Revision 1801940

fsfs: Add initial support for LZ4 compression.

This can significantly (up to 3 times) improve the speed of commits and
other operations with large binary or incompressible files, while still
maintaining a decent compression ratio.

Our current use of zlib compression — which, depending on the protocol,
can be used multiple times — heavily affects the speed of commits with
large binary or incompressible files.  According to the Squash benchmark
(https://quixdb.github.io/squash-benchmark/) and to my measurements, the
zlib compression speed with the default level is about 30-40 MiB/s, and
it doesn't matter if the file is incompressible or not.

This patch provides an alternative in the form of the LZ4 compression.
While still providing a decent compression ratio, LZ4 offers much faster
compression even than zlib with level=1, and can skip incompressible data
chunks.  Presumably, LZ4 is used for on-the-fly compression in different
file systems for these reasons.

With this patch, LZ4 compression will be enabled for fsfs repositories which
specify compression-level=1 in fsfs.conf.  The interoperability is implemented
by bumping the format of svndiff to 2 and the repository file system format
to 8.  From the client perspective, the patch starts using LZ4 compression
only for file:// protocol, and the support/negotiation of the use of svndiff2
with LZ4 compression for http:// and svn:// can be added later.

The tests for LZ4 compression can be run with one of the following commands:

    win-tests.py --fsfs-compression=1
    make check FSFS_COMPRESSION=1

* subversion/include/svn_delta.h
  (svn_txdelta_to_svndiff3): Update docstring.

* subversion/include/svn_error_codes.h
  (SVN_ERR_LZ4_COMPRESSION_FAILED,
   SVN_ERR_LZ4_DECOMPRESSION_FAILED): New error codes.

* subversion/include/private/svn_subr_private.h
  (svn__compress, svn__decompress): Rename to ...
  (svn__compress_zlib, svn__decompress_zlib): ..this.
  (svn__compress_lz4, svn__decompress_lz4): Declare new functions.

* subversion/libsvn_subr/compress.c
  (): Include LZ4 library header.
  (svn__compress, svn__decompress): Rename to ...
  (svn__compress_zlib, svn__decompress_zlib): ..this.
  (svn__compress_lz4, svn__decompress_lz4): Implement new functions.

* subversion/libsvn_subr/packed_data.c
  (write_stream_data, read_stream_data): Update usages of svn__compress()
   and svn__decompress().

* subversion/libsvn_delta/svndiff.c
  (SVNDIFF_V2): New.
  (get_svndiff_header): Update to support svndiff2 headers.
  (encode_window, decode_window, write_handler): Support svndiff2 with
   LZ4 compression.  Tweak the relevant comments.

* subversion/libsvn_fs_fs/fs.h
  (SVN_FS_FS__FORMAT_NUMBER): Bump to 8.
  (SVN_FS_FS__MIN_SVNDIFF2_FORMAT): New define.

* subversion/libsvn_fs_fs/fs_fs.c
  (write_config): Tweak the compression-level option description.
  (svn_fs_fs__create, svn_fs_fs__info_format): Update to handle the
   format bump.

* subversion/libsvn_fs_fs/transaction.c
  (txdelta_to_svndiff): New helper to call svn_txdelta_to_svndiff3() with
   appropriate svndiff version and compression level, depending on the
   file system configuration.
  (rep_write_get_baton, write_container_delta_rep): Use new helper.

* subversion/libsvn_fs_fs/revprops.c
  (parse_packed_revprops, repack_revprops, svn_fs_fs__copy_revprops):
   Update usages of svn__compress() and svn__decompress().

* subversion/libsvn_fs_fs/structure
  (Filesystem formats): Update to describe usage of svndiff2.

* subversion/tests/libsvn_subr/compress-test.c: New.

* subversion/tests/libsvn_delta/random_test.c
  (DEFAULT_ITERATIONS): Increase to 60.
  (do_random_test, do_random_combine_test): Test different svndiff versions
   and compresssion levels.

* build.conf
  (libsvn_subr): Build LZ4 library sources.
  (compress-test): Add new section.

* notes/svndiff: Describe svndiff2.

* NOTICE, LICENSE: Include license for LZ4.