build_image: export FLATCAR_BUILD_ID to ensure consistency

[zhijianli88]

If FLATCAR_BUILD_ID is not exported, child processes or subsequent scripts may not inherit the variable. This can lead to mismatched build IDs between ${BUILD_DIR}/version.txt and ${rootfs}/lib/os-release, causing the generated QEMU image to fail to start.

Export the variable so that all related contexts share the same FLATCAR_BUILD_ID.

Link: flatcar/Flatcar#2041

Cc: stable branch

[chewi]

I'd like to better understand how these got out of sync. I suspect you may be tripping up on a quoting issue we had a while back that I fixed in 874ed25.

[zhijianli88]

I'd like to better understand how these got out of sync. I suspect you may be tripping up on a quoting issue we had a while back that I fixed in 874ed25.

I'm not sure whether they are the same. because I'm building the flatcar based the the stable branch(4593.2.2), which encountered flatcar/Flatcar#2041

I will try to cherry-pick 874ed25 and try again.

Let's share more details

• during build_image, it will write FLATCAR_BUILD_ID to 2 files as following:
  ${BUILD_DIR}/version.txt and ${rootfs}/lib/os-release
• image_to_vm.sh will read ${BUILD_DIR}/version.txt and use FLATCAR_BUILD_ID as a part of the filename /oem/sysext/oem-qemu-4593.2.2+2026-05-28-0426.raw , FLATCAR_BUILD_ID=2026-05-28-0426 in here.
• Once FLATCAR_BUILD_ID in ${BUILD_DIR}/version.txt and ${rootfs}/lib/os-release are different, it will cause stable-4459.2.4 image qemu boot hang Flatcar#2041 to download a matched oem-qemu-4593.2.2+${FLATCAR_BUILD_ID}.raw
  which is almost impossible.

[zhijianli88]

I will try to cherry-pick 874ed25 and try again.

It doesn't work for me on the stable branch after cherry-pick 874ed25

...
[   50.822250] audit: type=1130 audit(1779961136.512:37): pid=1 uid=0 auid=4294967295 ses=4294967295 subj=kernel msg='unit=ignition-quench comm="systemd" exe="/usr/lib/systemd/systemd" hostname=? addr=? terminal=? res=success'     
[   50.830236] audit: type=1131 audit(1779961136.514:38): pid=1 uid=0 auid=4294967295 ses=4294967295 subj=kernel msg='unit=ignition-quench comm="systemd" exe="/usr/lib/systemd/systemd" hostname=? addr=? terminal=? res=success'     
[   50.882397] initrd-setup-root-after-ignition[1022]: Did not find /oem/sysext/oem-qemu-4593.2.2+2026-05-28-0920.raw nor /etc/flatcar/oem-sysext/oem-qemu-4593.2.2+2026-05-28-0920.raw, downloading                                   
[   51.001642] systemd[1]: afterburn-network-kargs.service - Afterburn Initrd Setup Network Kernel Arguments was skipped because no trigger condition checks were met.                                                                 
[   52.726724] initrd-setup-root-after-ignition[1039]: curl: (22) The requested URL returned error: 404                                                                                                                                
[ ***  ] Job initrd-setup-root-after-ignitio…ice/start running (37s / no limit)                                                                                                                                                        
[**    ] Job initrd-setup-root-after-ignitio…ice/start running (39s / no limit)                                                                                                                                                        
[   ***] Job initrd-setup-root-after-ignitio…ice/start running (41s / no limit)                                                                                                                                                        
[    **] Job initrd-setup-root-after-ignitio…ice/start running (43s / no limit)                                                                                                                                                        
[***   ] Job initrd-setup-root-after-ignitio…ice/start running (45s / no limit)                                                                                                                                                        
[***   ] Job initrd-setup-root-after-ignitio…ice/start running (47s / no limit)            
...

:/root# systemctl start sysroot.mount
:/root# systemctl start sysroot-oem.mount
:/root# ls /sysroot/oem/sysext/
active-oem-qemu  oem-qemu-4593.2.2+2026-05-28-0919.raw
:/root# cat /sysroot/lib/os-release 
NAME="Flatcar Container Linux by Kinvolk"
ID=flatcar
ID_LIKE=coreos
VERSION=4593.2.2+2026-05-28-0920
VERSION_ID=4593.2.2
BUILD_ID=2026-05-28-0920
SYSEXT_LEVEL=1.0
PRETTY_NAME="Flatcar Container Linux by Kinvolk 4593.2.2+2026-05-28-0920 (Oklo)"
ANSI_COLOR="38;5;75"
HOME_URL="https://flatcar.org/"
BUG_REPORT_URL="https://issues.flatcar.org"
FLATCAR_BOARD="arm64-usr"
CPE_NAME="cpe:2.3:o:flatcar-linux:flatcar_linux:4593.2.2+2026-05-28-0920:*:*:*:*:*:*:*"

[chewi]

Okay, but you still haven't explained how or when they come to be different. os-release is written by set_lsb_release. This is executed by build_image and they both read FLATCAR_BUILD_ID from the same place, so I cannot see how they could end up being different.

[zhijianli88]

Okay, but you still haven't explained how or when they come to be different. os-release is written by set_lsb_release. This is executed by build_image and they both read FLATCAR_BUILD_ID from the same place, so I cannot see how they could end up being different.

everytime commom.sh is being sourced, it will generate a new FLATCAR_BUILD_ID, during the build_image context, it will source commom.sh multiple times. These FLATCAR_BUILD_ID could be different.

[chewi]

FLATCAR_BUILD_ID isn't random though. It's based on the SDK version you're using and the git commit you were on when you started the SDK. Those should not change in the middle of a build. If you want it to stop changing version.txt based on the git commit, pass -U to run_sdk_container. We acknowledge that the default behaviour is slightly annoying, and we're considering alternatives. @t-lo, did you get anywhere with that?

[zhijianli88]

FLATCAR_BUILD_ID isn't random though. It's based on the SDK version you're using and the git commit you were on when you started the SDK. Those should not change in the middle of a build. If you want it to stop changing version.txt based on the git commit, pass -U to run_sdk_container. We acknowledge that the default behaviour is slightly annoying, and we're considering alternatives. @t-lo, did you get anywhere with that?

https://github.com/flatcar/scripts/blob/main/common.sh#L309-L309

it's a timestamp

[t-lo]

I didn't find time for anything yet, sorry. My general sentiment is that we set / overwrite version.txt far too eagerly and should probably refrain from touching it in run_sdk_container most of the time. Release builds should update the version, and maybe "qualified" dev builds; right now we're updating it far too often. While I'm not sure about the full implications of exposing FLATCAR_BUILD_ID from brief skimming, any simplification that prevents needless overwriting the versions would be welcome.

[zhijianli88]

I didn't find time for anything yet, sorry. My general sentiment is that we set / overwrite version.txt far too eagerly and should probably refrain from touching it in run_sdk_container most of the time. Release builds should update the version, and maybe "qualified" dev builds; right now we're updating it far too often. While I'm not sure about the full implications of exposing FLATCAR_BUILD_ID from brief skimming, any simplification that prevents needless overwriting the versions would be welcome.

Thanks for the feedback. I'd like to clarify a point that might clear things up.

The version.txt I'm mentioning was ${BUILD_DIR}/version.txt, for example /mnt/host/source/src/build/images/arm64-usr/developer-4593.2.2+2026-05-28-0919-a1/version.txt. It was generated by build_image, and every build_image will create a new ${BUILD_DIR}, so in theory, this version.txt will not be overwritten.

Back to this patch, export FLATCAR_BUILD_ID after the first sourcing common.sh in build_image so that the entire build_image context including all its child processes can see the same environment variable FLATCAR_BUILD_ID.
When common.sh is being sourced again, FLATCAR_BUILD_ID will not be updated again.
https://github.com/flatcar/scripts/blob/main/common.sh#L309-L309

This ensures the entire build_image context sees the same FLATCAR_BUILD_ID