Image-specific layers - yocto

Question

I am building multiple poky-based images for different projects but using the same build directory (because it seems a bit overkill to have a new clone of yocto for every project).
But I happen to have my images conflicting with each other because of the .bbappend they define. For instance, I have two images that each extend the hostapd recipe in order to define their own configuration file.
Let me illustrate with an example. I have two layers that each define an image: first-project and second-project. Each has its own layer, because they belong to a different repository: meta-first-project and meta-second-project. And both use hostapd. So they projects look like this:
meta-first-project:
.
├── conf
│   └── layer.conf
├── recipes-connectivity
│   └── hostapd
│   ├── hostapd
│   │   └── hostapd.conf
│   └── hostapd_%.bbappend
└── recipes-first-project
   └── images
   └── first-project-image.bb
meta-second-project:
.
├── conf
│ └── layer.conf
├── recipes-connectivity
│ └── hostapd
│ ├── hostapd
│ │ └── hostapd.conf
│ └── hostapd_%.bbappend
└── recipes-second-project
└── images
└── second-project-image.bb
With a different hostapd.conf for each.
The conf/bblayers.conf file looks like this, including both layers:
conf/bblayers.conf:
BBLAYERS ?= " \
/home/user/Documents/yocto/poky/meta \
/home/user/Documents/yocto/poky/meta-poky \
/home/user/Documents/yocto/poky/meta-yocto-bsp \
/home/user/Documents/yocto/meta-first-project \
/home/user/Documents/yocto/meta-second-project \
When I run bitbake first-project-image, I want it to use the hostapd_%.bbappend in meta-first-project and not the one in meta-second-project. But my understanding is that both will be evaluated, meaning that meta-second-project/hostapd_%.bbappend will have an impact on my first-project-image, which I don't want.
Is there a way to solve that, or should I have two build folders (and download, build and duplicate everything there)?
For instance, I thought about checking something like this in the bbappend of hostapd in meta-first-project (pseudo-code):
if (${IMAGE_BASENAME} == "first-project-image") {
// write the whole bbappend here
}
Such that this bbappend is only considered when building the image first-project-image. However, ${IMAGE_BASENAME} does not seem to be set in the hostapd recipe...
Another idea, similar to COMPATIBLE_HOST and COMPATIBLE_MACHINE would be to define a COMPATIBLE_IMAGE variable, but that does not exist.

Answer 1

You can't change the content of individual recipes based on an image, that is a simple and unavoidable constraint from the way the system is built. The reason why is that the packages are constructed from the recipes long before the rootfs/image is built so it has no influence over it. All images use the same package set. You can reuse the same downloads, build directory and tree and simply change bblayers.conf to use one layer or the other and things will work but there isn't a way you can make both images work in the same configuration.
There are other options. You could separate out the hostapd change into two different recipes and then have each image include the correct content that way by including the right recipe. You could also tweak the image at construction time (see rootfs-postcommands.bbclass). You could specify two different DISTRO settings and switch between those, or maybe switch between configs depending on a MACHINE setting (making the hostapd recipe machine specific) but those two do require configuration changes between the images too. Multiconfig may or may not also be something which could help, it is hard to say from the information here.
There is no COMPATIBLE_IMAGE option as recipes are common to all images, that is just the way the system is designed and works, it doesn't build a new recipe set per image.

Answer 2

You can create your own COMPATIBLE_IMAGE variable and use it to conditionally append the conf file.
For example, in your meta-first-project/recipes-connectivity/hostapd/hostapd_%.bbappend:
SRC_URI_append = " ${#bb.utils.contains('COMPATIBLE_IMAGE', 'first-project-image', 'file://hostapd.conf', '', d)}"
And for your second image as well in
meta-second-project/recipes-connectivity/hostapd/hostapd_%.bbappend:
SRC_URI_append = " ${#bb.utils.contains('COMPATIBLE_IMAGE', 'second-project-image', 'file://hostapd.conf', '', d)}"
Now, the first conf file will appended to SRC_URI only if COMPATIBLE_IMAGE contains first-project-image.
You can define COMPATIBLE_IMAGE now anywhere you want, you can set it in local.conf for testing purposes or in your image recipe:
COMPATIBLE_MACHINE = "first-project-image"
You can use the COMPATIBLE_MACHINE variable now anywhere you want with the same principle.
Now, if you are appending to some hostapd tasks and using the conf file make sure to test on the variable before using it, and if hostapd understands directly the conf file and used without overriding some task you don't need to test on the variable.

Answer 3

Hello your use case seems compatible with BBMASK.
"Prevents BitBake from processing recipes and recipe append files."
Taken from documentation
With this variable, you can hide the bbappend you don't want to interact with your layer. Starting from your example, you'll have the following line in conf file of meta-first-project:
BBMASK += "meta-second-project/recipes-connectivity/hostapd"
But, if your two layers doesn't depends on each other, it is simpler to not reference them in their respective bblayers.conf. If the meta-second-project
is not necessary to build images provided by meta-first-project, simply remove meta-second-project from bblayers.conf.

Answer 4

I'm guessing from your post that the two projects are using the same MACHINE?
If they were different MACHINEs you'd have a lot more tools...
What I would do is probably create a hostapd-foo.bb with require hostapd.bb.