signals: Add a simplified boost-compatible implementation

diff --git a/src/btcsignals.h b/src/btcsignals.h
index 415d8b86..f4c2530b 100644
--- a/src/btcsignals.h
+++ b/src/btcsignals.h
@@ -5,10 +5,252 @@
 #ifndef BITCOIN_BTCSIGNALS_H
 #define BITCOIN_BTCSIGNALS_H
 
-#include <boost/signals2/connection.hpp>
-#include <boost/signals2/optional_last_value.hpp>
-#include <boost/signals2/signal.hpp>
+#include <sync.h>
 
-namespace btcsignals = boost::signals2;
+#include <algorithm>
+#include <atomic>
+#include <functional>
+#include <memory>
+#include <optional>
+#include <type_traits>
+#include <utility>
+#include <vector>
+
+/**
+ * btcsignals is a simple mechanism for signaling events to multiple subscribers.
+ * It is api-compatible with a minimal subset of boost::signals2.
+ *
+ * Rather than using a custom slot type, and the features/complexity that they
+ * imply, std::function is used to store the callbacks. Lifetime management of
+ * the callbacks is left up to the user.
+ *
+ * All usage is thread-safe except for interacting with a connection while
+ * copying/moving it on another thread.
+ */
+
+namespace btcsignals {
+
+/*
+ * optional_last_value is the default and only supported combiner.
+ * As such, its behavior is embedded into the signal functor.
+ *
+ * Because optional<void> is undefined, void must be special-cased.
+ */
+
+template <typename T>
+class optional_last_value
+{
+public:
+    using result_type = std::conditional_t<std::is_void_v<T>, void, std::optional<T>>;
+};
+
+template <typename Signature, typename Combiner = optional_last_value<typename std::function<Signature>::result_type>>
+class signal;
+
+/*
+ * State object representing the liveness of a registered callback.
+ * signal::connect() returns an enabled connection which can be held and
+ * disabled in the future.
+ */
+class connection
+{
+    template <typename Signature, typename Combiner>
+    friend class signal;
+
+    /*
+     * Tag for the constructor used by signal.
+     */
+    struct enabled_tag_type {
+    };
+    static constexpr enabled_tag_type enabled_tag{};
+
+    /**
+     * connections have shared_ptr-like copy and move semantics.
+     */
+    std::shared_ptr<std::atomic_bool> m_connected{};
+
+    /**
+     * Only a signal can create an enabled connection.
+     */
+    explicit connection(enabled_tag_type /*unused*/) : m_connected{std::make_shared<std::atomic_bool>(true)} {}
+
+public:
+    /**
+     * The default constructor creates a connection with no associated signal
+     */
+    constexpr connection() noexcept = default;
+
+    /**
+     * If a callback is associated with this connection, prevent it from being
+     * called in the future.
+     *
+     * If a connection is disabled as part of a signal's callback function, it
+     * will _not_ be executed in the current signal invocation.
+     *
+     * Note that disconnected callbacks are not removed from their owning
+     * signals here. They are garbage collected in signal::connect().
+     */
+    void disconnect()
+    {
+        if (m_connected) {
+            m_connected->store(false);
+        }
+    }
+
+    /**
+     * Returns true if this connection was created by a signal and has not been
+     * disabled.
+     */
+    bool connected() const
+    {
+        return m_connected && m_connected->load();
+    }
+};
+
+/*
+ * RAII-style connection management
+ */
+class scoped_connection
+{
+    connection m_conn;
+
+public:
+    scoped_connection(connection rhs) noexcept : m_conn{std::move(rhs)} {}
+
+    scoped_connection(scoped_connection&&) noexcept = default;
+    scoped_connection& operator=(scoped_connection&&) noexcept = default;
+
+    /**
+     * For simplicity, disable copy assignment and construction.
+     */
+    scoped_connection& operator=(const scoped_connection&) = delete;
+    scoped_connection(const scoped_connection&) = delete;
+
+    void disconnect()
+    {
+        m_conn.disconnect();
+    }
+
+    ~scoped_connection()
+    {
+        disconnect();
+    }
+};
+
+/*
+ * Functor for calling zero or more connected callbacks
+ */
+template <typename Signature, typename Combiner>
+class signal
+{
+    using function_type = std::function<Signature>;
+
+    static_assert(std::is_same_v<Combiner, optional_last_value<typename function_type::result_type>>, "only the optional_last_value combiner is supported");
+
+    /*
+     * Helper struct for maintaining a callback and its associated connection
+     */
+    struct connection_holder {
+        template <typename Callable>
+        connection_holder(Callable&& callback) : m_callback{std::forward<Callable>(callback)}
+        {
+        }
+
+        connection m_connection{connection::enabled_tag};
+        function_type m_callback;
+    };
+
+    mutable Mutex m_mutex;
+
+    /* Store connection_holders as shared_ptrs to avoid having to copy them by
+     * value in operator().
+     */
+    std::vector<std::shared_ptr<connection_holder>> m_connections GUARDED_BY(m_mutex){};
+
+public:
+    using result_type = Combiner::result_type;
+
+    constexpr signal() noexcept = default;
+    ~signal() = default;
+
+    /*
+     * For simplicity, disable all moving/copying/assigning.
+     */
+    signal(const signal&) = delete;
+    signal(signal&&) = delete;
+    signal& operator=(const signal&) = delete;
+    signal& operator=(signal&&) = delete;
+
+    /*
+     * Execute all enabled callbacks for the signal. Rather than allowing for
+     * custom combiners, the behavior of optional_last_value is hard-coded
+     * here. Return the value of the last executed callback, or nullopt if none
+     * were executed.
+     *
+     * Callbacks which return void require special handling.
+     *
+     * In order to avoid locking during the callbacks, the list of callbacks is
+     * cached before they are called. This allows a callback to call connect(),
+     * but the newly connected callback will not be run during the current
+     * signal invocation.
+     *
+     * Note that the parameters are accepted as universal references, though
+     * they are not perfectly forwarded as that could cause a use-after-move if
+     * more than one callback is enabled.
+     */
+    template <typename... Args>
+    result_type operator()(Args&&... args) const EXCLUSIVE_LOCKS_REQUIRED(!m_mutex)
+    {
+        std::vector<std::shared_ptr<connection_holder>> connections;
+        {
+            LOCK(m_mutex);
+            connections = m_connections;
+        }
+        if constexpr (std::is_void_v<result_type>) {
+            for (const auto& connection : connections) {
+                if (connection->m_connection.connected()) {
+                    connection->m_callback(args...);
+                }
+            }
+        } else {
+            result_type ret{std::nullopt};
+            for (const auto& connection : connections) {
+                if (connection->m_connection.connected()) {
+                    ret.emplace(connection->m_callback(args...));
+                }
+            }
+            return ret;
+        }
+    }
+
+    /*
+     * Connect a new callback to the signal. A forwarding callable accepts
+     * anything that can be stored in a std::function.
+     */
+    template <typename Callable>
+    connection connect(Callable&& func) EXCLUSIVE_LOCKS_REQUIRED(!m_mutex)
+    {
+        LOCK(m_mutex);
+
+        // Garbage-collect disconnected connections to prevent unbounded growth
+        std::erase_if(m_connections, [](const auto& holder) { return !holder->m_connection.connected(); });
+
+        const auto& connection = m_connections.emplace_back(std::make_shared<connection_holder>(std::forward<Callable>(func)));
+        return connection->m_connection;
+    }
+
+    /*
+     * Returns true if there are no enabled callbacks
+     */
+    [[nodiscard]] bool empty() const EXCLUSIVE_LOCKS_REQUIRED(!m_mutex)
+    {
+        LOCK(m_mutex);
+        return std::ranges::none_of(m_connections, [](const auto& holder) {
+            return holder->m_connection.connected();
+        });
+    }
+};
+
+} // namespace btcsignals
 
 #endif // BITCOIN_BTCSIGNALS_H