introduce and use the generalized node::Warnings interface

Instead of having separate warning functions (and globals) for each
different warning that can be raised, encapsulate this logic into
a single class and allow to (un)set any number of warnings.

Introduces behaviour change:
- the `-alertnotify` command is executed for all
  `KernelNotifications::warningSet` calls, which now also covers the
  `LARGE_WORK_INVALID_CHAIN` warning.
- previously, warnings were returned based on a predetermined order,
  e.g. with the "pre-release test build" warning always first. This
  is no longer the case, and Warnings::GetMessages() will return
  messages sorted by the id of the warning.

Removes warnings.cpp from kernel.

src/node/warnings.h

@@ -6,26 +6,86 @@
 #ifndef BITCOIN_NODE_WARNINGS_H
 #define BITCOIN_NODE_WARNINGS_H
 
-#include <optional>
-#include <string>
+#include <sync.h>
+
+#include <map>
+#include <variant>
 #include <vector>
 
 class UniValue;
 struct bilingual_str;
 
+namespace kernel {
+enum class Warning;
+} // namespace kernel
+
 namespace node {
-void SetMiscWarning(const bilingual_str& warning);
-void SetfLargeWorkInvalidChainFound(bool flag);
-/** Pass std::nullopt to disable the warning */
-void SetMedianTimeOffsetWarning(std::optional<bilingual_str> warning);
-/** Return potential problems detected by the node. */
-std::vector<bilingual_str> GetWarnings();
+enum class Warning {
+    CLOCK_OUT_OF_SYNC,
+    PRE_RELEASE_TEST_BUILD,
+    FATAL_INTERNAL_ERROR,
+};
+
 /**
- * RPC helper function that wraps GetWarnings. Returns a UniValue::VSTR
- * with the latest warning if use_deprecated is set to true, or a
- * UniValue::VARR with all warnings otherwise.
+ * @class Warnings
+ * @brief Manages warning messages within a node.
+ *
+ * The Warnings class provides mechanisms to set, unset, and retrieve
+ * warning messages.
+ *
+ * This class is designed to be non-copyable to ensure warnings
+ * are managed centrally.
+ */
+class Warnings
+{
+    typedef std::variant<kernel::Warning, node::Warning> warning_type;
+
+    mutable Mutex m_mutex;
+    std::map<warning_type, bilingual_str> m_warnings GUARDED_BY(m_mutex);
+
+public:
+    Warnings();
+    //! A warnings instance should always be passed by reference, never copied.
+    Warnings(const Warnings&) = delete;
+    Warnings& operator=(const Warnings&) = delete;
+    /**
+     * @brief Set a warning message. If a warning with the specified
+     *        `id` is already active, false is returned and the new
+     *        warning is ignored. If `id` does not yet exist, the
+     *        warning is set, and true is returned.
+     *
+     * @param[in]   id  Unique identifier of the warning.
+     * @param[in]   message Warning message to be shown.
+     *
+     * @returns true if the warning was indeed set (i.e. there is no
+     *          active warning with this `id`), otherwise false.
+     */
+    bool Set(warning_type id, bilingual_str message) EXCLUSIVE_LOCKS_REQUIRED(!m_mutex);
+    /**
+     * @brief Unset a warning message. If a warning with the specified
+     *        `id` is active, it is unset, and true is returned.
+     *        Otherwise, no warning is unset and false is returned.
+     *
+     * @param[in]   id  Unique identifier of the warning.
+     *
+     * @returns true if the warning was indeed unset (i.e. there is an
+     *          active warning with this `id`), otherwise false.
+     */
+    bool Unset(warning_type id) EXCLUSIVE_LOCKS_REQUIRED(!m_mutex);
+    /** Return potential problems detected by the node, sorted by the
+     * warning_type id */
+    std::vector<bilingual_str> GetMessages() const EXCLUSIVE_LOCKS_REQUIRED(!m_mutex);
+};
+
+/**
+ * RPC helper function that wraps g_warnings.GetMessages().
+ *
+ * Returns a UniValue::VSTR with the latest warning if use_deprecated is
+ * set to true, or a UniValue::VARR with all warnings otherwise.
  */
 UniValue GetWarningsForRpc(bool use_deprecated);
+
+extern Warnings g_warnings;
 } // namespace node
 
 #endif // BITCOIN_NODE_WARNINGS_H