Glycin

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

• glycin – The image library
• glycin-utils – Utilities to write loaders for glycin
• loaders – Glycin loaders for several formats

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;

Image loader configuration

Loader configurations are read 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

Supported image formats

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

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

Distributions need to package the loader binaries and their configs independent of apps. The loaders build and installed via meson.

Apps will depend on the glycin crate to make use of the installed binary loaders.

License

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