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#41 in Operating systems

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Extension APIs for ostree that are written in Rust, using the Rust ostree bindings.

If you are writing tooling that uses ostree and Rust, this crate is intended for you. However, while the ostree core is very stable, the APIs and data models and this crate should be considered "slushy". An effort will be made to preserve backwards compatibility for data written by prior versions (e.g. of tar and container serialization), but if you choose to use this crate, please file an issue to let us know.

At the moment, the following projects are known to use this crate:

The intention of this crate is to be where new high level ostree-related features land. However, at this time it is kept separate from the core C library, which is in turn separate from the ostree-rs bindings.

High level features (more on this below):

  • ostree and opencontainers/image bridging/integration
  • Generalized tar import/export
  • APIs to diff ostree commits
flowchart TD
    ostree-rs-ext --- ostree-rs --- ostree
    ostree-rs-ext --- containers-image-proxy-rs --- skopeo --- containers/image

For more information on the container stack, see below.

module "tar": tar export/import

ostree's support for exporting to a tarball is lossy because it doesn't have e.g. commit metadata. This adds a new export format that is effectively a new custom repository mode combined with a hardlinked checkout.

This new export stream can be losslessly imported back into a different repository.

Filesystem layout

├── etc                # content is at traditional /etc, not /usr/etc
│   └── passwd
├── sysroot       
│   └── ostree         # ostree object store with hardlinks to destinations
│       ├── repo
│       │   └── objects
│       │       ├── 00
│       │       └── 8b
│       │           └── 7df143d91c716ecfa5fc1730022f6b421b05cedee8fd52b1fc65a96030ad52.file.xattrs
│       │           └── 7df143d91c716ecfa5fc1730022f6b421b05cedee8fd52b1fc65a96030ad52.file
│       └── xattrs    # A new directory with extended attributes, hardlinked with .xattr files
│           └── 58d523efd29244331392770befa2f8bd55b3ef594532d3b8dbf94b70dc72e674
└── usr
    ├── bin
    │   └── bash
    └── lib64
        └── libc.so

Think of this like a new ostree repository mode tar-stream or so, although right now it only holds a single commit.

A major distinction is the addition of special .xattr files; tar variants and support library differ too much for us to rely on this making it through round trips. And further, to support the webserver-in-container we need e.g. security.selinux to not be changed/overwritten by the container runtime.

module "diff": Compute the difference between two ostree commits

    let subdir: Option<&str> = None;
    let refname = "fedora/coreos/x86_64/stable";
    let diff = ostree_ext::diff::diff(repo, &format!("{}^", refname), refname, subdir)?;

This is used by rpm-ostree ex apply-live.

module "container": Bridging between ostree and OCI/Docker images

This module contains APIs to bidirectionally map between OSTree commits and the opencontainers ecosystem.

Because container images are just layers of tarballs, this builds on the crate::tar module.

This module builds on containers-image-proxy-rs and skopeo, which in turn is ultimately a frontend around the containers/image ecosystem.

In particular, the containers/image library is used to fetch content from remote registries, which allows building on top of functionality in that library, including signatures, mirroring and in general a battle tested codebase for interacting with both OCI and Docker registries.


For existing organizations which use ostree, APIs (and a CLI) are provided to "encapsulate" and "unencapsulate" an OSTree commit as as an OCI image.

$ ostree-ext-cli container encapsulate --repo=/path/to/repo exampleos/x86_64/stable docker://quay.io/exampleos/exampleos:stable

You can then e.g.

$ podman run --rm -ti --entrypoint bash quay.io/exampleos/exampleos:stable

Running the container directly for e.g. CI testing is one use case. But more importantly, this container image can be pushed to any registry, and used as part of ostree-based operating system release engineering.

However, this is a very simplistic model - it currently generates a container image with a single layer, which means every change requires redownloading that entire layer. As of recently, the underlying APIs for generating container images support "chunked" images. But this requires coding for a specific package/build system.

A good reference code base for generating "chunked" images is rpm-ostree compose container-encapsulate. This is used to generate the current Fedora CoreOS images.

Unencapsulate an ostree-container directly

A primary goal of this effort is to make it fully native to an ostree-based operating system to pull a container image directly too.

The CLI offers a method to "unencapsulate" - fetch a container image in a streaming fashion and import the embedded OSTree commit. Here, you must use a prefix scheme which defines signature verification.

  • ostree-remote-image:$remote:$imagereference: This declares that the OSTree commit embedded in the image reference should be verified using the ostree remote config $remote.
  • ostree-image-signed:$imagereference: Fetch via the containers/image stack, but require some signature verification (not via ostree).
  • ostree-unverified-image:$imagereference: Don't do any signature verification
$ ostree-ext-cli container unencapsulate --repo=/ostree/repo ostree-remote-image:someremote:docker://quay.io/exampleos/exampleos:stable

But a project like rpm-ostree could hence support:

$ rpm-ostree rebase ostree-remote-image:someremote:quay.io/exampleos/exampleos:stable

(Along with the usual rpm-ostree upgrade knowing to pull that container image)

To emphasize this, the current high level model is that this is a one-to-one mapping - an ostree commit can be exported (wrapped) into a container image, which will have exactly one layer. Upon import back into an ostree repository, all container metadata except for its digested checksum will be discarded.


OSTree supports GPG and ed25519 signatures natively, and it's expected by default that when booting from a fetched container image, one verifies ostree-level signatures. For ostree, a signing configuration is specified via an ostree remote. In order to pair this configuration together, this library defines a "URL-like" string schema: ostree-remote-registry:<remotename>:<containerimage> A concrete instantiation might be e.g.: ostree-remote-registry:fedora:quay.io/coreos/fedora-coreos:stable To parse and generate these strings, see OstreeImageReference.


A key feature of container images is support for layering. This functionality is handled via a separate container/store module.

These APIs are also exposed via the CLI:

$ ostree-ext-cli container image --help
ostree-ext-cli-container-image 0.4.0-alpha.0
Commands for working with (possibly layered, non-encapsulated) container images

    ostree-ext-cli container image <SUBCOMMAND>

    -h, --help       Prints help information
    -V, --version    Prints version information

    copy      Copy a pulled container image from one repo to another
    deploy    Perform initial deployment for a container image
    help      Prints this message or the help of the given subcommand(s)
    list      List container images
    pull      Pull (or update) a container image

More details about ostree and containers

See ostree-and-containers.md.


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