Forbid having multiple mains in the cluster (#1727)

src/coordination/coordinator_instance.cpp

@@ -206,6 +206,10 @@ auto CoordinatorInstance::SetReplicationInstanceToMain(std::string instance_name
     -> SetInstanceToMainCoordinatorStatus {
   auto lock = std::lock_guard{coord_instance_lock_};
 
+  if (std::ranges::any_of(repl_instances_, &ReplicationInstance::IsMain)) {
+    return SetInstanceToMainCoordinatorStatus::MAIN_ALREADY_EXISTS;
+  }
+
   auto const is_new_main = [&instance_name](ReplicationInstance const &instance) {
     return instance.InstanceName() == instance_name;
   };

src/coordination/include/coordination/register_main_replica_coordinator_status.hpp

@@ -39,6 +39,7 @@ enum class UnregisterInstanceCoordinatorStatus : uint8_t {
 
 enum class SetInstanceToMainCoordinatorStatus : uint8_t {
   NO_INSTANCE_WITH_NAME,
+  MAIN_ALREADY_EXISTS,
   NOT_COORDINATOR,
   SUCCESS,
   COULD_NOT_PROMOTE_TO_MAIN,

src/query/interpreter.cpp

@@ -563,6 +563,8 @@ class CoordQueryHandler final : public query::CoordinatorQueryHandler {
       using enum memgraph::coordination::SetInstanceToMainCoordinatorStatus;
       case NO_INSTANCE_WITH_NAME:
         throw QueryRuntimeException("No instance with such name!");
+      case MAIN_ALREADY_EXISTS:
+        throw QueryRuntimeException("Couldn't set instance to main since there is already a main instance in cluster!");
       case NOT_COORDINATOR:
         throw QueryRuntimeException("SET INSTANCE TO MAIN query can only be run on a coordinator!");
       case COULD_NOT_PROMOTE_TO_MAIN: