core: move to inotify for monitoring the config files

instead of manually polling every second which is not efficient, use inotify.

an added bonus is that inotify is much much faster

src/config/ConfigWatcher.cpp

@@ -0,0 +1,72 @@
+#include "ConfigWatcher.hpp"
+#include <sys/inotify.h>
+#include "../debug/Log.hpp"
+#include <ranges>
+#include <fcntl.h>
+
+CConfigWatcher::CConfigWatcher() : m_inotifyFd(inotify_init()) {
+    if (m_inotifyFd < 0) {
+        Debug::log(ERR, "CConfigWatcher couldn't open an inotify node. Config will not be automatically reloaded");
+        return;
+    }
+
+    const int FLAGS = fcntl(m_inotifyFd, F_GETFL, 0);
+    if (fcntl(m_inotifyFd, F_SETFL, FLAGS | O_NONBLOCK) < 0) {
+        Debug::log(ERR, "CConfigWatcher couldn't non-block inotify node. Config will not be automatically reloaded");
+        close(m_inotifyFd);
+        m_inotifyFd = -1;
+        return;
+    }
+}
+
+CConfigWatcher::~CConfigWatcher() {
+    if (m_inotifyFd >= 0)
+        close(m_inotifyFd);
+}
+
+int CConfigWatcher::getInotifyFD() {
+    return m_inotifyFd;
+}
+
+void CConfigWatcher::setWatchList(const std::vector<std::string>& paths) {
+
+    // we clear all watches first, because whichever fired is now invalid
+    // or that is at least what it seems to be.
+    // since we don't know which fired,
+    // plus it doesn't matter that much, these ops are done rarely and fast anyways.
+
+    // cleanup old paths
+    for (auto& watch : m_watches) {
+        inotify_rm_watch(m_inotifyFd, watch.wd);
+    }
+
+    m_watches.clear();
+
+    // add new paths
+    for (const auto& path : paths) {
+        m_watches.emplace_back(SInotifyWatch{
+            .wd   = inotify_add_watch(m_inotifyFd, path.c_str(), IN_MODIFY),
+            .file = path,
+        });
+    }
+}
+
+void CConfigWatcher::setOnChange(const std::function<void(const SConfigWatchEvent&)>& fn) {
+    m_watchCallback = fn;
+}
+
+void CConfigWatcher::onInotifyEvent() {
+    inotify_event ev;
+    while (read(m_inotifyFd, &ev, sizeof(ev)) > 0) {
+        const auto WD = std::ranges::find_if(m_watches.begin(), m_watches.end(), [wd = ev.wd](const auto& e) { return e.wd == wd; });
+
+        if (WD == m_watches.end()) {
+            Debug::log(ERR, "CConfigWatcher: got an event for wd {} which we don't have?!", ev.wd);
+            return;
+        }
+
+        m_watchCallback(SConfigWatchEvent{
+            .file = WD->file,
+        });
+    }
+}