chore: use condition variable to shutdown websocket pools (#5721)

src/util/OnceFlag.hpp

@@ -0,0 +1,41 @@
+#pragma once
+
+#include <chrono>
+#include <condition_variable>
+#include <mutex>
+
+namespace chatterino {
+
+/// @brief A flag that can only be set once which notifies waiters.
+///
+/// This can be used to synchronize with other threads. Note that waiting
+/// threads will be suspended.
+class OnceFlag
+{
+public:
+    OnceFlag();
+    ~OnceFlag();
+
+    /// Set this flag and notify waiters
+    void set();
+
+    /// @brief Wait for at most `ms` until this flag is set.
+    ///
+    /// The calling thread will be suspended during the wait.
+    ///
+    /// @param ms The maximum time to wait for this flag
+    /// @returns `true` if this flag was set during the wait or before
+    bool waitFor(std::chrono::milliseconds ms);
+
+    /// @brief Wait until this flag is set by another thread
+    ///
+    /// The calling thread will be suspended during the wait.
+    void wait();
+
+private:
+    std::mutex mutex;
+    std::condition_variable condvar;
+    bool flag = false;
+};
+
+}  // namespace chatterino