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diff --git a/lib/vfs/HACKING b/lib/vfs/HACKING new file mode 100644 index 0000000..c02e23d --- /dev/null +++ b/lib/vfs/HACKING @@ -0,0 +1,104 @@ +Intended audience +================= + +This document is intended for everybody who wants to understand VFS +code. Knowledge of programming is a must. + + +Preface +======= + +While VFS should be considered an excellent idea, which came ahead of +its time, the implementation used in GNU Midnight Commander is now +showing its age. + +The VFS code was left us without any decent documentation. Most +functions don't have comments explaining what they do. Most comments +describe quirks and implementation details, rather than the intended +functionality of the code. This document is an attempt to reconstruct +understanding of the VFS code and help its future developers. + +Being the part of GNU Midnight Commander most exposed to potential +security threats, the VFS code needs to be kept is a good shape. +Understanding the code is the key to making and keeping it secure. + + +Basics of code organization +=========================== + +VFS code it to a certain extent object oriented. The code dealing with +a certain type of data (e.g. tar archives) can be thought +of as a class in the terms of object oriented programming. They may +reuse some code from their parent classes. For instance, tar and cpio +archives have a common parent class direntry, which contains some common +code for archives. + +Individual archives or connections can be considered as instances of +those classes. They provide POSIX like interface to their structure, +but don't expose that structure directly to the common VFS layer. + +Each VFS object has a directory tree associated with it. The tree +consists of entries for files and directories. In some VFS classes, the +entries have names and a are associated with nameless inodes, which +contain information such as size, timestamps and other data normally +contained in POSIX "struct stat". + +File vfs.c serves as a multiplexor. It exports functions similar to +POSIX but with "mc_" prepended to them. For example, mc_open() will act +like open(), but will treat VFS names in a special way. + +Common utility functions not intended to be used outside the VFS code +should go to utilvfs.c and possibly to other files. Presently, there is +a lot of such code in vfs.c. + + +Hierarchy of classes +==================== + +vfs ---- direntry ---- cpio } archives + | | ---- tar } + | | + | | ---- fish } remote systems + | | ---- ftpfs } + | + |---- extfs ---- extfs archives + |---- localfs ---- sfs ---- sfs archives + |---- undelfs + + +Properties of classes +===================== + + read only inode->entry local cache full tree + mapping loaded + +cpio yes* yes* no yes +tar yes* yes* no yes +fish no yes yes no +ftpfs no yes yes no +extfs no no yes yes +localfs no no N/A N/A +sfs no yes yes N/A +undelfs no yes no yes + + +"*" means that this property should change during further development. +Mapping from inode to entry prevents implementing hard links. It is +permissible for directories, which cannot be hardlinked. Not loading +the full tree speeds up access to large archives and conserves memory. + + +Stamping +======== + +Stamping is the VFS equivalent of garbage collection. It's purpose is +to destroy unreferenced VFS objects, in other words close archives or +connections once they are unused for some time. There is a tree of +items representing VFS objects. The common layer doesn't know the +structure of the pointers, but it knows the class that should handle the +pointer. Every item has a timestamp. Once the timestamp becomes too +old, the object is freed. + +There are ways to keep objects alive if they are used. Also, objects +can have parent objects, which are freed together with there original +object if they are otherwise unreferenced. |