storage: add struct StoragePlugin and a plugin registry

src/storage/plugins/LocalStorage.cxx

@@ -19,6 +19,7 @@
 
 #include "config.h"
 #include "LocalStorage.hxx"
+#include "storage/StoragePlugin.hxx"
 #include "storage/StorageInterface.hxx"
 #include "storage/FileInfo.hxx"
 #include "util/Error.hxx"
@@ -210,3 +211,8 @@ CreateLocalStorage(Path base_fs)
 {
 	return new LocalStorage(base_fs);
 }
+
+const StoragePlugin local_storage_plugin = {
+	"local",
+	nullptr,
+};

src/storage/plugins/LocalStorage.hxx

@@ -23,9 +23,12 @@
 #include "check.h"
 #include "Compiler.h"
 
+struct StoragePlugin;
 class Storage;
 class Path;
 
+extern const StoragePlugin local_storage_plugin;
+
 gcc_malloc gcc_nonnull_all
 Storage *
 CreateLocalStorage(Path base_fs);

src/storage/plugins/SmbclientStorage.cxx

@@ -19,6 +19,7 @@
 
 #include "config.h"
 #include "SmbclientStorage.hxx"
+#include "storage/StoragePlugin.hxx"
 #include "storage/StorageInterface.hxx"
 #include "storage/FileInfo.hxx"
 #include "lib/smbclient/Init.hxx"
@@ -178,9 +179,12 @@ SmbclientDirectoryReader::GetInfo(gcc_unused bool follow, FileInfo &info,
 	return ::GetInfo(path.c_str(), info, error);
 }
 
-Storage *
-CreateSmbclientStorage(const char *base, Error &error)
+static Storage *
+CreateSmbclientStorageURI(const char *base, Error &error)
 {
+	if (memcmp(base, "smb://", 6) != 0)
+		return nullptr;
+
 	if (!SmbclientInit(error))
 		return nullptr;
 
@@ -200,3 +204,8 @@ CreateSmbclientStorage(const char *base, Error &error)
 
 	return new SmbclientStorage(base, ctx2);
 }
+
+const StoragePlugin smbclient_storage_plugin = {
+	"smbclient",
+	CreateSmbclientStorageURI,
+};

src/storage/plugins/SmbclientStorage.hxx

@@ -22,10 +22,8 @@
 
 #include "check.h"
 
-class Error;
-class Storage;
+struct StoragePlugin;
 
-Storage *
-CreateSmbclientStorage(const char *base, Error &error);
+extern const StoragePlugin smbclient_storage_plugin;
 
 #endif