[#28,!20] Implemented Config Backend pool for DHCPv4.

Author: msiodelski

src/lib/config_backend/base_config_backend_pool.h

@@ -12,6 +12,7 @@
 #include <database/backend_selector.h>
 #include <database/db_exceptions.h>
 #include <database/server_selector.h>
+#include <util/optional_value.h>
 #include <functional>
 #include <list>
 #include <string>
@@ -75,7 +76,7 @@ class BaseConfigBackendPool {
     ///                       const BackendSelector& selector,
     ///                       const ServerSelector& server_selector) const {
     ///     Subnet4Ptr subnet;
-    ///     getPropertyPtrConst<Subnet4Ptr, const SubnetID&, ConfigBackendDHCPv4::getSubnet4>
+    ///     getPropertyPtrConst<Subnet4Ptr, const SubnetID&>
     ///         (&ConfigBackendDHCPv4::getSubnet4, selector, subnet, subnet_id);
     ///     return (subnet);
     /// }
@@ -142,6 +143,84 @@ class BaseConfigBackendPool {
         }
     }
 
+    /// @brief Retrieve a single value encapsulated in the @c OptionalValue
+    /// template.
+    ///
+    /// This is common method for retrieving a single configuration property
+    /// from the databases. The property is encapsulated in the @c OptionalValue
+    /// class. The value is set to "unspecified" if it is null in the database.
+    /// The following is the example implementation of the method retrieving
+    /// global conifguration value:
+    ///
+    /// @code
+    /// OptionalValue<std::string>
+    /// getGlobalParameter4(const std::string& parameter_name,
+    ///                     const BackendSelector& selector,
+    ///                     const ServerSelector& server_selector) const {
+    ///     std::string parameter;
+    ///     getOptionalPropertyConst<std::string, const std::string&>
+    ///         (&ConfigBackendDHCPv4::getGlobalParameter4, selector, server_selector,
+    ///          parameter, parameter_name);
+    ///     return (parameter);
+    /// }
+    /// @endcode
+    ///
+    /// where @c ConfigBackendDHCPv4::getGlobalParameter has the following signature:
+    ///
+    /// @code
+    /// std::string getGlobalParameter4(const ServerSelector&, const std::string&) const;
+    /// @endcode
+    ///
+    ///
+    /// @tparam PropertyType Type of the object returned by the backend call.
+    /// @tparam InputType Type of the objects used as input to the backend call.
+    ///
+    /// @param MethodPointer Pointer to the backend method to be called.
+    /// @param selector Backend selector.
+    /// @param server_selector Server selector.
+    /// @param [out] property Reference to the shared pointer where retrieved
+    /// property should be assigned.
+    /// @param input Values to be used as input to the backend call.
+    ///
+    /// @throw db::NoSuchDatabase if no database matching the given selector
+    /// was found.
+    template<typename PropertyType, typename... InputType>
+    void getOptionalPropertyConst(util::OptionalValue<PropertyType>
+                                  (ConfigBackendType::*MethodPointer)
+                                  (const db::ServerSelector&, InputType...) const,
+                                  const db::BackendSelector& selector,
+                                  const db::ServerSelector& server_selector,
+                                  util::OptionalValue<PropertyType>& property,
+                                  InputType... input) const {
+
+        // If no particular backend is selected, call each backend and return
+        // the first non-null (non zero) value.
+        if (selector.amUnspecified()) {
+            for (auto backend : backends_) {
+                property = ((*backend).*MethodPointer)(server_selector, input...);
+                if (property.isSpecified()) {
+                    break;
+                }
+            }
+
+        } else {
+            // Backend selected, find the one that matches selection.
+            auto backends = selectBackends(selector);
+            if (!backends.empty()) {
+                for (auto backend : backends) {
+                    property = ((*backend).*MethodPointer)(server_selector, input...);
+                    if (property.isSpecified()) {
+                        break;
+                    }
+                }
+
+            } else {
+                isc_throw(db::NoSuchDatabase, "no such database found for selector: "
+                          << selector.toText());
+            }
+        }
+    }
+
     /// @brief Retrieve multiple configuration properties from the pool.
     ///
     /// This is a common method for retrieving multiple configuration properties

src/lib/dhcpsrv/Makefile.am

@@ -108,6 +108,8 @@ libkea_dhcpsrv_la_SOURCES += cfg_subnets6.cc cfg_subnets6.h
 libkea_dhcpsrv_la_SOURCES += cfg_mac_source.cc cfg_mac_source.h
 libkea_dhcpsrv_la_SOURCES += cfgmgr.cc cfgmgr.h
 libkea_dhcpsrv_la_SOURCES += client_class_def.cc client_class_def.h
+libkea_dhcpsrv_la_SOURCES += config_backend_dhcp4.h
+libkea_dhcpsrv_la_SOURCES += config_backend_pool_dhcp4.cc config_backend_pool_dhcp4.h
 libkea_dhcpsrv_la_SOURCES += csv_lease_file4.cc csv_lease_file4.h
 libkea_dhcpsrv_la_SOURCES += csv_lease_file6.cc csv_lease_file6.h
 libkea_dhcpsrv_la_SOURCES += d2_client_cfg.cc d2_client_cfg.h
@@ -211,6 +213,7 @@ if HAVE_CQL
 libkea_dhcpsrv_la_LIBADD  += $(top_builddir)/src/lib/cql/libkea-cql.la
 endif
 
+libkea_dhcpsrv_la_LIBADD  += $(top_builddir)/src/lib/config_backend/libkea-cb.la
 libkea_dhcpsrv_la_LIBADD  += $(top_builddir)/src/lib/database/libkea-database.la
 libkea_dhcpsrv_la_LIBADD  += $(top_builddir)/src/lib/asiolink/libkea-asiolink.la
 libkea_dhcpsrv_la_LIBADD  += $(top_builddir)/src/lib/cc/libkea-cc.la
@@ -270,6 +273,8 @@ libkea_dhcpsrv_include_HEADERS = \
 	cfg_subnets6.h \
 	cfgmgr.h \
 	client_class_def.h \
+	config_backend_dhcp4.h \
+	config_backend_pool_dhcp4.h \
 	csv_lease_file4.h \
 	csv_lease_file6.h \
 	dhcpsrv_exceptions.h \