kernel: Add warning method to notifications

This commit is part of the libbitcoinkernel project and seeks to remove
the ChainstateManager's and, more generally, the kernel library's
dependency on interface_ui with options methods in this and the following
few commits. By removing interface_ui from the kernel library, its
dependency on boost is reduced to just boost::multi_index.

The DoWarning and AlertNotify functions are moved out of the
validation.cpp file, which removes its dependency on interface_ui as
well as util/system.

src/node/kernel_notifications.cpp

@@ -4,8 +4,51 @@
 
 #include <node/kernel_notifications.h>
 
+#if defined(HAVE_CONFIG_H)
+#include <config/bitcoin-config.h>
+#endif
+
+#include <common/args.h>
 #include <node/interface_ui.h>
+#include <util/strencodings.h>
+#include <util/string.h>
+#include <util/system.h>
 #include <util/translation.h>
+#include <warnings.h>
+
+#include <cstdint>
+#include <string>
+#include <thread>
+
+static void AlertNotify(const std::string& strMessage)
+{
+    uiInterface.NotifyAlertChanged();
+#if HAVE_SYSTEM
+    std::string strCmd = gArgs.GetArg("-alertnotify", "");
+    if (strCmd.empty()) return;
+
+    // Alert text should be plain ascii coming from a trusted source, but to
+    // be safe we first strip anything not in safeChars, then add single quotes around
+    // the whole string before passing it to the shell:
+    std::string singleQuote("'");
+    std::string safeStatus = SanitizeString(strMessage);
+    safeStatus = singleQuote+safeStatus+singleQuote;
+    ReplaceAll(strCmd, "%s", safeStatus);
+
+    std::thread t(runCommand, strCmd);
+    t.detach(); // thread runs free
+#endif
+}
+
+static void DoWarning(const bilingual_str& warning)
+{
+    static bool fWarned = false;
+    SetMiscWarning(warning);
+    if (!fWarned) {
+        AlertNotify(warning.original);
+        fWarned = true;
+    }
+}
 
 namespace node {
 
@@ -24,4 +67,9 @@ void KernelNotifications::progress(const bilingual_str& title, int progress_perc
     uiInterface.ShowProgress(title.translated, progress_percent, resume_possible);
 }
 
+void KernelNotifications::warning(const bilingual_str& warning)
+{
+    DoWarning(warning);
+}
+
 } // namespace node