Intro

Sometimes there are occasions where direct internet access is not possible (proxy/offline/airgapped). Even in this setups it is possible to deploy and use Kubernetes with openSUSE Kubic and a local private registry.

In this blog I will explain how to setup a local server which acts as private registry providing all the container images needed to deploy Kubernetes with openSUSE Kubic.

Installation

OS

As OS for our host serving the required services openSUSE MicroOS is used, which needs to be installed with the MicroOS Container Host system role.

The following additional packages are required and need to be installed:

• container-registry-systemd
• mirror-registry
• skopeo with the latest sync patches as provided in openSUSE Tumbleweed
• reg (optional, for testing and debugging)

# transactional-update pkg install mirror-registry skopeo container-registry-systemd reg
# systemctl reboot

After the reboot the tools are available and we can start with the setup of the registry.

Registry Setup

There is a script to setup a local private registry running in containers. By default, everybody can list and pull images from it, but only the admin is allowed to push new images.

# setup-container-registry

There is no default password for the admin account, which means it is necessary to set one to be able to sync the containers to this registries. The password must be a bcrypt (blowfish) hash:

# htpasswd -nB admin

Modify the password entry for the admin account and set the new hash:

# nano /etc/registry/auth_config.yml

Additional, the ACL rules can be adjusted.

Start the containers for the registry:

# systemctl start container-registry
# systemctl start registry-auth_server

Now your registry is running and should work. This can be verified with:

# reg ls localhost
Repositories for localhost
REPO                TAGS

Certificates

By default setup-container-registry will create self signed certificates, which are valid for 1,5 year and stored in /etc/registry/certs. They can be replaced with official certificates.

If the self signed certificates are used, the public CA certificate needs to be installed on all machines, which should access the registry. Copy /etc/registry/certs/ContainerRegistryCA.crt to /etc/pki/trust/anchors/ and run update-ca-certificates on every machine.

List of container images

The tool mirror-registry will be used to create a list of containers which needs to be mirrored. It will analyse a remote registry and create a yaml file with all containers and tags matching a regex to sync with skopeo to a private registry.

This tool can run on any architecture, the target platform can be specified as argument. In contrast to this, skopeo needs to run on the target architecture. Else it will fail or sync the wrong container image if there are multi-architecture container images in the source repository.

All container images below registry.opensuse.org/kubic/ are required. Additional, it could be from help to mirror all official openSUSE container images from registry.opensuse.org/opensuse/, too. Else utilities like toolbox will not work.

The commnd to create a list of container images is:

# mirror-registry --out kubic-full.yaml registry.opensuse.org "(^kubic|^opensuse/([^/]+)$)"

This command will mirror all builds of all versions of the required container images. But most of the time it is enough to mirror the latest build of every version:

# mirror-registry --out kubic-small.yaml --minimize registry.opensuse.org "(^kubic|^opensuse/([^/]+)$)"

kubic-small.yaml can now be further tuned. If e.g. weave is used for the POD network, flannel and the cilium containers don’t need to be mirrored and can be deleted from the list.

Private registry

From a host, which has access to the source registry and the internal private registry, the container images can now be synced with skopeo:

# skopeo sync --scoped --src yaml --dest docker --dest-creds admin:password kubic-small.yaml registry.local

The --scoped option will prefix the internal registry name to the current external repository name, so the repository will be registry.opensuse.org/... and the pull command podman pull registry.local/registry.opensuse.org/... to avoid clashes with the namespace of the internal registry or by mirroring additional registries.

The result can be verified with:

# reg ls registry.local

skopeo needs recent enough patches adding the sync option. This patches are already part of openSUSE Tumbleweed, openSUSE MicroOS and openSUSE Kubic, but not of SUSE Linux Enterprise Server 15.

Airgapped/offline registry

If there is no host which can access both registries, the external and the internal one, the container images needs to be copied to a disk, which then needs to be transfered into the internal network.

Copy the container images to an external disk:

# skopeo sync --scoped --src yaml --dest dir kubic-small.yaml /media/external-disk

Take the external disk to the internal registry machine and import the container images:

# skopeo sync --scoped --src dir --dest docker --dest-creds admin:password /media/external-disk localhost

Verify the result:

# reg ls registry.local

Node configuration

After the private registry is running and contains all container images, cri-o and podman need to be aware of this registry and pull the images from it.

Both tools share the same configuration file for this: /etc/containers/registries.conf.

The default configuration file is using the v1 syntax, but v2 is needed. The documentation can be found here. A working configuration template, which needs to be deployed on every kubernetes node, could look like:

[[registry]]
prefix = "registry.opensuse.org/kubic"
location = "registry.local/registry.opensuse.org/kubic"
insecure = false
blocked = false

[[registry]]
prefix = "registry.opensuse.org/opensuse"
location = "registry.local/registry.opensuse.org/opensuse"
insecure = false
blocked = false

• insecure: true or false. By default TLS is required when retrieving images from a registry. If insecure is set to true, unencrypted HTTP as well as TLS connections with untrusted certificates are allowed.
• blocked : true or false. If true, pulling images with matching names is forbidden.

After updating the configuration file, cri-o needs to be restarted on that node if it is running:

# systemctl status crio
* crio.service - Open Container Initiative Daemon
   Loaded: loaded (/usr/lib/systemd/system/crio.service; disabled; vendor preset: disabled)
   Active: active (running) since Sat 2019-11-02 18:09:23 UTC; 8min ago
[...]
# systemctl stop crio
# systemctl start crio

Now you can deploy kubernetes!

Configuration files

Following configuration files define the behavior of the registry and the authentication server:

• /etc/registry/config.yml - configuration file for registry
• /etc/registry/auth_config.yml - configuration file for auth server
• /etc/sysconfig/container-registry - configuration file for the two containers

I keep getting this same error message when I tried to run vagrant up on my machine

$ vagrant up
Bringing machine 'homestead' up with 'virtualbox' provider...
==> homestead: Importing base box 'laravel/homestead'...
Progress: 90%
There was an error while executing `VBoxManage`, a CLI used by Vagrant
for controlling VirtualBox. The command and stderr is shown below.

Command: ["import", "/home/rnm/.vagrant.d/boxes/laravel-VAGRANTSLASH-homestead/8.2.1/virtualbox/box.ovf", "--vsys", "0", "--vmname", "ubuntu-18.04-amd64_1573673848596_76505", "--vsys", "0", "--unit", "11", "--disk", "/home/rnm/VirtualBox VMs/ubuntu-18.04-amd64_1573673848596_76505/ubuntu-18.04-amd64-disk001.vmdk"]

Stderr: 0%...10%...20%...30%...40%...50%...60%...70%...80%...90%...100%
Interpreting /home/rnm/.vagrant.d/boxes/laravel-VAGRANTSLASH-homestead/8.2.1/virtualbox/box.ovf...
OK.
0%...
Progress state: NS_ERROR_INVALID_ARG
VBoxManage: error: Appliance import failed
VBoxManage: error: Code NS_ERROR_INVALID_ARG (0x80070057) - Invalid argument value (extended info not available)
VBoxManage: error: Context: "RTEXITCODE handleImportAppliance(HandlerArg*)" at line 957 of file VBoxManageAppliance.cpp

Arghh..this is quite anoyying.. I tried executing same command again and again but no luck. I check my disk space and there is plenty.

I have no choice, I will delete ~/.vagrant.d and execute vagrant command again

$ rm -rf ~/.vagrant.d
removed '/home/rnm/.vagrant.d/insecure_private_key'
removed '/home/rnm/.vagrant.d/rgloader/loader.rb'
removed directory '/home/rnm/.vagrant.d/rgloader'
removed directory '/home/rnm/.vagrant.d/tmp'
removed directory '/home/rnm/.vagrant.d/gems/2.4.9'
removed directory '/home/rnm/.vagrant.d/gems'
removed '/home/rnm/.vagrant.d/setup_version'
removed '/home/rnm/.vagrant.d/boxes/laravel-VAGRANTSLASH-homestead/8.2.1/virtualbox/ubuntu-18.04-amd64-disk001.vmdk'
removed '/home/rnm/.vagrant.d/boxes/laravel-VAGRANTSLASH-homestead/8.2.1/virtualbox/Vagrantfile'
removed '/home/rnm/.vagrant.d/boxes/laravel-VAGRANTSLASH-homestead/8.2.1/virtualbox/box.ovf'
removed '/home/rnm/.vagrant.d/boxes/laravel-VAGRANTSLASH-homestead/8.2.1/virtualbox/metadata.json'
removed directory '/home/rnm/.vagrant.d/boxes/laravel-VAGRANTSLASH-homestead/8.2.1/virtualbox'
removed directory '/home/rnm/.vagrant.d/boxes/laravel-VAGRANTSLASH-homestead/8.2.1'
removed '/home/rnm/.vagrant.d/boxes/laravel-VAGRANTSLASH-homestead/metadata_url'
removed directory '/home/rnm/.vagrant.d/boxes/laravel-VAGRANTSLASH-homestead'
removed directory '/home/rnm/.vagrant.d/boxes'
removed '/home/rnm/.vagrant.d/data/machine-index/index.lock'
removed directory '/home/rnm/.vagrant.d/data/machine-index'
removed '/home/rnm/.vagrant.d/data/lock.dotlock.lock'
removed '/home/rnm/.vagrant.d/data/checkpoint_signature'
removed '/home/rnm/.vagrant.d/data/checkpoint_cache'
removed '/home/rnm/.vagrant.d/data/lock.machine-action-13105a2995d25174a20c9b1d206a4f8a.lock'
removed directory '/home/rnm/.vagrant.d/data'
removed directory '/home/rnm/.vagrant.d'

and when I execute vagrant, re-downloading the box then suddenly it worked!

$ vagrant up
Bringing machine 'homestead' up with 'virtualbox' provider...
==> homestead: Box 'laravel/homestead' could not be found. Attempting to find and install...
    homestead: Box Provider: virtualbox
    homestead: Box Version: >= 8.2.0
==> homestead: Loading metadata for box 'laravel/homestead'
    homestead: URL: https://vagrantcloud.com/laravel/homestead
==> homestead: Adding box 'laravel/homestead' (v8.2.1) for provider: virtualbox
    homestead: Downloading: https://vagrantcloud.com/laravel/boxes/homestead/versions/8.2.1/providers/virtualbox.box
    homestead: Download redirected to host: vagrantcloud-files-production.s3.amazonaws.com
==> homestead: Successfully added box 'laravel/homestead' (v8.2.1) for 'virtualbox'!
==> homestead: Importing base box 'laravel/homestead'...
==> homestead: Matching MAC address for NAT networking...
==> homestead: Checking if box 'laravel/homestead' version '8.2.1' is up to date...
==> homestead: Setting the name of the VM: homestead
Vagrant is currently configured to create VirtualBox synced folders with
the `SharedFoldersEnableSymlinksCreate` option enabled. If the Vagrant
guest is not trusted, you may want to disable this option. For more
information on this option, please refer to the VirtualBox manual:

  https://www.virtualbox.org/manual/ch04.html#sharedfolders

This option can be disabled globally with an environment variable:

  VAGRANT_DISABLE_VBOXSYMLINKCREATE=1

or on a per folder basis within the Vagrantfile:

  config.vm.synced_folder '/host/path', '/guest/path', SharedFoldersEnableSymlinksCreate: false
==> homestead: Clearing any previously set network interfaces...
==> homestead: Preparing network interfaces based on configuration...
    homestead: Adapter 1: nat
    homestead: Adapter 2: hostonly
==> homestead: Forwarding ports...
    homestead: 80 (guest) => 8000 (host) (adapter 1)
    homestead: 443 (guest) => 44300 (host) (adapter 1)
    homestead: 3306 (guest) => 33060 (host) (adapter 1)
    homestead: 4040 (guest) => 4040 (host) (adapter 1)
    homestead: 5432 (guest) => 54320 (host) (adapter 1)
    homestead: 8025 (guest) => 8025 (host) (adapter 1)
    homestead: 9600 (guest) => 9600 (host) (adapter 1)
    homestead: 27017 (guest) => 27017 (host) (adapter 1)
    homestead: 22 (guest) => 2222 (host) (adapter 1)
==> homestead: Running 'pre-boot' VM customizations...
==> homestead: Booting VM...
==> homestead: Waiting for machine to boot. This may take a few minutes...
    homestead: SSH address: 127.0.0.1:2222
    homestead: SSH username: vagrant
    homestead: SSH auth method: private key
    homestead: Warning: Connection reset. Retrying...
    homestead: Warning: Remote connection disconnect. Retrying...
    homestead: Warning: Remote connection disconnect. Retrying...
    homestead: Warning: Remote connection disconnect. Retrying...
    homestead: Warning: Connection reset. Retrying...
    homestead: Warning: Remote connection disconnect. Retrying...
    homestead: 
    homestead: Vagrant insecure key detected. Vagrant will automatically replace
    homestead: this with a newly generated keypair for better security.
    homestead: 
    homestead: Inserting generated public key within guest...
    homestead: Removing insecure key from the guest if it's present...
    homestead: Key inserted! Disconnecting and reconnecting using new SSH key...
==> homestead: Machine booted and ready!
==> homestead: Checking for guest additions in VM...
==> homestead: Setting hostname...
==> homestead: Configuring and enabling network interfaces...
==> homestead: Mounting shared folders...
    homestead: /vagrant => /home/rnm/Homestead
    homestead: /home/vagrant/project1 => /home/rnm/workplace/laravel/project1
==> homestead: Running provisioner: file...
    homestead: /home/rnm/Homestead/aliases => /tmp/bash_aliases
==> homestead: Running provisioner: shell...
    homestead: Running: inline script
==> homestead: Running provisioner: shell...
    homestead: Running: inline script
    homestead: 
    homestead: ssh-rsa
==> homestead: Running provisioner: shell...
    homestead: Running: inline script
==> homestead: Running provisioner: shell...
    homestead: Running: inline script
==> homestead: Running provisioner: shell...
    homestead: Running: inline script
==> homestead: Running provisioner: shell...
    homestead: Running: inline script
    homestead: Ignoring feature: mariadb because it is set to false
==> homestead: Running provisioner: shell...
    homestead: Running: inline script
    homestead: Ignoring feature: ohmyzsh because it is set to false
==> homestead: Running provisioner: shell...
    homestead: Running: inline script
    homestead: Ignoring feature: webdriver because it is set to false
==> homestead: Running provisioner: shell...
    homestead: Running: /tmp/vagrant-shell20191114-1822-10idfmj.sh
==> homestead: Running provisioner: shell...
    homestead: Running: /tmp/vagrant-shell20191114-1822-1pewpva.sh
==> homestead: Running provisioner: shell...
    homestead: Running: script: Creating Certificate: project1.test
==> homestead: Running provisioner: shell...
    homestead: Running: script: Creating Site: project1.test
==> homestead: Running provisioner: shell...
    homestead: Running: inline script
==> homestead: Running provisioner: shell...
    homestead: Running: /tmp/vagrant-shell20191114-1822-yobfu3.sh
==> homestead: Running provisioner: shell...
    homestead: Running: script: Checking for old Schedule
==> homestead: Running provisioner: shell...
    homestead: Running: script: Clear Variables
==> homestead: Running provisioner: shell...
    homestead: Running: script: Restarting Cron
==> homestead: Running provisioner: shell...
    homestead: Running: script: Restarting Nginx
==> homestead: Running provisioner: shell...
    homestead: Running: script: Creating MySQL Database: project1
==> homestead: Running provisioner: shell...
    homestead: Running: script: Creating Postgres Database: project1
==> homestead: Running provisioner: shell...
    homestead: Running: script: Update Composer
    homestead: Updating to version 1.9.1 (stable channel).
    homestead:    
    homestead: Use composer self-update --rollback to return to version 1.9.0
==> homestead: Running provisioner: shell...
    homestead: Running: /tmp/vagrant-shell20191114-1822-o4c7za.sh
==> homestead: Running provisioner: shell...
    homestead: Running: script: Update motd
==> homestead: Running provisioner: shell...
    homestead: Running: /tmp/vagrant-shell20191114-1822-13ajt5r.sh

nice!

$ vagrant global-status
id       name      provider   state   directory                           
--------------------------------------------------------------------------
dd946c7  homestead virtualbox running /home/rnm/Homestead                 
 
The above shows information about all known Vagrant environments
on this machine. This data is cached and may not be completely
up-to-date (use "vagrant global-status --prune" to prune invalid
entries). To interact with any of the machines, you can go to that
directory and run Vagrant, or you can use the ID directly with
Vagrant commands from any directory. For example:
"vagrant destroy 1a2b3c4d"

$ vagrant ssh
Welcome to Ubuntu 18.04.3 LTS (GNU/Linux 4.15.0-64-generic x86_64)

Thanks for using 
 _                               _                 _ 
| |                             | |               | |
| |__   ___  _ __ ___   ___  ___| |_ ___  __ _  __| |
| '_ \ / _ \| '_ ` _ \ / _ \/ __| __/ _ \/ _` |/ _` |
| | | | (_) | | | | | |  __/\__ \ ||  __/ (_| | (_| |
|_| |_|\___/|_| |_| |_|\___||___/\__\___|\__,_|\__,_|

* Homestead v9.2.2 released
* Settler v8.2.0 released

0 packages can be updated.
0 updates are security updates.


vagrant@homestead:~$ 

Seems, my vagrant box was corrupted and re-download the box is the solution.

Until now, it was not easy to run debugging tools on openSUSE MicroOS or Kubic for the simple reason that installing any package using the transactional-update in package_name command would require a reboot for the program to be available on the system. Why so? MicroOS & Kubic use transactional updates and thus, the root filesystem is mounted as read-only. Packages that are installed are available in a "snapshot" which is selected at the next boot. openSUSE MicroOS & Kubic fall in the category of immutable operating systems.

One couldn't just reboot the system every time a simple program was needed - e.g using Vi to edit a YAML config or using database client utils for a rapid inspection.

But now we have a toolbox script available in MicroOS & Kubic that allows us to launch a container and use debugging utilities that can interact with the host. The script is written in Bash and it is based on CoreOS/toolbox.

Thorsten Kukuk, SUSE Engineer & Architect (SLES / MicroOS), announced the availability of the toolbox container on the openSUSE Kubic mailing list in October.

The toolbox script resides in the system $PATH and thus can be called directly. On its first execution, the script will pull the toolbox container from the openSUSE Container Registry.

microos:~ # toolbox
Trying to pull registry.opensuse.org/opensuse/toolbox...
Getting image source signatures
Copying blob 92a022f7c0d8 [>-------------------------------------] 2.8MiB / 122.9MiB
Copying blob f0c8107beec9 [==>-----------------------------------] 3.0MiB / 36.4MiB

The host root filesystem can be accessed at /media/root/ and since the toolbox container is spawned as the root user, the files will be modifiable.

toolbox:/ # ls /media/root/
bin   dev  home  lib64	opt   root  sbin     srv  tmp  var
boot  etc  lib	 mnt	proc  run   selinux  sys  usr

Non-root toolbox containers will have read-only access to the host files, unless the --root flag is provided.

ish@microos:~> toolbox --root

Next, use zypper to install the required utilities and interact with the host and other containers.

Exiting toolbox will stop and the container and next time the script is called, the container is started again. Therefore, previously installed utilities remain available.

Each toolbox container is named by appending the username calling it, e.g toolbox-ish.

microos:~ # podman ps --format "{{.ID}} {{.Status}} {{.Names}}"
7f61fba95487 Up 10 minutes ago toolbox-ish
00ada606ec4f Up 33 minutes ago toolbox-root

Summary: after an epic amount of refactoring, librsvg now does all CSS parsing and matching in Rust, without using libcroco. In addition, the CSS engine comes from Mozilla Servo, so it should be able to handle much more complex CSS than librsvg ever could before.

This is the story of CSS support in librsvg.

Introduction

The first commit to introduce CSS parsing in librsvg dates from 2002. It was as minimal as possible, written to support a small subset of what was then CSS2.

Librsvg handled CSS stylesheets more "piecing them apart" than "parsing them". You know, when g_strsplit() is your best friend. The basic parsing algorithm was to turn a stylesheet like this:

rect { fill: blue; }

.classname {
    fill: green;
    stroke-width: 4;
}

Into a hash table whose keys are strings like rect and .classname, and whose values are everything inside curly braces.

The selector matching phase was equally simple. The code only handled a few possible match types as follows. If it wanted to match a certain kind of CSS selector, it would say, "what would this selector look like in CSS syntax", it would make up a string with that syntax, and compare it to the key strings it had stored in the hash table from above.

So, to match an element name selector, it would sprintf("%s", element->name), obtain something like rect and see if the hash table had such a key.

To match a class selector, it would sprintf(".%s", element->class), obtain something like .classname, and look it up in the hash table.

This scheme supported only a few combinations. It handled tag, .class, tag.class, and a few combinations with #id in them. This was enough to support very simple stylesheets.

The value corresponding to each key in the hash table was the stuff between curly braces in the stylesheet, so the second rule from the example above would contain fill: green; stroke-width: 4;. Once librsvg decided that an SVG element matched that CSS rule, it would re-parse the string with the CSS properties and apply them to the element's style.

I'm amazed that so little code was enough to deal with a good number of SVG files with stylesheets. I suspect that this was due to a few things:

• While people were using complex CSS in HTML all the time, it was less common for SVG...

• ... because CSS2 was somewhat new, and the SVG spec was still being written...

• ... and SVGs created with illustration programs don't really use stylesheets; they include the full style information inside each element instead of symbolically referencing it from a stylesheet.

From the kinds of bugs that librsvg has gotten around "CSS support is too limited", it feels like SVGs which use CSS features are either hand-written, or machine-generated from custom programs like data plotting software. Illustration programs tend to list all style properties explicitly in each SVG element, and don't use CSS.

Libcroco appears

The first commit to introduce libcroco was to do CSS parsing, from March 2003.

At the same time, libcroco was introducing code to do CSS matching. However, this code never got used in librsvg; it still kept its simple string-based matcher. Maybe libcroco's API was not ready?

Libcroco fell out of maintainership around the first half of 2005, and volunteers have kept fixing it since then.

Problems with librsvg's string matcher for CSS

The C implementation of CSS matching in librsvg remained basically untouched until 2018, when Paolo Borelli and I started porting the surrounding code to Rust.

I had a lot of trouble figuring out the concepts from the code. I didn't know all the terminology of CSS implementations, and librsvg didn't use it, either.

I think that librsvg's code suffered from what the refactoring literature calls primitive obsession. Instead of having a parsed representation of CSS selectors, librsvg just stored a stringified version of them. So, a selector like rect#classname really was stored with a string like that, instead of an actual decomposition into structs.

Moreover, things were misnamed. This is the field that stored stylesheet data inside an RsvgHandle:

    GHashTable *css_props;

From just looking at the field declaration, this doesn't tell me anything about what kind of data is stored there. One has to grep the source code for where that field is used:

static void
rsvg_css_define_style (RsvgHandle * ctx,
                       const gchar * selector,
                       const gchar * style_name,
                       const gchar * style_value,
                       gboolean important)
{
    GHashTable *styles;

    styles = g_hash_table_lookup (ctx->priv->css_props, selector);

Okay, it looks up a selector by name in the css_props, and it gives back... another hash table styles? What's in there?

        g_hash_table_insert (styles,
                             g_strdup (style_name),
                             style_value_data_new (style_value, important));

Another string key called style_name, whose key is a StyleValueData; what's in it?

typedef struct _StyleValueData {
    gchar *value;
    gboolean important;
} StyleValueData;

The value is another string. Strings all the way!

At the time, I didn't really figure out what each level of nested hash tables was supposed to mean. I didn't understand why we handled style properties in a completely different part of the code, and yet this part had a css_props field that didn't seem to store properties at all.

It took a while to realize that css_props was misnamed. It wasn't storing a mapping of selector names to properties; it was storing a mapping of selector names to declaration lists, which are lists of property/value pairs.

So, when I started porting the CSS parsing code to Rust, I started to create real types with for each concept.

// Maps property_name -> Declaration
type DeclarationList = HashMap<String, Declaration>;

pub struct CssStyles {
    selectors_to_declarations: HashMap<String, DeclarationList>,
}

Even though the keys of those HashMaps are still strings, because librsvg didn't have a better way to represent their corresponding concepts, at least those declarations let one see what the hell is being stored without grepping the rest of the code. This is a part of the code that I didn't really touch very much, so it was nice to have that reminder.

The first port of the CSS matching code to Rust kept the same algorithm as the C code, the one that created strings with element.class and compared them to the stored selector names. Ugly, but it still worked in the same limited fashion.

Rustifying the CSS parsers

It turns out that CSS parsing is divided in two parts. One can have a style attribute inside an element, for example

<rect x="0" y="0" width="100" height="100"
      style="fill: green; stroke: magenta; stroke-width: 4;"/>

This is a plain declaration list which is not associated to any selectors, and which is applied directly to just the element in which it appears.

Then, there is the <style> element itself, with a normal-looking CSS stylesheet

<style type="text/css">
  rect {
    fill: green;
    stroke: magenta;
    stroke-width: 4;
  }
</style>

This means that all <rect> elements will get that style applied.

I started to look for existing Rust crates to parse and handle CSS data. The cssparser and selectors crates come from Mozilla, so I thought they should do a pretty good job of things.

And they do! Except that they are not a drop-in replacement for anything. They are what gets used in Mozilla's Servo browser engine, so they are optimized to hell, and the code can be pretty intimidating.

Out of the box, cssparser provides a CSS tokenizer, but it does not know how to handle any properties/values in particular. One must use the tokenizer to implement a parser for each kind of CSS property one wants to support — Servo has mountains of code for all of HTML's style properties, and librsvg had to provide a smaller mountain of code for SVG style properties.

Thus started the big task of porting librsvg's string-based parsers for CSS properties into ones based on cssparser tokens. Cssparser provides a Parser struct, which extracts tokens out of a CSS stream. Out of this, librsvg defines a Parse trait for parsable things:

use cssparser::Parser;

pub trait Parse: Sized {
    type Err;

    fn parse(parser: &mut Parser<'_, '_>) -> Result<Self, Self::Err>;
}

What's with those two default lifetimes in Parser<'_, '_>? Cssparser tries very hard to be a zero-copy tokenizer. One of the lifetimes refers to the input string which is wrapped in a Tokenizer, which is wrapped in a ParserInput. The other lifetime is for the ParserInput itself.

In the actual implementation of that trait, the Err type also uses the lifetime that refers to the input string. For example, there is a BasicParseErrorKind::UnexpectedToken(Token<'i>), which one returns when there is an unexpected token. And to avoid copying the substring into the error, one returns a slice reference into the original string, thus the lifetime.

I was more of a Rust newbie back then, and it was very hard to make sense of how cssparser was meant to be used.

The process was more or less this:

• Port the C parsers to Rust; implement types for each CSS property.

• Port the &str-based parsers into ones that use cssparser.

• Fix the error handling scheme to match what cssparser's high-level traits expect.

This last point was... hard. Again, I wasn't comfortable enough with Rust lifetimes and nested generics; in the end it was all right.

Moving declaration lists to Rust

With the individual parsers for CSS properties done, and with them already using a different type for each property, the next thing was to implement cssparser's traits to parse declaration lists.

Again, a declaration list looks like this:

fill: blue;
stroke-width: 4;

It's essentially a key/value list.

The trait that cssparser wants us to implement is this:

pub trait DeclarationParser<'i> {
    type Declaration;
    type Error: 'i;

    fn parse_value<'t>(
        &mut self,
        name: CowRcStr<'i>,
        input: &mut Parser<'i, 't>,
    ) -> Result<Self::Declaration, ParseError<'i, Self::Error>>;
}

That is, define a type for a Declaration, and implement a parse_value() method that takes a name and a Parser, and outputs a Declaration or an error.

What this really means is that the type you implement for Declaration needs to be able to represent all the CSS property types that you care about. Thus, a struct plus a big enum like this:

pub struct Declaration {
    pub prop_name: String,
    pub property: ParsedProperty,
    pub important: bool,
}

pub enum ParsedProperty {
    BaselineShift(SpecifiedValue<BaselineShift>),
    ClipPath(SpecifiedValue<ClipPath>),
    ClipRule(SpecifiedValue<ClipRule>),
    Color(SpecifiedValue<Color>),
    ColorInterpolationFilters(SpecifiedValue<ColorInterpolationFilters>),
    Direction(SpecifiedValue<Direction>),
    ...
}

This gives us declaration lists (the stuff inside curly braces in a CSS stylesheet), but it doesn't give us qualified rules, which are composed of selector names plus a declaration list.

Refactoring towards real CSS concepts

Paolo Borelli has been steadily refactoring librsvg and fixing things like the primitive obsession I mentioned above. We now have real concepts like a Document, Stylesheet, QualifiedRule, Rule, AtRule.

This refactoring took a long time, because it involved redoing the XML loading code and its interaction with the CSS parser a few times.

Implementing traits from the selectors crate

The selectors crate contains Servo's code for parsing CSS selectors and doing matching. However, it is extremely generic. Using it involves implementing a good number of concepts.

For example, this SelectorImpl trait has no methods, and is just a collection of types that refer to your implementation of an element tree. How do you represent an attribute/value? How do you represent an identifier? How do you represent a namespace and a local name?

pub trait SelectorImpl {
    type ExtraMatchingData: ...;
    type AttrValue: ...;
    type Identifier: ...;
    type ClassName: ...;
    type PartName: ...;
    type LocalName: ...;
    type NamespaceUrl: ...;
    type NamespacePrefix: ...;
    type BorrowedNamespaceUrl: ...;
    type BorrowedLocalName: ...;
    type NonTSPseudoClass: ...;
    type PseudoElement: ...;
}

A lot of those can be String, but Servo has smarter things in store. I ended up using the markup5ever crate, which provides a string interning framework for markup and XML concepts like a LocalName, a Namespace, etc. This reduces memory consumption, because instead of storing string copies of element names everywhere, one just stores tokens for interned strings.

(In the meantime I had to implement support for XML namespaces, which the selectors code really wants, but which librsvg never supported.)

Then, the selectors crate wants you to say how your code implements an element tree. It has a monster trait Element:

pub trait Element {
    type Impl: SelectorImpl;

    fn opaque(&self) -> OpaqueElement;

    fn parent_element(&self) -> Option<Self>;

    fn parent_node_is_shadow_root(&self) -> bool;

    ...

    fn prev_sibling_element(&self) -> Option<Self>;
    fn next_sibling_element(&self) -> Option<Self>;

    fn has_local_name(
        &self, 
        local_name: &<Self::Impl as SelectorImpl>::BorrowedLocalName
    ) -> bool;

    fn has_id(
        &self,
        id: &<Self::Impl as SelectorImpl>::Identifier,
        case_sensitivity: CaseSensitivity,
    ) -> bool;

    ...
}

That is, when you provide an implementation of Element and SelectorImpl, the selectors crate will know how to navigate your element tree and ask it questions like, "does this element have the id #foo?"; "does this element have the name rect?". It makes perfect sense in the end, but it is quite intimidating when you are not 100% comfortable with webs of traits and associated types and generics with a bunch of trait bounds!

I tried implementing that trait twice in the last year, and failed. It turns out that its API needed a key fix that landed last June, but I didn't notice until a couple of weeks ago.

So?

Two days ago, Paolo and I committed the last code to be able to completely replace libcroco.

And, after implementing CSS specificity (which was easy now that we have real CSS concepts and a good pipeline for the CSS cascade), a bunch of very old bugs started falling down (1 2 3 4 5 6).

Now it is going to be easy to implement things like letting the application specify a user stylesheet. In particular, this should let GTK remove the rather egregious hack it has to recolor SVG icons while using librsvg indirectly.

Conclusion

This will appear in librsvg 2.47.1 — that version will no longer require libcroco.

As far as I know, the only module that still depends on libcroco (in GNOME or otherwise) is gnome-shell. It uses libcroco to parse CSS and get the basic structure of selectors so it can implement matching by hand.

Gnome-shell has some code which looks awfully similar to what librsvg had when it was written in C:

• StTheme has the high-level CSS stylesheet parser and the selector matching code.

• StThemeNode has the low-level CSS property parsers.

... and it turns out that those files come all the way from HippoCanvas, the CSS-aware canvas that Mugshot used! Mugshot was a circa-2006 pre-Facebook aggregator for social media data like blogs, Flickr pictures, etc. HippoCanvas also got used in Sugar, the GUI for One Laptop Per Child. Yes, our code is that old.

Libcroco is unmaintained, and has outstanding CVEs. I would be very happy to assist someone in porting gnome-shell's CSS code to Rust :)

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