[PATCH v2] feat: add zstd support to image conversions

[Michael Adler <michael.adler@siemens.com>]

This patch enables ISAR to build zstd-compressed images.

ISAR already supports building xz-compressed images but there are several key
differences between the two that may make one more suitable than the other,
depending on the specific use case:

1. Compression speed: zstd is generally faster than xz when it comes to both
compression and decompression times.

2. Compression ratio: "zstd and xz trade blows in their compression ratio.
Recompressing all [ArchLinux] packages to zstd with our options yields a total
~0.8% increase in package size on all of our packages combined, but the
decompression time for all packages saw a ~1300% speedup." [1]

3. Memory usage: zstd uses a smaller amount of memory compared to xz.

[1] https://archlinux.org/news/now-using-zstandard-instead-of-xz-for-package-compression/

Signed-off-by: Michael Adler <michael.adler@siemens.com>

---
 meta/classes/imagetypes.bbclass | 5 ++++-
 meta/conf/bitbake.conf          | 3 +++
 2 files changed, 7 insertions(+), 1 deletion(-)

diff --git a/meta/classes/imagetypes.bbclass b/meta/classes/imagetypes.bbclass
index 0cff440..f11ddc6 100644
--- a/meta/classes/imagetypes.bbclass
+++ b/meta/classes/imagetypes.bbclass
@@ -93,10 +93,13 @@ IMAGE_CMD:ubi() {
 IMAGE_CMD:ubi[depends] = "${PN}:do_transform_template"
 
 # image conversions
-IMAGE_CONVERSIONS = "gz xz"
+IMAGE_CONVERSIONS = "gz xz zst"
 
 CONVERSION_CMD:gz = "${SUDO_CHROOT} sh -c 'gzip -f -9 -n -c --rsyncable ${IMAGE_FILE_CHROOT} > ${IMAGE_FILE_CHROOT}.gz'"
 CONVERSION_DEPS:gz = "gzip"
 
 CONVERSION_CMD:xz = "${SUDO_CHROOT} sh -c 'xz -c ${XZ_OPTIONS} ${IMAGE_FILE_CHROOT} > ${IMAGE_FILE_CHROOT}.xz'"
 CONVERSION_DEPS:xz = "xz-utils"
+
+CONVERSION_CMD:zst = "${SUDO_CHROOT} sh -c 'zstd -c ${ZSTD_OPTIONS} ${IMAGE_FILE_CHROOT} > ${IMAGE_FILE_CHROOT}.zst'"
+CONVERSION_DEPS:zst = "zstd"
diff --git a/meta/conf/bitbake.conf b/meta/conf/bitbake.conf
index 522241a..a1bad71 100644
--- a/meta/conf/bitbake.conf
+++ b/meta/conf/bitbake.conf
@@ -140,6 +140,9 @@ XZ_DEFAULTS[vardepsexclude] += "XZ_MEMLIMIT XZ_THREADS"
 # Default parallelism for zstd
 ZSTD_THREADS ?= "${@oe.utils.cpu_count(at_least=2)}"
 ZSTD_THREADS[vardepvalue] = "1"
+ZSTD_LEVEL ?= "19"
+ZSTD_OPTIONS ?= "-${ZSTD_LEVEL} --threads=${ZSTD_THREADS}"
+ZSTD_OPTIONS[vardepsexclude] += "ZSTD_LEVEL ZSTD_THREADS"
 
 BBINCLUDELOGS ??= "yes"
 
-- 
2.39.1