DatabasePlugin: add FLAG_REQUIRE_STORAGE

Ignore the storage configuration if FLAG_REQUIRE_STORAGE is not set in
the DatabasePlugin.

src/db/DatabasePlugin.hxx

@@ -33,6 +33,12 @@ class DatabaseListener;
 class Database;
 
 struct DatabasePlugin {
+	/**
+	 * This plugin requires a #Storage instance.  It contains only
+	 * cached metadata from files in the #Storage.
+	 */
+	static constexpr unsigned FLAG_REQUIRE_STORAGE = 0x1;
+
 	const char *name;
 
 	unsigned flags;
@@ -43,6 +49,10 @@ struct DatabasePlugin {
 	Database *(*create)(EventLoop &loop, DatabaseListener &listener,
 			    const config_param &param,
 			    Error &error);
+
+	constexpr bool RequireStorage() const {
+		return flags & FLAG_REQUIRE_STORAGE;
+	}
 };
 
 #endif

src/db/plugins/ProxyDatabasePlugin.cxx

@@ -780,6 +780,6 @@ ProxyDatabase::GetStats(const DatabaseSelection &selection,
 
 const DatabasePlugin proxy_db_plugin = {
 	"proxy",
-	0,
+	DatabasePlugin::FLAG_REQUIRE_STORAGE,
 	ProxyDatabase::Create,
 };

src/db/plugins/SimpleDatabasePlugin.cxx

@@ -337,6 +337,6 @@ SimpleDatabase::Save(Error &error)
 
 const DatabasePlugin simple_db_plugin = {
 	"simple",
-	0,
+	DatabasePlugin::FLAG_REQUIRE_STORAGE,
 	SimpleDatabase::Create,
 };