enhanced connection pool to acquire connection by specific id

include/matador/sql/connection_pool.hpp

@@ -1,12 +1,11 @@
 #ifndef QUERY_CONNECTION_POOL_HPP
 #define QUERY_CONNECTION_POOL_HPP
 
-#include <utility>
-#include <queue>
+#include <chrono>
 #include <mutex>
 #include <string>
+#include <thread>
 #include <unordered_map>
-#include <unordered_set>
 
 namespace matador::sql {
 
@@ -81,7 +80,6 @@ public:
       return {nullptr, *this};
     }
 
-
     pointer next_connection{nullptr};
     for (auto &item : idle_connections_) {
       next_connection = item.second;
@@ -93,8 +91,24 @@ public:
   }
 
   connection_ptr<Connection> acquire(size_t id) {
-    std::lock_guard<std::mutex> guard(mutex_);
-    return {nullptr, *this};
+    using namespace std::chrono_literals;
+    pointer next_connection{nullptr};
+    auto try_count{0};
+    std::unique_lock<std::mutex> lock(mutex_);
+
+    do {
+      if (auto it = idle_connections_.find(id); it != idle_connections_.end()) {
+        next_connection = it->second.second;
+        auto node = idle_connections_.extract(it);
+        inuse_connections_.insert(std::move(node));
+      } else {
+        lock.unlock();
+        std::this_thread::sleep_for(500ms);
+        lock.lock();
+      }
+    } while(try_count++ < 5);
+
+    return {next_connection, *this};
   }
 
   void release(IdConnection<Connection> *c) {
@@ -130,10 +144,7 @@ private:
   mutable std::mutex mutex_;
   std::vector<IdConnection<Connection>> connection_repo_;
   using pointer = IdConnection<Connection>*;
-  using connections = std::queue<pointer>;
   using connection_map = std::unordered_map<size_t, pointer>;
-  using connection_set = std::unordered_set<pointer>;
-//  connections idle_connections_;
   connection_map inuse_connections_;
   connection_map idle_connections_;