XZ_THREADS changing do_image_cpio task hash

[Mike Crowe <mac@mcrowe.com>]

It looks like this problem was raised previously[1] but there was some
confusion that made it look like it had been resolved.

Using current master or scarthgap, if I add:

 XZ_THREADS = "32"
 INITRAMFS_FSTYPES = "cpio.xz"

to local.conf, build core-image-minimal-initramfs (for qemuarm64) and then
tweak this to:

 XZ_THREADS = "8"
 INITRAMFS_FSTYPES = "cpio.xz"

and build core-image-minimal-initramfs again then the task hash for
do_image_cpio changes:

 tmp-glibc/stamps/qemuarm64-oe-linux/core-image-minimal-initramfs/1.0.do_image_cpio.sigdata.b1d5686160dfefe3c2aeddc5caa6a7d3e14e6478392abf81ddaf04d5cbaeaa1e
 tmp-glibc/stamps/qemuarm64-oe-linux/core-image-minimal-initramfs/1.0.do_image_cpio.sigdata.28a20d80e712a003b71146fd37ed1158dc99f4ef68f714e19d25fe6f5c83091b

basehash changed from ea836e918b7b1d732979d58e1e1d249355c59dc3e5a7d15e4be1ae66ff5dfda0 to 0029ad7eb64839cb1d6ac8de22d94676ddf9503b685517225546e651fb49935b
Variable do_image_cpio value changed:
@@ -17,5 +17,5 @@
 	fi

 	cd ${TMPDIR}/work/qemuarm64-oe-linux/core-image-minimal-initramfs/1.0/deploy-core-image-minimal-initramfs-image-complete
-	xz -f -k -c -6 --memlimit=50% --threads=32 --check=crc32 core-image-minimal-initramfs-qemuarm64${IMAGE_VERSION_SUFFIX}.cpio > core-image-minimal-initramfs-qemuarm64${IMAGE_VERSION_SUFFIX}.cpio.xz
+	xz -f -k -c -6 --memlimit=50% --threads=8 --check=crc32 core-image-minimal-initramfs-qemuarm64${IMAGE_VERSION_SUFFIX}.cpio > core-image-minimal-initramfs-qemuarm64${IMAGE_VERSION_SUFFIX}.cpio.xz
 	rm core-image-minimal-initramfs-qemuarm64${IMAGE_VERSION_SUFFIX}.cpio

My understanding of vardepsexclude and vardepvalue means that bitbake.conf
containing:

 XZ_THREADS ?= "${@oe.utils.cpu_count(at_least=2)}"
 XZ_THREADS[vardepvalue] = "1"
 XZ_DEFAULTS ?= "--memlimit=${XZ_MEMLIMIT} --threads=${XZ_THREADS}"
 XZ_DEFAULTS[vardepsexclude] += "XZ_MEMLIMIT XZ_THREADS"

ought to avoid the task hash changing. But it doesn't.

Am I missing an obvious reason why the task hash should change? Our build
hosts and developer machines have varying numbers of CPUs and this is
causing a lack of sstate sharing. :(

Thanks.

Mike.

[1] openembedded-core@lists.openembedded.org