os: add File_system_session::num_entries RPC

This patch splits the querying of the number of directory entries from
the directory's 'status' information. Subsuming the number of directory
entries as part of the status makes 'stat' calls too costly for some
file systems that need to read a directory for determining the number of
entries. So when stat'ing the entries of one directory that contains sub
directories, all entries of each sub directory are visited.

Thanks to Cedric Degea for pointing out this performance bottleneck!

With this change, the 'status' function returns a 'Status::size' value
of 0 when called for a directory handle.

Fixes #4603

repos/os/include/file_system_session/client.h

@@ -112,6 +112,11 @@ class File_system::Session_client : public Genode::Rpc_client<Session>
 		{
 			call<Rpc_move>(from_dir, from_name, to_dir, to_name);
 		}
+
+		unsigned num_entries(Dir_handle dir) override
+		{
+			return call<Rpc_num_entries>(dir);
+		}
 };
 
 #endif /* _INCLUDE__FILE_SYSTEM_SESSION__CLIENT_H_ */

repos/os/include/file_system_session/file_system_session.h

@@ -474,6 +474,13 @@ struct File_system::Session : public Genode::Session
 	virtual void move(Dir_handle, Name const &from,
 	                  Dir_handle, Name const &to) = 0;
 
+	/**
+	 * Return number of directory entries
+	 *
+	 * \throw Invalid_handle     the directory handle is invalid
+	 */
+	virtual unsigned num_entries(Dir_handle) = 0;
+
 
 	/*******************
 	 ** RPC interface **
@@ -525,12 +532,14 @@ struct File_system::Session : public Genode::Session
 	                 GENODE_TYPE_LIST(Invalid_handle, Invalid_name,
 	                                  Lookup_failed, Permission_denied, Unavailable),
 	                 Dir_handle, Name const &, Dir_handle, Name const &);
+	GENODE_RPC_THROW(Rpc_num_entries, unsigned, num_entries,
+	                 GENODE_TYPE_LIST(Invalid_handle),
+	                 Dir_handle);
 
 	GENODE_RPC_INTERFACE(Rpc_tx_cap,
-	                     Rpc_file, Rpc_symlink, Rpc_dir,
-	                     Rpc_node, Rpc_watch,
+	                     Rpc_file, Rpc_symlink, Rpc_dir, Rpc_node, Rpc_watch,
 	                     Rpc_close, Rpc_status, Rpc_control, Rpc_unlink,
-	                     Rpc_truncate, Rpc_move);
+	                     Rpc_truncate, Rpc_move, Rpc_num_entries);
 };
 
 #endif /* _INCLUDE__FILE_SYSTEM_SESSION__FILE_SYSTEM_SESSION_H_ */

repos/os/src/lib/vfs/fs_file_system.h

@@ -754,13 +754,11 @@ class Vfs::Fs_file_system : public File_system
 				path = "/";
 
 			try {
-				::File_system::Node_handle node = _fs.node(path);
-				Fs_handle_guard node_guard(*this, _fs, node,
+				::File_system::Dir_handle dir = _fs.dir(path, false);
+				Fs_handle_guard node_guard(*this, _fs, dir,
 				                           _handle_space, _fs);
 
-				::File_system::Status status = _fs.status(node);
-
-				return status.size / sizeof(::File_system::Directory_entry);
+				return _fs.num_entries(dir);
 			}
 			catch (...) { }
 			return 0;

repos/os/src/server/lx_fs/directory.h

@@ -85,7 +85,7 @@ class Lx_fs::Directory : public Node
 			return fd;
 		}
 
-		size_t _num_entries() const
+		unsigned _num_entries() const
 		{
 			unsigned num = 0;
 
@@ -250,7 +250,7 @@ class Lx_fs::Directory : public Node
 				st.st_mtime = 0;
 
 			return {
-				.size  = _num_entries() * sizeof(File_system::Directory_entry),
+				.size  = 0,
 				.type  = Node_type::DIRECTORY,
 				.rwx   = { .readable   = (st.st_mode & S_IRUSR) != 0,
 				           .writeable  = (st.st_mode & S_IWUSR) != 0,
@@ -260,6 +260,11 @@ class Lx_fs::Directory : public Node
 			};
 		}
 
+		unsigned num_entries() override
+		{
+			return _num_entries();
+		}
+
 		Path path() const override
 		{
 			return _path;

repos/os/src/server/lx_fs/main.cc

@@ -441,6 +441,19 @@ class Lx_fs::Session_component : private Session_resources,
 			}
 		}
 
+		unsigned num_entries(Dir_handle dir_handle) override
+		{
+			auto fn = [&] (Open_node &open_node) {
+				return open_node.node().num_entries();
+			};
+
+			try {
+				return _open_node_registry.apply<Open_node>(dir_handle, fn);
+			} catch (Id_space<File_system::Node>::Unknown_id const &) {
+				throw Invalid_handle();
+			}
+		}
+
 		void control(Node_handle, Control) override
 		{
 			Genode::error(__func__, " not implemented");

repos/os/src/server/lx_fs/node.h

@@ -74,6 +74,8 @@ class Lx_fs::Node : public File_system::Node_base
 
 		virtual Status status() = 0;
 
+		virtual unsigned num_entries() { return 0; }
+
 		/*
 		 * File functionality
 		 */

repos/os/src/server/vfs/main.cc

@@ -660,8 +660,6 @@ class Vfs_server::Session_component : private Session_resources,
 				{
 					switch (vfs_stat.type) {
 					case Vfs::Node_type::DIRECTORY:
-						return _vfs.num_dirent(node.path()) * sizeof(Directory_entry);
-
 					case Vfs::Node_type::SYMLINK:
 						return 0ULL;
 
@@ -688,6 +686,12 @@ class Vfs_server::Session_component : private Session_resources,
 			return fs_stat;
 		}
 
+		unsigned num_entries(Dir_handle dir_handle) override
+		{
+			return _apply(dir_handle, [&] (Directory &dir) {
+				return (unsigned)_vfs.num_dirent(dir.path()); });
+		}
+
 		void unlink(Dir_handle dir_handle, Name const &name) override
 		{
 			if (!_writeable) throw Permission_denied();