Glycin

Glycin allows to decode images into gdk::Textures and to extract image metadata. The decoding happens in sandboxed modular image loaders.

• glycin – The Rust image library
  • libglycin – C-Bindings for the library
  • libglycin-gtk4 – C-Bindings to convert glycin frames to GDK Textures
• glycin-utils – Utilities to write loaders for glycin
• loaders – Glycin loaders for several formats

Usage and Packaging

The Rust client library is available as glycin on crates.io. For other programming languages, the libglycin C client library can be used. For the client libraries to work, loader binaries must also be installed. The loader binaries provided by the glycin project cover a lot of common image formats (see below). Both, the loader binaries and libglycin can be built from the released glycin tarballs. By using -Dlibglycin=false or -Dglycin-loaders=false it is possible to build only one of these components. In distributions, the loaders are usually packaged as glycin-loaders, and libglycin as libglycin-1. However, each loader binary could be also packaged as its own package.

Example

let file = gio::File::for_path("image.jpg");
let image = Loader::new(file).load().await?;

let height = image.info().height();
let texture = image.next_frame().await?.texture();

Limitations

Glycin is based on technologies like memfds, unix sockets, and linux namespaces. It currently only works on Linux. An adoption to other unixoid systems could be possible without usage of the sandbox mechanism. Windows support is currently not planned and might not be feasible.

Supported Image Formats

The following features are supported by the glycin loaders provided in the loaders directory.

Image Loader Configuration

Loader configurations are read by the client library from XDG_DATA_DIRS and XDG_DATA_HOME. The location is typically of the from

<data-dir>/share/glycin/<compat-version>+/conf.d/<loader-name>.conf

so for example

<data-dir>/share/glycin/0+/conf.d/glyicn-image-rs.conf

The configs are glib KeyFiles of the the form

[loader:image/png]
Exec = /usr/libexec/glycin/1+/glycin-image-rs

Where the part behind loader is a mime-type and the value of Exec can be any executable path.

Existing Compatibility Versions

Not every new major version of the library has to break compatibility with the loaders. If a glycin version X breaks compatibility, the new compativility version will be called X+. Only glycin X and newer version will be compatible with X+ until a new compatibilityv version is used. The definition of the API of each compatibility version is available in docs/. The following compatibility versions currently exist

Sandboxing and Inner Workings

Glycin spawns one process per image file. The communication between glycin and the loader takes place via peer-to-peer D-Bus over a Unix socket.

Glycin supports a sandbox mechanism inside and outside of Flatpaks. Outside of Flatpaks, the following mechanisms are used: The image loader binary is spawned via bwrap. The bubblewrap configuration only allows for minimal interaction with the host system. Only necessary parts of the filesystem are mounted and only with read access. There is no direct network access. Environment variables are not passed to the sandbox. Before forking the process the memory usage is limited via calling setrlimit and syscalls are limited to an allow-list via seccomp filters.

Inside of Flatpaks the flatpak-spawn --sandbox command is used. This restricts the access to the filesystem in a similar way as the direct bwrap call. The memory usage is limited by wrapping the loader call into a prlimit command. No additional seccomp filters are applied to the existing Flatpak seccomp rules.

The GFile content is streamed to the loader via a Unix socket. This way, loaders can load contents that require network access, without having direct network access themselves. Formats like SVG set the ExposeBaseDir = true option in their config. This option causes the original image file's directory to be mounted into the sandbox to include external image files from there. The ExposeBaseDir option has no effect for flatpak-spawn sandboxes since they don't support this feature.

The loaders provide the texture data via a memfd that is sealed by glycin and then given as an mmap to GDK. For animations and SVGs the sandboxed process is kept alive for new frames or tiles as long as needed.

For information on how to implement a loader, please consult the glycin-utils docs.

Building and Testing

• The -Dloaders option allows to only build certain loaders.
• The -Dtest_skip_ext option allows to skip certain image filename extensions during tests. The option -Dtest_skip_ext=heic might be needed if x265 is not available.
• Running integration tests requires the glycin loaders to be installed. By default, meson test creates an separate installation against which the tests are run. This behavior can be changed by setting -Dtest_skip_install=true, requiring to manually calling meson install before running the tests.
• The glycin crate has an example, glycin-render that will load the image passed as a parameter and render it as a PNG into output.png in the current directory.

Packaging Status

License

SPDX-License-Identifier: MPL-2.0 OR LGPL-2.1-or-later