image,shared: Allow specifying compression level

[gnustomp]

Allow specifying compression levels when the underlying tool or mksquashfs understands it. For example, gzip-1 or zstd-22. Compression levels are checked pre-run.

Signed-off-by: James Ye jamesye@google.com

[monstermunchkin]

@gnustomp a couple of things are still missing:

• support for lxc images
• update docs (building.md)
• update compressionDescription variable in main.go
• lzo compression level support

Also, I believe we should do early validation here, here, here, and here. Some builds take quite a while, and it'd be a shame if they failed late due to a typo in the compression string.

[gnustomp]

Thanks for the feedback, @monstermunchkin. I have refactored the PR so that it supports specifying levels for most compression methods and have added tests.

• As I understand, LXC images are handled by the tarball code, so it should not require any special support, unlike LXD which can use squashfs?
• Since distrobuilder currently doesn't validate the compression method, I have retained this behaviour for tarball outputs in this PR. A compression level cannot be specified for unknown methods, however, and the output is uncompressed.
• While mksquashfs supports specifying a level for its lzo method, the corresponding tool for this algorithm is named lzop, so a split squashfs image currently can't be generated while specifying a compression level. (Without specifying a compression level, the metadata tarball is left uncompressed.) This could be worked around by redirecting squashfs lzo to the lzop binary or vice versa, but I think this should be a separate PR?

[monstermunchkin]

This looks really good. Just a few changes regarding lzo and lzop to avoid confusion.

[monstermunchkin]

• This could be worked around by redirecting squashfs lzo to the lzop binary or vice versa, but I think this should be a separate PR?

Let's add it to this PR, just a separate commit.

[gnustomp]

Let's add it to this PR, just a separate commit.

Done.