To-do list for nbdkit
======================================================================

General ideas for improvements
------------------------------

* Listen on specific interfaces or protocols (eg. only IPv6).

* Performance - measure and improve it.  Chart it over various buffer
  sizes and threads, as that should make it easier to identify
  systematic issues.

* Exit on last connection (the default behaviour of qemu-nbd unless
  you use -t).

* Limit number of incoming connections (like qemu-nbd -e).

* For parallel plugins, only create threads on demand from parallel
  client requests, rather than pre-creating all threads at connection
  time, up to the thread pool size limit.  Of course, once created, a
  thread is reused as possible until the connection closes.

* Async callbacks.  The current parallel support requires one thread
  per pending message; a solution with fewer threads would split
  low-level code between request and response, where the callback has
  to inform nbdkit when the response is ready:
  https://www.redhat.com/archives/libguestfs/2018-January/msg00149.html

* More NBD protocol features.  The currently missing features are
  structured replies for sparse reads, block size constraints, and
  online resize.

* Add a callback to let plugins request minimum alignment for the
  buffer to pread/pwrite; useful for a plugin utilizing O_DIRECT or
  other situation where pre-aligned buffers are more efficient.
  Ideally, a blocksize filter would honor strict alignment below and
  advertise loose alignment above; all other filters (particularly
  ones like offset) can fail to initialize if they can't guarantee
  strict alignment and don't want to deal with bounce buffers.

* Test that zero-length read/write/extents requests behave sanely
  (NBD protocol says they are unspecified).

* If a client negotiates structured replies, and issues a read/extents
  call that exceeds EOF (qemu 3.1 is one such client, when nbdkit
  serves non-sector-aligned images), return the valid answer for the
  subset of the request in range and then NBD_REPLY_TYPE_ERROR_OFFSET
  for the tail, rather than erroring the entire request.

* Test and document how to run nbdkit from inetd and xinetd in
  nbdkit-service(1).

* Audit the code base to get rid of strerror() usage (the function is
  not thread-safe); however, using geterror_r() can be tricky as it
  has a different signature in glibc than in POSIX.

* Teach nbdkit_error() to have smart newline appending (for existing
  inconsistent clients), while fixing internal uses to omit the
  newline. Commit ef4f72ef has some ideas on smart newlines, but that
  should probably be factored into a utility function.

* We may need a way to lock nbdkit into memory and adjust the OOM
  killer score.  See this LKML discussion:
  https://www.mail-archive.com/linux-kernel@vger.kernel.org/msg1933394.html
  and also look at the implementation of the -swap option in
  nbd-client.

* Add a mode of operation where nbdkit is handed a pre-opened fd to be
  used immediately in transmission phase (skipping handshake).  There
  are already third-party clients of the kernel's /dev/nbdX which rely
  on their own protocol instead of NBD handshake, before calling
  ioctl(NBD_SET_SOCK); this mode would let the third-party client
  continue to keep their non-standard handshake while utilizing nbdkit
  to prototype new behaviors in serving the kernel.

* Clients should be able to list export names supported by plugins.
  Current behaviour is not really correct: We only list the -e
  parameter from the command line, which is different from the export
  name(s) that a plugin might want to support.  Probably we should
  deprecate the -e option entirely since it does nothing useful.

* Add plugin "connect" method.  This would be called on a connection
  before handshaking or TLS negotiation, and could be used (with
  nbdkit_peer_name) to accept or reject connections based on IP
  address, rather like a poor man's TCP wrappers.  See also commit
  c05686f9577f.

* Background thread for filters.  Some filters (readahead, cache and
  proposed scan filter - see below) could be more effective if they
  were able to defer work to a background thread.  However it's not as
  simple as just creating a background thread, because you only have
  access to the connection / next_ops from the context of a filter
  callback.  (Also you can't "save" next_ops because it becomes
  invalid outside the callback).  The background thread would need to
  have its own connection to the plugin, which would be independent of
  any client connection, and this requires some care because it breaks
  an existing assumption.

* Add scan filter.  This would be placed on top of cache filters and
  would scan (read) the whole disk in the background, ensuring it is
  copied into the cache.  Useful if you have a slow plugin, limited
  size device, and lots of local disk space, especially if you know
  that the NBD clients will eventually read all of the device.  RWMJ
  wrote an implementation of this but it doesn't work well without a
  background thread.

* "nbdkit.so": nbdkit as a loadable shared library.  The aim of nbdkit
  is to make it reusable from other programs (see nbdkit-captive(1)).
  If it was a loadable shared library it would be even more reusable.
  API would allow you to create an nbdkit instance, configure it (same
  as the current command line), start it serving on a socket, etc.
  However perhaps the current ability to work well as a subprocess is
  good enough?  Also allowing multiple instances of nbdkit to be
  loaded in the same process is probably impossible.

* Examine other fuzzers: https://gitlab.com/akihe/radamsa

* Analyze code with Coverity.

Suggestions for plugins
-----------------------

Note: qemu supports other formats such as libnfs, iscsi, gluster and
ceph/rbd, and while similar plugins could be written for nbdkit there
is no compelling reason unless the result is better than qemu-nbd.
For the majority of users it would be better if they were directed to
qemu-nbd for these use cases.

* XVA files

  https://lists.gnu.org/archive/html/qemu-devel/2017-11/msg02971.html
  is a partial solution but it needs cleaning up.

nbdkit-floppy-plugin:

* Add boot sector support.  In theory this is easy (eg. using
  SYSLINUX), but the practical reality of making a fully bootable
  floppy is rather more complex.

* Add multiple dir merging.

nbdkit-linuxdisk-plugin:

* Add multiple dir merging (in e2fsprogs mke2fs).

Suggestions for language plugins
--------------------------------

Python:

* Get the __docstring__ from the module and print it in --help output.
  This requires changes to the core API so that config_help is a
  function rather than a variable (see V3 suggestions below).

Rust:

* Consider supporting a more idiomatic style for writing Rust plugins.

* Better documentation.

* Add tests.

* There is no attempt to ‘make install’ or otherwise package the
  crate.  Since it looks as if Rust code is normally distributed as
  source it's not clear what that would even mean.

Suggestions for filters
-----------------------

* tar plugin should really be a filter

* gzip plugin should really be a filter

* libarchive could be used to implement a general tar/zip filter

* LUKS encrypt/decrypt filter, bonus points if compatible with qemu
  LUKS-encrypted disk images

* masking plugin features for testing clients (see 'nozero' and 'fua'
  filters for examples)

* nbdkit-cache-filter should handle ENOSPC errors automatically by
  reclaiming blocks from the cache

* zstd filter was requested as a way to do what we currently do with
  xz but saving many hours on compression (at the cost of hundreds of
  MBs of extra data)
  https://github.com/facebook/zstd/issues/395#issuecomment-535875379

nbdkit-rate-filter:

* allow other kinds of traffic shaping such as VBR

* limit traffic per client (ie. per IP address)

* split large requests to avoid long, lumpy sleeps when request size
  is much larger than rate limit

nbdkit-retry-filter:

* allow user to specify which errors cause a retry and which ones are
  passed through; for example there's probably no point retrying on
  ENOMEM

* implement a softer mode (retry-reopen=no) where we don't reopen the
  plugin, we just retry the data command that failed

* there are all kinds of extra complications possible here,
  eg. specifying a pattern of retrying and reopening:
  retry-method=RRRORRRRRORRRRR meaning to retry the data command 3
  times, reopen, retry 5 times, etc.

* subsecond times

nbdkit-extentlist-filter:

* read the extents generated by qemu-img map, allowing extents to be
  ported from a qemu block device

* make non-read-only access safe by updating the extent list when the
  filter sees writes and trims

Filters for security
--------------------

Things like blacklisting or whitelisting IP addresses can be done
using external wrappers (TCP wrappers, systemd).

However it might be nice to have a configurable filter for preventing
valid but not sensible requests.  The server already filters invalid
requests.  This would be like seccomp, and could be implemented using
an eBPF-based parser.  Unfortunately actual eBPF is difficult to use
for userspace processes.  The "standard" isn't solidly defined - the
Linux kernel implementation is the standard - and Linux has by far the
best implementation, particularly around bytecode verification and
JITting.  There is a userspace VM (ubpf) but it has very limited
capabilities compared to Linux.

Composing nbdkit
----------------

Filters allow certain types of composition, but others would not be
possible, for example RAIDing over multiple nbd sources.  Because the
plugin API limits us to loading a single plugin to the server, the
best way to do this (and the most robust) is to compose multiple
nbdkit processes.  Perhaps libnbd will prove useful for this purpose.

Build-related
-------------

* Figure out how to get 'make distcheck' working. VPATH builds are
  working, but various pkg-config results that try to stick
  bash-completion and ocaml add-ons into their system-wide home do
  not play nicely with --prefix builds for a non-root user.

* Port to Windows.

V3 plugin protocol
------------------

From time to time we may update the plugin protocol.  This section
collects ideas for things which might be fixed in the next version of
the protocol.

Note that we keep the old protocol(s) around so that source
compatibility is retained.  Plugins must opt in to the new protocol
using ‘#define NBDKIT_API_VERSION <version>’.

* All methods taking a ‘count’ field should be uint64_t (instead of
  uint32_t).  Although the NBD protocol does not support 64 bit
  lengths, it might do in future.

* pread could be changed to allow it to support Structured Replies
  (SRs).  This could mean allowing it to return partial data, holes,
  zeroes, etc.  For a client that negotiates SR coupled with a plugin
  that supports .extents, the v2 protocol would allow us to at least
  synthesize NBD_REPLY_TYPE_OFFSET_HOLE for less network traffic, even
  though the plugin will still have to fully populate the .pread
  buffer; the v3 protocol should make sparse reads more direct.

* Parameters should be systematized so that they aren't just (key,
  value) strings.  nbdkit should know the possible keys for the plugin
  and filters, and the type of the values, and both check and parse
  them for the plugin.

* Modify open() API so it takes an export name parameter.

* Change config_help from a variable to a function.
