vfs: extend <zero> fs to limit amount of zeros

Setting the 'size' attribute will restrict a reader to consume
0s up to the specified amount per VFS handle. The attribute is
parsed as 'Number_of_bytes', which will limit the amount to the
maximum of the addressable memory on the platform.

This addition comes in handy when needing to provide a synthetic
empty file with a fixed size.

Issue #3781.

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

@@ -22,10 +22,14 @@ namespace Vfs { class Zero_file_system; }
 
 struct Vfs::Zero_file_system : Single_file_system
 {
+	Genode::size_t const _size;
+
 	Zero_file_system(Vfs::Env&, Genode::Xml_node config)
 	:
 		Single_file_system(Node_type::CONTINUOUS_FILE, name(),
-		                   Node_rwx::rw(), config)
+		                   Node_rwx::rw(), config),
+		_size(config.attribute_value("size",
+		                             Genode::Number_of_bytes(0)))
 	{ }
 
 	static char const *name()   { return "zero"; }
@@ -33,14 +37,36 @@ struct Vfs::Zero_file_system : Single_file_system
 
 	struct Zero_vfs_handle : Single_vfs_handle
 	{
+		Genode::size_t const _size;
+
 		Zero_vfs_handle(Directory_service &ds,
 		                File_io_service   &fs,
-		                Genode::Allocator &alloc)
+		                Genode::Allocator &alloc,
-		: Single_vfs_handle(ds, fs, alloc, 0) { }
+		                Genode::size_t     size)
+		:
+			Single_vfs_handle(ds, fs, alloc, 0),
+			_size(size)
+		{ }
 
 		Read_result read(char *dst, file_size count,
 		                 file_size &out_count) override
 		{
+			if (_size) {
+
+				/* current read offset */
+				file_size const read_offset = seek();
+
+				/* maximum read offset */
+				file_size const end_offset = min(count + read_offset, _size);
+
+				if (read_offset >= end_offset) {
+					out_count = 0;
+					return READ_OK;
+				}
+
+				count = end_offset - read_offset;
+			}
+
 			memset(dst, 0, count);
 			out_count = count;
 
@@ -70,13 +96,24 @@ struct Vfs::Zero_file_system : Single_file_system
 			return OPEN_ERR_UNACCESSIBLE;
 
 		try {
-			*out_handle = new (alloc) Zero_vfs_handle(*this, *this, alloc);
+			*out_handle = new (alloc) Zero_vfs_handle(*this, *this, alloc,
+			                                          _size);
 			return OPEN_OK;
 		}
 		catch (Genode::Out_of_ram)  { return OPEN_ERR_OUT_OF_RAM; }
 		catch (Genode::Out_of_caps) { return OPEN_ERR_OUT_OF_CAPS; }
 	}
 
+	Stat_result stat(char const *path, Stat &out) override
+	{
+		Stat_result const result = Single_file_system::stat(path, out);
+
+		if (_size) {
+			out.size = _size;
+		}
+
+		return result;
+	}
 };
 
 #endif /* _INCLUDE__VFS__ZERO_FILE_SYSTEM_H_ */