[yocto] cannot re-use shared state cache between build hosts

[Andrea Galbusera]

Hi!

I was trying to share sstate between different hosts, but the consumer
build system seems to be unable to use re-use any sstate object. My
scenario is setup as follows:

* The cache was populated by a pristine qemux86 core-image-minimal build of
morty. This was done in a crops/poky container (running in docker on Mac)
* The cache was then served via HTTP
* The second host is a VM running Ubuntu 16.04 where I set SSTATE_MIRRORS
to point to the hosted sstate cache like this:

SSTATE_MIRRORS ?= "\
file://.* http://192.168.33.1:8000/sstate-cache/PATH;downloadfilename=PATH"

* I checked with curl that the VM can successfully get sstate objects from
the server.
* Then I start a new build (same metadata revisions, default configuration
for core-image-minimal) and each and every task run from scratch with no
sstate cache re-use.

Here are the two configurations from bitbake and /etc/lsb-release files:

On the container used to seed sstate cache:

Build Configuration:
BB_VERSION        = "1.32.0"
BUILD_SYS         = "x86_64-linux"
NATIVELSBSTRING   = "universal"
TARGET_SYS        = "i586-poky-linux"
MACHINE           = "qemux86"
DISTRO            = "poky"
DISTRO_VERSION    = "2.2.2"
TUNE_FEATURES     = "m32 i586"
TARGET_FPU        = ""
meta
meta-poky
meta-yocto-bsp    = "morty:2a70e84643381eca0e7bf7928d4a3d56f9651128"

$ cat /etc/lsb-release
DISTRIB_ID=Ubuntu
DISTRIB_RELEASE=16.04
DISTRIB_CODENAME=xenial
DISTRIB_DESCRIPTION="Ubuntu 16.04.2 LTS"

On the VM that should consume the cache:

Build Configuration:
BB_VERSION        = "1.32.0"
BUILD_SYS         = "x86_64-linux"
NATIVELSBSTRING   = "Ubuntu-16.04"
TARGET_SYS        = "i586-poky-linux"
MACHINE           = "qemux86"
DISTRO            = "poky"
DISTRO_VERSION    = "2.2.2"
TUNE_FEATURES     = "m32 i586"
TARGET_FPU        = ""
meta
meta-poky
meta-yocto-bsp    = "morty:2a70e84643381eca0e7bf7928d4a3d56f9651128"

$ cat /etc/lsb-release
DISTRIB_ID=Ubuntu
DISTRIB_RELEASE=16.04
DISTRIB_CODENAME=xenial
DISTRIB_DESCRIPTION="Ubuntu 16.04.3 LTS"


To me, the only differing bit that in my understanding can lead to sstate
cache objects invalidation is the value of NATIVELSBSTRING which is
"universal" inside the container and "Ubuntu-16.04". This sounds strange to
me, since both underlying systems are Ubuntu 16.04 (although not exactly
the same dot release) as confirmed by /etc/lsb-release contents.

Is the different NATIVELSBSTRING the root cause for everything being
re-built? If so, what's causing them being different in the end and what
does "universal" exactly mean (to me it looks like a more generic and
incluse term than any distro label, so I'm confused...)?
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