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Re: Skippable Frames in zstd files

From: Antonio Diaz Diaz
Subject: Re: Skippable Frames in zstd files
Date: Sat, 20 Aug 2022 17:29:40 +0200
User-agent: Mozilla/5.0 (X11; U; Linux i586; en-US; rv: Gecko/20110420 SeaMonkey/2.0.14

Hi Chris,

Jamboretz, Chris wrote:
The zstd compression format allows for skippable frames

pzstd uses this and puts the skippable frame at the beginning of the file.

Thank you for reporting this.

I think pzstd is making an improper use of skippable frames.

The link above states that "Skippable frames defined in this specification are compatible with LZ4 ones".

So it can't be known from the magic bytes if a file starting with a skippable frame is in zstd format or in lz4 format.

In fact, https://github.com/lz4/lz4/blob/dev/doc/lz4_Frame_format.md#skippable-frames advises not to do what pzstd is doing:

"For the purpose of facilitating identification, it is discouraged to start a flow of concatenated frames with a skippable frame. If there is a need to start such a flow with some user data encapsulated into a skippable frame, it's recommended to start with a zero-byte LZ4 frame followed by a skippable frame. This will make it easier for file type identifiers."

The function test_format in zutils.cc doesn't take this magic_number into 
account and so reports the file as uncompressed.

Detecting the 16 (!) magic numbers of skippable frames in test_format would be wrong because they do not really identify the zstd format. IMHO pzstd should follow the documentation of lz4 and start its files with a zero-byte zstd frame.

Best regards,

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