Additional exportable layers #181
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| # Drawbacks | ||
| [drawbacks]: #drawbacks | ||
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I actually think one of the drawbacks is any buildpack using this feature is highly tied to the underlying stack image. With #167, does this make it harder for buildpacks to have that tight coupling? How does one know what VOLUMEs are required before building?
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What happens if the |
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I do think the use cases are compelling, but my concerns are around exposing this information upfront and I think this impacts #167. |
| # Optionally users can override the default application workspace | ||
| # by specifying it through the WORKDIR directive in the build | ||
| # and run images. | ||
| WORKDIR /app |
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Should we instead use CNB_APP_DIR for this? Although workdir=appdir in the buildpack API, in the platform API we don't use the working directory to indicate this and instead use an environment variable.
| USER ${CNB_USER_ID}:${CNB_GROUP_ID} | ||
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| Buildpacks will be passed a list of volumes through the `CNB_EXPORT_VOLUMES` variable and this variable would be a JSON list. Buildpacks may read this environment variable during `detect` or `build`. |
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| Buildpacks will be passed a list of volumes through the `CNB_EXPORT_VOLUMES` variable and this variable would be a JSON list. Buildpacks may read this environment variable during `detect` or `build`. | |
| Buildpacks will be passed a list of volumes through the `CNB_EXPORT_DIRS` variable and this variable would be a JSON list. Buildpacks may read this environment variable during `detect` or `build`. |
Given we are exporting the contents of these build-time volumes into real layers in the resulting app image maybe this makes more sense from the buildpack perspective? It is irrelevent to the buildpack that these are build time volumes.
| Users would be able to define stack images with `VOLUME` directives in Dockerfiles. | ||
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| For example a build/run base image may look like - |
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Is it confusing to add these volumes to the run image, given that when we extend the run image to create the app image these directories will have buildpack-contributed contents and thus will not behave as volumes?
If did wanted these as volumes on the run image (to prevent folks from extending the run image in a way that breaks rebase) I think the exporter should remove these volumes from the final app image.
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| # Drawbacks | ||
| [drawbacks]: #drawbacks | ||
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It would be hard for a simple platform like tekton to implement this.
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Is it possible this could open a new attack vector? I can't think of anything specific, but i wonder what's possible if a buildpack can drop files in /etc/ or /usr/bin/ for example.
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@samj1912 will update with an alternative proposal from the last WG. |
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| The lifecycle changes would involve exporting the files present in the above locations which should be similar to the logic that currently exists for exporting application workspace. Buildpacks could also take advantage of `slices` to specify paths in these additional directories that should exist as separate layers. | ||
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| The lifecycle `analysis` phase would also be responsible for validating that the above list of `export_dirs` is valid for the provided `run` image. |
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How would analyzer validate this? We would need to be able to do this quickly, so some sort of pre-processing would need to occur.
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Answered below. It would make it fairly slow if the platform has not put additional metadata in the run image.
| # Unresolved Questions | ||
| [unresolved-questions]: #unresolved-questions | ||
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| TBD. |
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Are there any security ramifications for certain paths? Do platforms or builders need a way to deny paths?
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It might be good to have a block list of paths.
| export_dirs = ["/opt", "/home/cnb/.config"] | ||
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| The above would only be valid for normal buildpacks and not meta-buildpacks. Based on the `volumes` field, a platform would be responsible for validating that the `build` and `run` image do not have any content on these paths. Additionally, the builder image would be tagged with a label `io.buildpacks.export_dirs` with the list of paths that need to be exported. |
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| The above would only be valid for normal buildpacks and not meta-buildpacks. Based on the `volumes` field, a platform would be responsible for validating that the `build` and `run` image do not have any content on these paths. Additionally, the builder image would be tagged with a label `io.buildpacks.export_dirs` with the list of paths that need to be exported. | |
| The above would only be valid for normal buildpacks and not meta-buildpacks. Based on the `export_dirs` field, a platform would be responsible for validating that the `build` and `run` image do not have any content on these paths. Additionally, the builder image would be tagged with a label `io.buildpacks.export_dirs` with the list of paths that need to be exported. |
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| The lifecycle changes would involve exporting the files present in the above locations which should be similar to the logic that currently exists for exporting application workspace. Buildpacks could also take advantage of `slices` to specify paths in these additional directories that should exist as separate layers. | ||
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| The lifecycle `analysis` phase would also be responsible for validating that the above list of `export_dirs` is valid for the provided `run` image. |
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It would make the analysis phase fairly expensive. There are a couple of ways -
- During the builder creation, the platform add a label to the run images with a flag making sure that the buildpacks are compatible with the run image.
- The other option is to download the run images and check all the paths across the layers to confirm that the specified folders are empty.
Not ideal, but we could skip the validation as discussed in the WG if the operator trusts the builder to be valid.
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Could validation be just the Deny path list that is set at builder creation time and stored as a label? Or is that not validating the image in the way you would expect?
| [buildpack] | ||
| id = "<buildpack ID>" | ||
| name = "<buildpack name>" | ||
| export_dirs = ["/opt", "/home/cnb/.config"] |
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Are we worried about this coupling between a buildpack and the stack? Does this hurt portability of buildpacks between stacks?
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It probably will, but it is an optional feature to implement. If a buildpack is aiming for portability it could choose not to do things this way.
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@samj1912 will update with single input / multiple output proposal from the discussion. |
Signed-off-by: Sambhav Kothari <[email protected]>
Signed-off-by: Sambhav Kothari <[email protected]>
Signed-off-by: Sambhav Kothari <[email protected]>
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| # Motivation | ||
| [motivation]: #motivation | ||
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| The main motivation behind this RFC is to unlock exporting application images that require libraries or software to be installed in certain directories apart from the default application directory and these installations are rebase safe and self-contained. This unlocks use cases where buildpacks may install software in `/opt` directories (for AWS Lambda extensions or other common standalone software) for example or preserve some settings in a `/home/$USER/.config` for the output application. This provides a rebase safe extension point without major changes to the buildpacks API. |
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If the motivation is to allow for "libraries or software that install in certain directories", it goes at odds with requiring that "these exportable volumes be empty". After all, we can't guarantee that these "libraries or software" aren't installing to / or something.
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| ```json | ||
| [ | ||
| {"name": "user-config-dir", "value": "/home/cnb/.config"}, |
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I like this mapping concept being done by the base image author.
Could this list be something buildpack authors could get at and then name layers that match user-config-dir that gets exported to /home/cnb/.config during export phase? Or perhaps a key in the layer.toml has to be set so it is more explicit.
The requirement for an empty dir and symlinking for builder could potentially be avoided. A buildpack author could put a new binary in the resulting app image's /usr/bin by putting the binary in /layers/<bp-dir>/usr-bin/. If a new stack is used that doesn't support it, the layer would still exist on the resulting app image but not in the overridden path.
Giving the buildpack a dictionary so they can easily check if "usr/bin" is available to use would allow buildpack authors to either put the binary in "usr/bin" or do more work like altering PATH and whatnot to get it to work on other base images.
{
"usr/bin" : "usr-bin",
"~/.config": "user-config-dir"
}
| # What it is | ||
| [what-it-is]: #what-it-is | ||
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| Buildpack authors would be able to buildpacks with `buildpack.exportable-volumes` keys in their `buildpack.toml` files. |
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| Buildpack authors would be able to buildpacks with `buildpack.exportable-volumes` keys in their `buildpack.toml` files. | |
| Buildpack authors would be able define a set of `buildpack.exportable-volumes` keys in their `buildpack.toml` files. |
Signed-off-by: Sambhav Kothari [email protected]
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