--- a/src/kernel/bitcoinkernel.cpp
+++ b/src/kernel/bitcoinkernel.cpp
@@ -12,6 +12,7 @@
 #include <kernel/context.h>
 #include <kernel/cs_main.h>
 #include <kernel/notifications_interface.h>
+#include <kernel/warning.h>
 #include <logging.h>
 #include <primitives/transaction.h>
 #include <script/interpreter.h>
@@ -23,6 +24,7 @@
 #include <util/result.h>
 #include <util/signalinterrupt.h>
 #include <util/translation.h>
+#include <validation.h>
 
 #include <cassert>
 #include <cstddef>
@@ -36,6 +38,8 @@
 #include <utility>
 #include <vector>
 
+class CBlockIndex;
+
 // Define G_TRANSLATION_FUN symbol in libbitcoinkernel library so users of the
 // library aren't required to export this symbol
 extern const std::function<std::string(const char*)> G_TRANSLATION_FUN{nullptr};
@@ -120,6 +124,12 @@ struct Handle {
     }
 };
 
+} // namespace
+
+struct btck_BlockTreeEntry: Handle<btck_BlockTreeEntry, CBlockIndex> {};
+
+namespace {
+
 BCLog::Level get_bclog_level(btck_LogLevel level)
 {
     switch (level) {
@@ -176,6 +186,30 @@ BCLog::LogFlags get_bclog_flag(btck_LogCategory category)
     assert(false);
 }
 
+btck_SynchronizationState cast_state(SynchronizationState state)
+{
+    switch (state) {
+    case SynchronizationState::INIT_REINDEX:
+        return btck_SynchronizationState_INIT_REINDEX;
+    case SynchronizationState::INIT_DOWNLOAD:
+        return btck_SynchronizationState_INIT_DOWNLOAD;
+    case SynchronizationState::POST_INIT:
+        return btck_SynchronizationState_POST_INIT;
+    } // no default case, so the compiler can warn about missing cases
+    assert(false);
+}
+
+btck_Warning cast_btck_warning(kernel::Warning warning)
+{
+    switch (warning) {
+    case kernel::Warning::UNKNOWN_NEW_RULES_ACTIVATED:
+        return btck_Warning_UNKNOWN_NEW_RULES_ACTIVATED;
+    case kernel::Warning::LARGE_WORK_INVALID_CHAIN:
+        return btck_Warning_LARGE_WORK_INVALID_CHAIN;
+    } // no default case, so the compiler can warn about missing cases
+    assert(false);
+}
+
 struct LoggingConnection {
     std::unique_ptr<std::list<std::function<void(const std::string&)>>::iterator> m_connection;
     void* m_user_data;
@@ -224,9 +258,61 @@ struct LoggingConnection {
     }
 };
 
+class KernelNotifications final : public kernel::Notifications
+{
+private:
+    btck_NotificationInterfaceCallbacks m_cbs;
+
+public:
+    KernelNotifications(btck_NotificationInterfaceCallbacks cbs)
+        : m_cbs{cbs}
+    {
+    }
+
+    ~KernelNotifications()
+    {
+        if (m_cbs.user_data && m_cbs.user_data_destroy) {
+            m_cbs.user_data_destroy(m_cbs.user_data);
+        }
+        m_cbs.user_data_destroy = nullptr;
+        m_cbs.user_data = nullptr;
+    }
+
+    kernel::InterruptResult blockTip(SynchronizationState state, const CBlockIndex& index, double verification_progress) override
+    {
+        if (m_cbs.block_tip) m_cbs.block_tip(m_cbs.user_data, cast_state(state), btck_BlockTreeEntry::ref(&index), verification_progress);
+        return {};
+    }
+    void headerTip(SynchronizationState state, int64_t height, int64_t timestamp, bool presync) override
+    {
+        if (m_cbs.header_tip) m_cbs.header_tip(m_cbs.user_data, cast_state(state), height, timestamp, presync ? 1 : 0);
+    }
+    void progress(const bilingual_str& title, int progress_percent, bool resume_possible) override
+    {
+        if (m_cbs.progress) m_cbs.progress(m_cbs.user_data, title.original.c_str(), title.original.length(), progress_percent, resume_possible ? 1 : 0);
+    }
+    void warningSet(kernel::Warning id, const bilingual_str& message) override
+    {
+        if (m_cbs.warning_set) m_cbs.warning_set(m_cbs.user_data, cast_btck_warning(id), message.original.c_str(), message.original.length());
+    }
+    void warningUnset(kernel::Warning id) override
+    {
+        if (m_cbs.warning_unset) m_cbs.warning_unset(m_cbs.user_data, cast_btck_warning(id));
+    }
+    void flushError(const bilingual_str& message) override
+    {
+        if (m_cbs.flush_error) m_cbs.flush_error(m_cbs.user_data, message.original.c_str(), message.original.length());
+    }
+    void fatalError(const bilingual_str& message) override
+    {
+        if (m_cbs.fatal_error) m_cbs.fatal_error(m_cbs.user_data, message.original.c_str(), message.original.length());
+    }
+};
+
 struct ContextOptions {
     mutable Mutex m_mutex;
     std::unique_ptr<const CChainParams> m_chainparams GUARDED_BY(m_mutex);
+    std::shared_ptr<KernelNotifications> m_notifications GUARDED_BY(m_mutex);
 };
 
 class Context
@@ -234,7 +320,7 @@ class Context
 public:
     std::unique_ptr<kernel::Context> m_context;
 
-    std::unique_ptr<kernel::Notifications> m_notifications;
+    std::shared_ptr<KernelNotifications> m_notifications;
 
     std::unique_ptr<util::SignalInterrupt> m_interrupt;
 
@@ -242,7 +328,6 @@ public:
 
     Context(const ContextOptions* options, bool& sane)
         : m_context{std::make_unique<kernel::Context>()},
-          m_notifications{std::make_unique<kernel::Notifications>()},
           m_interrupt{std::make_unique<util::SignalInterrupt>()}
     {
         if (options) {
@@ -250,11 +335,18 @@ public:
             if (options->m_chainparams) {
                 m_chainparams = std::make_unique<const CChainParams>(*options->m_chainparams);
             }
+            if (options->m_notifications) {
+                m_notifications = options->m_notifications;
+            }
         }
 
         if (!m_chainparams) {
             m_chainparams = CChainParams::Main();
         }
+        if (!m_notifications) {
+            m_notifications = std::make_shared<KernelNotifications>(btck_NotificationInterfaceCallbacks{
+                nullptr, nullptr, nullptr, nullptr, nullptr, nullptr, nullptr, nullptr, nullptr});
+        }
 
         if (!kernel::SanityChecks(*m_context)) {
             sane = false;
@@ -511,6 +603,13 @@ void btck_context_options_set_chainparams(btck_ContextOptions* options, const bt
     btck_ContextOptions::get(options).m_chainparams = std::make_unique<const CChainParams>(btck_ChainParameters::get(chain_parameters));
 }
 
+void btck_context_options_set_notifications(btck_ContextOptions* options, btck_NotificationInterfaceCallbacks notifications)
+{
+    // The KernelNotifications are copy-initialized, so the caller can free them again.
+    LOCK(btck_ContextOptions::get(options).m_mutex);
+    btck_ContextOptions::get(options).m_notifications = std::make_shared<KernelNotifications>(notifications);
+}
+
 void btck_context_options_destroy(btck_ContextOptions* options)
 {
     delete options;
diff --git a/src/kernel/bitcoinkernel.h b/src/kernel/bitcoinkernel.h
index f901dedd..25f39a11 100644
--- a/src/kernel/bitcoinkernel.h
+++ b/src/kernel/bitcoinkernel.h
@@ -67,6 +67,11 @@ extern "C" {
  * functions, e.g. for scripts, may communicate more detailed error information
  * through status code out parameters.
  *
+ * The kernel notifications issue callbacks for errors. These are usually
+ * indicative of a system error. If such an error is issued, it is recommended
+ * to halt and tear down the existing kernel objects. Remediating the error may
+ * require system intervention by the user.
+ *
  * @section pointer Pointer and argument conventions
  *
  * The user is responsible for de-allocating the memory owned by pointers
@@ -148,6 +153,30 @@ typedef struct btck_ContextOptions btck_ContextOptions;
  */
 typedef struct btck_Context btck_Context;
 
+/**
+ * Opaque data structure for holding a block tree entry.
+ *
+ * This is a pointer to an element in the block index currently in memory of
+ * the chainstate manager. It is valid for the lifetime of the chainstate
+ * manager it was retrieved from. The entry is part of a tree-like structure
+ * that is maintained internally. Every entry, besides the genesis, points to a
+ * single parent. Multiple entries may share a parent, thus forming a tree.
+ * Each entry corresponds to a single block and may be used to retrieve its
+ * data and validation status.
+ */
+typedef struct btck_BlockTreeEntry btck_BlockTreeEntry;
+
+/** Current sync state passed to tip changed callbacks. */
+typedef uint8_t btck_SynchronizationState;
+#define btck_SynchronizationState_INIT_REINDEX ((btck_SynchronizationState)(0))
+#define btck_SynchronizationState_INIT_DOWNLOAD ((btck_SynchronizationState)(1))
+#define btck_SynchronizationState_POST_INIT ((btck_SynchronizationState)(2))
+
+/** Possible warning types issued by validation. */
+typedef uint8_t btck_Warning;
+#define btck_Warning_UNKNOWN_NEW_RULES_ACTIVATED ((btck_Warning)(0))
+#define btck_Warning_LARGE_WORK_INVALID_CHAIN ((btck_Warning)(1))
+
 /** Callback function types */
 
 /**
@@ -161,6 +190,43 @@ typedef void (*btck_LogCallback)(void* user_data, const char* message, size_t me
  */
 typedef void (*btck_DestroyCallback)(void* user_data);
 
+/**
+ * Function signatures for the kernel notifications.
+ */
+typedef void (*btck_NotifyBlockTip)(void* user_data, btck_SynchronizationState state, const btck_BlockTreeEntry* entry, double verification_progress);
+typedef void (*btck_NotifyHeaderTip)(void* user_data, btck_SynchronizationState state, int64_t height, int64_t timestamp, int presync);
+typedef void (*btck_NotifyProgress)(void* user_data, const char* title, size_t title_len, int progress_percent, int resume_possible);
+typedef void (*btck_NotifyWarningSet)(void* user_data, btck_Warning warning, const char* message, size_t message_len);
+typedef void (*btck_NotifyWarningUnset)(void* user_data, btck_Warning warning);
+typedef void (*btck_NotifyFlushError)(void* user_data, const char* message, size_t message_len);
+typedef void (*btck_NotifyFatalError)(void* user_data, const char* message, size_t message_len);
+
+/**
+ * A struct for holding the kernel notification callbacks. The user data
+ * pointer may be used to point to user-defined structures to make processing
+ * the notifications easier.
+ *
+ * If user_data_destroy is provided, the kernel will automatically call this
+ * callback to clean up user_data when the notification interface object is destroyed.
+ * If user_data_destroy is NULL, it is the user's responsibility to ensure that
+ * the user_data outlives the kernel objects. Notifications can
+ * occur even as kernel objects are deleted, so care has to be taken to ensure
+ * safe unwinding.
+ */
+typedef struct {
+    void* user_data;                        //!< Holds a user-defined opaque structure that is passed to the notification callbacks.
+                                            //!< If user_data_destroy is also defined ownership of the user_data is passed to the
+                                            //!< created context options and subsequently context.
+    btck_DestroyCallback user_data_destroy; //!< Frees the provided user data structure.
+    btck_NotifyBlockTip block_tip;          //!< The chain's tip was updated to the provided block entry.
+    btck_NotifyHeaderTip header_tip;        //!< A new best block header was added.
+    btck_NotifyProgress progress;           //!< Reports on current block synchronization progress.
+    btck_NotifyWarningSet warning_set;      //!< A warning issued by the kernel library during validation.
+    btck_NotifyWarningUnset warning_unset;  //!< A previous condition leading to the issuance of a warning is no longer given.
+    btck_NotifyFlushError flush_error;      //!< An error encountered when flushing data to disk.
+    btck_NotifyFatalError fatal_error;      //!< A un-recoverable system error encountered by the library.
+} btck_NotificationInterfaceCallbacks;
+
 /**
  * A collection of logging categories that may be encountered by kernel code.
  */
@@ -572,6 +638,17 @@ BITCOINKERNEL_API void btck_context_options_set_chainparams(
     btck_ContextOptions* context_options,
     const btck_ChainParameters* chain_parameters) BITCOINKERNEL_ARG_NONNULL(1, 2);
 
+/**
+ * @brief Set the kernel notifications for the context options. The context
+ * created with the options will be configured with these notifications.
+ *
+ * @param[in] context_options Non-null, previously created by @ref btck_context_options_create.
+ * @param[in] notifications   Is set to the context options.
+ */
+BITCOINKERNEL_API void btck_context_options_set_notifications(
+    btck_ContextOptions* context_options,
+    btck_NotificationInterfaceCallbacks notifications) BITCOINKERNEL_ARG_NONNULL(1);
+
 /**
  * Destroy the context options.
  */
@@ -591,7 +668,7 @@ BITCOINKERNEL_API void btck_context_options_destroy(btck_ContextOptions* context
  * kernel notification callbacks.
  *
  * @param[in] context_options Nullable, created by @ref btck_context_options_create.
- * @return                    The allocated kernel context, or null on error.
+ * @return                    The allocated context, or null on error.
  */
 BITCOINKERNEL_API btck_Context* BITCOINKERNEL_WARN_UNUSED_RESULT btck_context_create(
     const btck_ContextOptions* context_options);
diff --git a/src/kernel/bitcoinkernel_wrapper.h b/src/kernel/bitcoinkernel_wrapper.h
index b550c3a5..8e30b6ac 100644
--- a/src/kernel/bitcoinkernel_wrapper.h
+++ b/src/kernel/bitcoinkernel_wrapper.h
@@ -46,6 +46,17 @@ enum class ChainType : btck_ChainType {
     REGTEST = btck_ChainType_REGTEST
 };
 
+enum class SynchronizationState : btck_SynchronizationState {
+    INIT_REINDEX = btck_SynchronizationState_INIT_REINDEX,
+    INIT_DOWNLOAD = btck_SynchronizationState_INIT_DOWNLOAD,
+    POST_INIT = btck_SynchronizationState_POST_INIT
+};
+
+enum class Warning : btck_Warning {
+    UNKNOWN_NEW_RULES_ACTIVATED = btck_Warning_UNKNOWN_NEW_RULES_ACTIVATED,
+    LARGE_WORK_INVALID_CHAIN = btck_Warning_LARGE_WORK_INVALID_CHAIN
+};
+
 enum class ScriptVerifyStatus : btck_ScriptVerifyStatus {
     OK = btck_ScriptVerifyStatus_OK,
     ERROR_INVALID_FLAGS_COMBINATION = btck_ScriptVerifyStatus_ERROR_INVALID_FLAGS_COMBINATION,
@@ -531,6 +542,35 @@ public:
     }
 };
 
+class BlockTreeEntry : View<btck_BlockTreeEntry>
+{
+public:
+    BlockTreeEntry(const btck_BlockTreeEntry* entry)
+        : View{entry}
+    {
+    }
+};
+
+class KernelNotifications
+{
+public:
+    virtual ~KernelNotifications() = default;
+
+    virtual void BlockTipHandler(SynchronizationState state, BlockTreeEntry entry, double verification_progress) {}
+
+    virtual void HeaderTipHandler(SynchronizationState state, int64_t height, int64_t timestamp, bool presync) {}
+
+    virtual void ProgressHandler(std::string_view title, int progress_percent, bool resume_possible) {}
+
+    virtual void WarningSetHandler(Warning warning, std::string_view message) {}
+
+    virtual void WarningUnsetHandler(Warning warning) {}
+
+    virtual void FlushErrorHandler(std::string_view error) {}
+
+    virtual void FatalErrorHandler(std::string_view error) {}
+};
+
 class ChainParams : public Handle<btck_ChainParameters, btck_chain_parameters_copy, btck_chain_parameters_destroy>
 {
 public:
@@ -547,6 +587,27 @@ public:
     {
         btck_context_options_set_chainparams(get(), chain_params.get());
     }
+
+    template <typename T>
+    void SetNotifications(std::shared_ptr<T> notifications)
+    {
+        static_assert(std::is_base_of_v<KernelNotifications, T>);
+        auto heap_notifications = std::make_unique<std::shared_ptr<T>>(std::move(notifications));
+        using user_type = std::shared_ptr<T>*;
+        btck_context_options_set_notifications(
+            get(),
+            btck_NotificationInterfaceCallbacks{
+                .user_data = heap_notifications.release(),
+                .user_data_destroy = +[](void* user_data) { delete static_cast<user_type>(user_data); },
+                .block_tip = +[](void* user_data, btck_SynchronizationState state, const btck_BlockTreeEntry* entry, double verification_progress) { (*static_cast<user_type>(user_data))->BlockTipHandler(static_cast<SynchronizationState>(state), BlockTreeEntry{entry}, verification_progress); },
+                .header_tip = +[](void* user_data, btck_SynchronizationState state, int64_t height, int64_t timestamp, int presync) { (*static_cast<user_type>(user_data))->HeaderTipHandler(static_cast<SynchronizationState>(state), height, timestamp, presync == 1); },
+                .progress = +[](void* user_data, const char* title, size_t title_len, int progress_percent, int resume_possible) { (*static_cast<user_type>(user_data))->ProgressHandler({title, title_len}, progress_percent, resume_possible == 1); },
+                .warning_set = +[](void* user_data, btck_Warning warning, const char* message, size_t message_len) { (*static_cast<user_type>(user_data))->WarningSetHandler(static_cast<Warning>(warning), {message, message_len}); },
+                .warning_unset = +[](void* user_data, btck_Warning warning) { (*static_cast<user_type>(user_data))->WarningUnsetHandler(static_cast<Warning>(warning)); },
+                .flush_error = +[](void* user_data, const char* error, size_t error_len) { (*static_cast<user_type>(user_data))->FlushErrorHandler({error, error_len}); },
+                .fatal_error = +[](void* user_data, const char* error, size_t error_len) { (*static_cast<user_type>(user_data))->FatalErrorHandler({error, error_len}); },
+            });
+    }
 };
 
 class Context : public Handle<btck_Context, btck_context_copy, btck_context_destroy>
diff --git a/src/test/kernel/test_kernel.cpp b/src/test/kernel/test_kernel.cpp
index 8fc0308c..ef669cb6 100644
--- a/src/test/kernel/test_kernel.cpp
+++ b/src/test/kernel/test_kernel.cpp
@@ -60,6 +60,35 @@ public:
     }
 };
 
+class TestKernelNotifications : public KernelNotifications
+{
+public:
+    void HeaderTipHandler(SynchronizationState state, int64_t height, int64_t timestamp, bool presync) override
+    {
+        BOOST_CHECK_GT(timestamp, 0);
+    }
+
+    void WarningSetHandler(Warning warning, std::string_view message) override
+    {
+        std::cout << "Kernel warning is set: " << message << std::endl;
+    }
+
+    void WarningUnsetHandler(Warning warning) override
+    {
+        std::cout << "Kernel warning was unset." << std::endl;
+    }
+
+    void FlushErrorHandler(std::string_view error) override
+    {
+        std::cout << error << std::endl;
+    }
+
+    void FatalErrorHandler(std::string_view error) override
+    {
+        std::cout << error << std::endl;
+    }
+};
+
 void run_verify_test(
     const ScriptPubkey& spent_script_pubkey,
     const Transaction& spending_tx,
@@ -401,6 +430,7 @@ BOOST_AUTO_TEST_CASE(btck_context_tests)
         ChainParams regtest_params{ChainType::REGTEST};
         CheckHandle(params, regtest_params);
         options.SetChainParams(params);
+        options.SetNotifications(std::make_shared<TestKernelNotifications>());
         Context context{options};
     }
 }