feat: Graceful shutdown (#1801)

Fixes #442.

src/etl/LoadBalancer.cpp

@@ -26,6 +26,7 @@
 #include "feed/SubscriptionManagerInterface.hpp"
 #include "rpc/Errors.hpp"
 #include "util/Assert.hpp"
+#include "util/CoroutineGroup.hpp"
 #include "util/Random.hpp"
 #include "util/ResponseExpirationCache.hpp"
 #include "util/log/Logger.hpp"
@@ -336,6 +337,16 @@ LoadBalancer::getETLState() noexcept
     return etlState_;
 }
 
+void
+LoadBalancer::stop(boost::asio::yield_context yield)
+{
+    util::CoroutineGroup group{yield};
+    std::ranges::for_each(sources_, [&group, yield](auto& source) {
+        group.spawn(yield, [&source](boost::asio::yield_context innerYield) { source->stop(innerYield); });
+    });
+    group.asyncWait(yield);
+}
+
 void
 LoadBalancer::chooseForwardingSource()
 {