What's bad with V1 images

Current images format is great, but sometimes we find problems with it. This page collects all we don't like about them and would like to change some time in the future (once we change this, image format version will be changed as well).

• vma.proto vma_entry->fd is unused
• ipc-msg.proto ipc_msg/ipc_msg_entry names mess
• sk-packet.proto vs packet-sock.proto names mess (all sockets .proto files are sk-<something>)
• core.proto blk_sigset for master thread is on task_core_entry, but should be in respective thread_core_entry
• pstree.proto: sid and pgid should NOT be there, but in core image
• core.proto -- padding for fpu is not used
• what to do with absent image files? Backward compatibility requires we still restore, common sense -- abort. Flooding inventory with every single new image doesn't sound that great.
• core.proto -- unused (though optional) ids field
• clear_thread_info in per-arch members
• it would be great to unify inotify and fanotify images into one fsnotify file
• currently we write checkpoint files with *.img extension, which might be confusing since this extension is a way overloaded by meaning and quite widespreaded, thus it might be worth to change extension to *.criu
• mnt_entry::fstype (in mnt.proto) should have enum fstype type
• all IDs in image should be in uint64 format (not uint32, like in reg_file_entry), this allows us to use unified ID generator over all images
• IPC mqueue sysctls are required, while in kernel they can be missing
• In proto files for registers (and more) it's better to use fixed- types, rather than plain
• use fixed32 and fixed64 where approiate instead of uint32 and uint64 https://developers.google.com/protocol-buffers/docs/proto
• flock's flag and type fields is actually too much for what is stored there
• remap image uses older bit od ID to distinguish one remap type from another, while it should be a typed field
• evenpoll's tfd ID field is always zero
• Images structure is not natural. Some contain pb entries of a same type, some have a header of one pb type and then an array of another type or some raw data.