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Added tests/files/README.

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.lzma Test Files
----------------
0. Introduction
This directory contains bunch of files to test handling of .lzma files
in .lzma decoder implementations. Many of the files have been created
by hand with a hex editor, thus there is no better "source code" than
the files themselves. All the test files (*.lzma) and this README have
been put into the public domain.
1. File Types
Good files (good-*.lzma) must decode successfully without requiring
a lot of CPU time or RAM. If the decoder supports only Single-Block
Streams, then good-multi-*.lzma won't decode, of course.
Bad files (bad-*.lzma) must cause the decoder to give an error. Like
with the good files, these files must not require a lot of CPU time
or RAM before they get detected to be broken.
Malicious files (malicious-*.lzma) are good in terms of the file format
specification, but try to trigger excessive CPU, RAM or disk usage in
the decoder. To prevent malicious files from putting the decoder in
inifinite loop (*), eating all available RAM or disk space, decoders
should have internal limitters that catch these situations.
(*) Strictly speaking not infinite, but if decoding of a small file
would take a few weeks or even years, it's an infinite loop in
practice.
2. Descriptions of Individual Files
2.1. Good Files
good-single-none.lzma uses implicit Copy filter with known Uncompressed
Size.
good-single-none-pad.lzma is good-single-none.lzma with Footer Padding.
good-cat-single-none-pad.lzma is two good-single-none-pad.lzma files
concatenated as is. Fully decoding this file requires that the decoder
supports decoding concatenated files.
good-single-lzma.lzma is LZMA compressed file with EOPM.
good-single-subblock-lzma.lzma has basic combination of Subblock and
LZMA filters.
good-single-subblock_rle.lzma takes advantage of Subblock filter's
run-length encoding.
good-single-delta-lzma.tiff.lzma is an image file that compresses
better with Delta+LZMA than with plain LZMA.
2.2. Bad Files
bad-single-data_after_eopm.lzma has LZMA+Subblock, where the Subblock
filter gives one byte of data to LZMA after LZMA has detected EOPM.
bad-single-data_after_eopm_2.lzma is like
bad-single-data_after_eopm.lzma but Subblock gives 256 MiB of data to
LZMA after LZMA has detected EOPM.
bad-single-subblock_subblock.lzma has Subblock+Subblock, where the
Subblock decoder is given End of Input in the middle of a Subblock.
bad-single-subblock-padding_loop.lzma contains huge amount of
consecutive Padding bytes, which isn't allowed by the Subblock filter
format. If it were allowed, this file would hang the decoder for very
long time (weeks to years).
bad-single-subblock1023-slow.lzma is similar to
malicious-single-subblock31-slow.lzma except that this uses 1023 bytes
of Padding in every place instead of 31 bytes. The Subblock filter
format specification allows only 31-byte Padings, thus this file must
get detected as bad without producing any output. Allowing larger
Padding than 31 bytes was considered (so this test file was created),
but it seemed to be a bad idea since it would increase worst-case CPU
usage.
2.3. Malicious Files
malicious-single-subblock31-slow.lzma requires quite a bit of CPU time
per decoded byte. It contains LZMA compressed Subblock filter data that
has as much Padding as the specification allows. LZMA is also used as
a Subfilter, to further slowdown the decoder. Every Subfilter instance
produces only one byte of output. If you can create a file that wastes
notably more CPU cycles than this file, please contact Lasse Collin.
malicious-single-subblock-256MiB.lzma is a tiny file that produces
256 MiB of output. It uses Subblock filter's run-length encoding
to achieve this.
malicious-single-subblock-64PiB.lzma is a tiny file that produces
64 PiB of output (if you have patience to wait). This is done by
chaining two Subblock filters and using their run-length encoders.
malicious-multi-metadata-64PiB.lzma is like
malicious-single-subblock-64PiB.lzma but the huge amount of output
is in a Metadata Block. Trying to decode this file may take years
unless the decoder catches that the Metadata has unreasonable size.