[OE-core] Problems with INC_RPM_IMAGE_GEN

[ChenQi]

Hi All,

Recently I've enabled INC_RPM_IMAGE_GEN on my old and slow build 
machine. Here are two problems I can see with this feature.

1. INC_RPM_IMAGE_GEN and 'rm_work'
If we enable INC_RPM_IMAGE_GEN and 'rm_work' at the same time, we would 
get build failures.
This is because the original rootfs has been removed because of rm_work.

2. INC_RPM_IMAGE_GEN and XXX_POSTPROCESS_COMMAND
Let's first look at a case.
1. build core-image-minimal with 'extrausers' inherited and root 
password set in local.conf
2. build core-image-minimal without 'extrausers' inherited
The image generated in the second step still has the root password set. 
This is not what we want.

The problem in incremental rpm image generation is that it chooses the 
final image as a starting point.
However, the final image has been tweaked by all those 
XXX_POSTPROCESS_COMMAND.
For example, the 'debug-tweaks' IMAGE_FEATURE will lead to several 
additonal ROOTFS_POSTPROCESS_COMMAND.
meta/classes/image.bbclass:# Allow dropbear/openssh to accept logins 
from accounts with an empty password string if debug-tweaks is enabled
meta/classes/image.bbclass:ROOTFS_POSTPROCESS_COMMAND += 
'${@base_contains("IMAGE_FEATURES", "debug-tweaks", 
"ssh_allow_empty_password; ", "",d)}'
meta/classes/image.bbclass:# Enable postinst logging if debug-tweaks is 
enabled
meta/classes/image.bbclass:ROOTFS_POSTPROCESS_COMMAND += 
'${@base_contains("IMAGE_FEATURES", "debug-tweaks", 
"postinst_enable_logging; ", "",d)}'

IMHO, the rootfs after all packages installed and before all postprocess 
commands running should serve as the reference rootfs for the 
incremental rpm image generation.
But I cannot figure out a reasonable solution which doesn't have much 
performance impact.

Any idea or suggestion?

Best Regards,
Chen Qi