I had the following in my image recipe
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set_root_pwd() {                                                                
    # we know we have /etc/shadows because we install openssh
    sed -i 's|^root:[^:]*:|root:XXXXXXXX:|' \
        ${IMAGE_ROOTFS}/etc/shadow
}
# execute function on image creation
ROOTFS_POSTPROCESS_COMMAND += "set_root_pwd;
----
then I run bitbake to set the password and it worked. So I have a current image.

Now I disabled the function by commenting out the one line that actually executes it.
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#ROOTFS_POSTPROCESS_COMMAND += "set_root_pwd;
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If I try now to build the image again it wont build
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NOTE: Tasks Summary: Attempted 2655 tasks of which 2655 didn't need to be rerun and all succeeded.
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I would have assumed that every change would rebuild the changed component. This does not seem to be the case with the image recipe.



Some more details:

Base of my image recipe:
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IMAGE_INSTALL = "packagegroup-core-boot ${ROOTFS_PKGMANAGE_BOOTSTRAP} ${CORE_IMAGE_EXTRA_INSTALL}"
inherit core-image                                                              
[other options skipped]
----

My distro.conf contains the following line. This is not relevant if you modify the set_root_pwd() to run some other commands. If you don't modify it, you need th ssh-serveropenssh image feature.
---
IMAGE_FEATURES = "read-only-rootfs ssh-serveropenssh package-management"
---