DatabasePlugin: add FLAG_REQUIRE_STORAGE

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

src/Main.cxx

@@ -67,6 +67,7 @@
 #ifdef ENABLE_DATABASE
 #include "db/update/Service.hxx"
 #include "db/Configured.hxx"
+#include "db/DatabasePlugin.hxx"
 #include "db/plugins/SimpleDatabasePlugin.hxx"
 #include "storage/Configured.hxx"
 #include "storage/CompositeStorage.hxx"
@@ -177,6 +178,7 @@ glue_db_init_and_load(void)
 			return true;
 	}
 
+	if (instance->database->GetPlugin().flags & DatabasePlugin::FLAG_REQUIRE_STORAGE) {
 		if (!InitStorage(error))
 			FatalError(error);
 
@@ -188,6 +190,12 @@ glue_db_init_and_load(void)
 				   "music_directory - disabling database");
 			return true;
 		}
+	} else {
+		if (IsStorageConfigured())
+			LogDefault(config_domain,
+				   "Ignoring the storage configuration "
+				   "because the database does not need it");
+	}
 
 	if (!instance->database->Open(error))
 		FatalError(error);

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,
 };

src/storage/Configured.cxx

@@ -76,3 +76,9 @@ CreateConfiguredStorage(Error &error)
 
 	return CreateConfiguredStorageLocal(error);
 }
+
+bool
+IsStorageConfigured()
+{
+	return config_get_string(CONF_MUSIC_DIR, nullptr) != nullptr;
+}

src/storage/Configured.hxx

@@ -21,6 +21,7 @@
 #define MPD_STORAGE_CONFIG_HXX
 
 #include "check.h"
+#include "Compiler.h"
 
 class Error;
 class Storage;
@@ -33,4 +34,11 @@ class Storage;
 Storage *
 CreateConfiguredStorage(Error &error);
 
+/**
+ * Returns true if there is configuration for a #Storage instance.
+ */
+gcc_const
+bool
+IsStorageConfigured();
+
 #endif