[2821] Small memory leak issue in MySQLLeaseMgr constructor

the mysql lease manager wouldn't close the mysql database immediately if initialization failed at certain points.

This proposed fix is not entirely complete; it would probably be better to either use such a holder class as the member, or to slightly clean it up so that the return value from release() can be given to mysql_ (and therefore use the holder in the construction code instead of mysql_), but I did not want to touch too much existing code at this point.

src/lib/dhcpsrv/mysql_lease_mgr.cc

@@ -193,6 +193,32 @@ TaggedStatement tagged_statements[] = {
     {MySqlLeaseMgr::NUM_STATEMENTS, NULL}
 };
 
+// Small RAII object for safer initialization, will close the database
+// connection upon destruction, unless release() has been called.
+class MySQLHolder {
+public:
+    MySQLHolder(MYSQL* mysql) : mysql_(mysql) {}
+
+    ~MySQLHolder() {
+        if (mysql_) {
+            mysql_close(mysql_);
+        }
+    }
+
+    MYSQL* get_ptr() {
+        return (mysql_);
+    }
+
+    MYSQL* release() {
+        MYSQL* ptr = mysql_;
+        mysql_ = NULL;
+        return (ptr);
+    }
+
+private:
+    MYSQL* mysql_;
+};
+
 };  // Anonymous namespace
 
 
@@ -870,14 +896,15 @@ private:
     MYSQL_STMT*     statement_;     ///< Statement for which results are freed
 };
 
-
 // MySqlLeaseMgr Constructor and Destructor
 
 MySqlLeaseMgr::MySqlLeaseMgr(const LeaseMgr::ParameterMap& parameters)
     : LeaseMgr(parameters), mysql_(NULL) {
 
-    // Allocate context for MySQL - it is destroyed in the destructor.
-    mysql_ = mysql_init(NULL);
+    // Allocate context for MySQL
+    MySQLHolder mysql_holder(mysql_init(NULL));
+
+    mysql_ = mysql_holder.get_ptr();
     if (mysql_ == NULL) {
         isc_throw(DbOpenError, "unable to initialize MySQL");
     }
@@ -902,6 +929,9 @@ MySqlLeaseMgr::MySqlLeaseMgr(const LeaseMgr::ParameterMap& parameters)
     // program and the database.
     exchange4_.reset(new MySqlLease4Exchange());
     exchange6_.reset(new MySqlLease6Exchange());
+
+    // Let the real destructor take care of cleaning up now
+    mysql_holder.release();
 }