Distributed File System

In this assignment, you will be developing a working *distributed file
server.* We provide you with only the bare minimal UDP communication
code; you have to build the rest. 

   A Basic File Server

Your file server is built as a stand-alone UDP-based server. It should wait
for a message and then process the message as need be, replying to the given
client.

Your file server will store all of its data in an on-disk, fixed-sized
file which will be referred to as the *file system image*. This image
contains the on-disk representation of your data structures; you
should use these system calls to access it: `open(), read(), write(),
lseek(), close(), fsync().`

To access the file server, you will be building a client library. The
interface that the library supports is defined in [mfs.h](mfs.h).  The
library should be called `libmfs.so`, and any programs that wish to access
your file server will link with it and call its various routines.

   On-Disk File System: A Basic Unix File System

The on-disk file system structures follow that of the
very simple file system discussed
[here](https://pages.cs.wisc.edu/~remzi/OSTEP/file-implementation.pdf). On-disk,
the structures are as follows:
- A single block (4KB) super block
- An inode bitmap (can be one or more 4KB blocks, depending on the number of inodes)
- A data bitmap (can be one or more 4KB blocks, depending on the number of data blocks)
- The inode table (a multiple of 4KB-sized blocks, depending on the number of inodes)
- The data region (some number of 4KB blocks, depending on the number of data blocks)

More details about on-disk structures can be found in the header [ufs.h](https://github.com/remzi-arpacidusseau/ostep-projects/blob/master/filesystems-distributed-ufs/ufs.h), which you should
use. Specifically, this has a very specific format for the super
block, inode, and directory entries. Bitmaps just have one bit per
allocated unit as described in the book.

As for directories, here is a little more detail.  Each directory has
an inode, and points to one or more data blocks that contain directory
entries. Each directory entry should be simple, and consist of 32
bytes: a name and an inode number pair. The name should be a
fixed-length field of size 28 bytes; the inode number is just an
integer (4 bytes). When a directory is created, it should contain two
entries: the name `.` (dot), which refers to this new directory's
inode number, and `..` (dot-dot), which refers to the parent
directory's inode number. For directory entries that are not yet in
use (in an allocated 4-KB directory block), the inode number should be
set to -1. This way, utilities can scan through the entries to check
if they are valid.

When your server is started, it is passed the name of the file system
image file. The image is created by a tool we provide, called `mkfs`.
It is pretty self-explanatory and can be found
[here](https://github.com/remzi-arpacidusseau/ostep-projects/blob/master/filesystems-distributed-ufs/mkfs.c). 

When booting off of an existing image, your server should read in the
superblock, bitmaps, and inode table, and keep in-memory versions of
these. When writing to the image, you should update these on-disk
structures accordingly.

Importantly, you cannot change the file-system on-disk format. 

   Client library

The client library should export the following interfaces:

- `int MFS_Init(char *hostname, int port)`: `MFS_Init()` takes a host name
and port number and uses those to find the server exporting the file system.
- `int MFS_Lookup(int pinum, char *name)`: `MFS_Lookup()` takes the parent
inode number (which should be the inode number of a directory) and looks up
the entry `name` in it. The inode number of `name` is returned. Success: 
return inode number of name; failure: return -1. Failure modes: invalid pinum,
name does not exist in pinum.
- `int MFS_Stat(int inum, MFS_Stat_t *m)`: `MFS_Stat()` returns some
information about the file specified by inum. Upon success, return 0,
otherwise -1. The exact info returned is defined by `MFS_Stat_t`. Failure modes:
inum does not exist. File and directory sizes are described below.
- `int MFS_Write(int inum, char *buffer, int offset, int nbytes)`:
`MFS_Write()` writes a buffer of size `nbytes` (max size: 4096 bytes) at the byte
offset specified by `offset`. Returns 0 on success, -1 on
failure. Failure modes: invalid inum, invalid nbytes, invalid offset, not a 
regular file (because you can't write to directories). 
- `int MFS_Read(int inum, char *buffer, int offset, int nbytes)`:
`MFS_Read()` reads `nbytes` of data (max size 4096 bytes) specified by the
byte offset `offset` into the buffer from file specified by
`inum`. The routine should work for either a file or directory;
directories should return data in the format specified by
`MFS_DirEnt_t`. Success: 0, failure: -1. Failure modes: invalid inum,
invalid offset, invalid nbytes.
- `int MFS_Creat(int pinum, int type, char *name)`: `MFS_Creat()` makes a
file (`type == MFS_REGULAR_FILE`) or directory (`type == MFS_DIRECTORY`)
in the parent directory specified by `pinum` of name `name`. Returns 0 on
success, -1 on failure. Failure modes: pinum does not exist, or name is too
long. If `name` already exists, return success.
- `int MFS_Unlink(int pinum, char *name)`: `MFS_Unlink()` removes the file or
directory `name` from the directory specified by `pinum`. 0 on success, -1
on failure. Failure modes: pinum does not exist, directory is NOT empty. Note
that the name not existing is NOT a failure by our definition (think about why
this might be). 
- `int MFS_Shutdown()`: `MFS_Shutdown()` just tells the server to force all
of its data structures to disk and shutdown by calling `exit(0)`. This interface
will mostly be used for testing purposes.

Size: The size of a file is the offset of the last valid byte written
to the file. Specifically, if you write 100 bytes to an empty file at
offset 0, the size is 100; if you write 100 bytes to an empty file at
offset 10, the size is 110. For a directory, it is the same (i.e., the
byte offset of the last byte of the last valid entry).

   Server Idempotency

The key behavior implemented by the server is *idempotency*.
Specifically, on any change to the file system state (such as a
`MFS_Write`, `MFS_Creat`, or `MFS_Unlink`), all the dirtied buffers in the
server are committed to the disk.  The server can achieved this end by
calling `fsync()` on the file system image. Thus, before returning a
success code, the file system should always `fsync()` the image.

Now you might be wondering: why do this? Simple: if the server crashes, the
client can simply timeout and retry the operation and know that it is OK to do
so. Read [this chapter](https://pages.cs.wisc.edu/~remzi/OSTEP/dist-nfs.pdf) on NFS
for details. 

Now you might be wondering: how do I implement a timeout? Simple, with the
`select()` interface. The `select()` calls allows you to wait for a reply
on a certain socket descriptor (or more than one, though that is not needed
here). You can even specify a timeout so that the client does not block
forever waiting for data to be returned from the server. By doing so, you can
wait for a reply for a certain amount of time, and if nothing is returned, try
the operation again until it is successful.

   Program Specifications

Your server program must be invoked exactly as follows:

prompt> server [portnum] [file-system-image]

The command line arguments to your file server are to be interpreted as follows.  

- portnum: the port number that the file server should listen on.
- file-system-image: a file that contains the file system image.

If the file system image does not exist, you should print out an error
message (`image does not exist\n`) and exit with exit code 1.

Your client library should be called `libmfs.so`. It should implement
the interface as specified by `mfs.h`, and in particular deal with
the case where the server does not reply in a timely fashion; the way
it deals with that is simply by retrying the operation, after a
timeout of some kind (default: five second timeout).

   Relevant Chapters

Read these:
- [File System Implementation](https://pages.cs.wisc.edu/~remzi/OSTEP/file-implementation.pdf)
- [Distributed Systems](https://pages.cs.wisc.edu/~remzi/OSTEP/dist-intro.pdf)
- [Distributed File System: NFS](https://pages.cs.wisc.edu/~remzi/OSTEP/dist-nfs.pdf)


   Some Helper Code

To get you going, we have written some simple UDP code that can send a
message and then receive a reply from a client to a server. It can be found in
[here](https://github.com/remzi-arpacidusseau/ostep-code/tree/master/dist-intro).

There is also other code as mentioned above:
- [mfs.h](https://github.com/remzi-arpacidusseau/ostep-projects/blob/master/filesystems-distributed-ufs/mfs.h)
- [ufs.h](https://github.com/remzi-arpacidusseau/ostep-projects/blob/master/filesystems-distributed-ufs/ufs.h)
- [mkfs.c](https://github.com/remzi-arpacidusseau/ostep-projects/blob/master/filesystems-distributed-ufs/mkfs.c)

You'll also have to learn how to make a shared library. Read [here](https://tldp.org/HOWTO/Program-Library-HOWTO/shared-libraries.html) for more information.