• Remove support for parsing CVS $Id$ strings and emitting \id commands.
• Stop wrapping output text.
• Fix internal section links.
• Fix conversion of nested =over blocks.
• Fix conversion of =over blocks without an =item.
• Fix links to headings containing markup.
• Fix metacharacter escaping in headings, contents, and navbar.
• Fix handling of =for thread and =begin thread.
• Fix non-breaking spaces in navbar section names.
• Fix internal links whose anchors are wrapped.
• Output the document before erroring out if POD errors were seen.
• Suppress \signature if the input document was empty.
• Add bugtracker, homepage, and repository package metadata.

Tell me, Mr. Anderson, what good is a phone call when you are unable to speak?

Logger.js:154 2021-09-10Txx:xx:xx.xxxZ [features/base/tracks] Failed to create local tracks
Array(2)
DOMException: Could not start audio source

I had the similar problem once with chromium. i could solve it by switching in preferences->microphone-> from "default" to "intern analog stereo".

• Introduction to Free Software and Debian,
• Packaging, policy, and Debian infrastructure,
• Systems administration, automation and orchestration,
• Cloud and containers,
• Security,
• Community, diversity, local outreach and social context,
• Internationalization, Localization and Accessibility,
• Embedded and Kernel,
• Debian Blends and Debian derived distributions,
• Debian in Arts and Science
• and other.

• [DLA 2738-1] c-ares security update for one CVE
• [DLA 2746-1] scrollz security update for one CVE
• [DLA 2747-1] ircii security update for one CVE
• [DLA 2748-1] tnef security update for one CVE
• [DLA 2749-1] gthumb security update for one CVE
• [DLA 2752-1] squashfs-tools security update for one CVE
• buster-pu for gthumb #993228
• prepared debdiffs for squashfs-tools in Buster and Bullseye, which will result in DSA 4967
• prepared debdiffs for btrbk in Buster and Bullseye

• ELA-474-1 for c-ares
• ELA-480-1 for squashfs-tools

• duktape
• pywws
• libosmo-abis
• osmo-ggsn
• osmo-libasn1c
• nuspell

The record extension problem Initially, the upgrade rules of Candid were a straight-forward application of the canonical subtyping rules. This worked and was sound, but it forbid one very commonly requested use case: Extending records in argument position. The subtyping rule for records say that

   record   old_field : nat; new_field : int  
<: record   old_field : nat  

or, in words, that subtypes can have more field than supertypes. This makes intuitive sense: A user of a such a record value that only expects old_field to be there is not bothered if suddenly a new_field appears. But a user who expects new_field to be there is thrown off if it suddenly isn t anymore. Therefore it is ok to extend the records returned by a service with new fields, but not to extend the record in your methods s argument types. But users want to extend their record types over time, also in argument position! In fact, they often want to extend them in both positions. Consider a service with the following interface, where the CUser record appears both in argument and result position:

type CUser = record   login : text; name : text  ;
service C :  
  register_user : (CUser) -> ();
  get_user_data : (text) -> (CUser);
 

It seems quite natural to want to extend the record with a new field (say, the age of the user). But simply changing the definition of CUser to

type CUser = record   login : text; name : text; age : nat  

is not ok, because now register_user requires the age field, but old clients don t provide it. So how can we allow developers to make changes like this (because they really really want that), while keeping the soundness guarantees made by Candid (because we really really want that)? This question has bothered us for over two years, and we even created a meta issue that records the dozen approached we considered, discussed and eventually ditched.

Dynamic subtyping checks in opt I will spare you the history lesson, though, and explain the solution we eventually came up with. In the example above it seems unreasonable to let the developer add a field age of type nat. Since there may be old clients around, the service unavoidably has to deal with records that don t have an age field. If the code expects an age value, what should it do in that case? But one can argue that changing the record type to

type CUser = record   login : text; name : text; age : opt nat  

could work: If the age field is present, use the value. If the field is absent, inject a null value during decoding. In the first-order case, this rather obvious solution would work just fine, and we d be done. But Candid aims to solve the higher order case, and I said things get tricky here, didn t I? Consider another, independent service D with the following interface:

type DUser = record   login : text; name : text  ;
service D :  
  on_user_added : (func (DUser) -> ()) -> ()
 

This service has a method that takes a method reference, presumably with the intention of calling it whenever a new user was added to service D. And because the types line up nicely, we can compose these two services, by once calling D.on_user_added(C.register_user), so that from now on D calls C.register_user( ). These kind of service mesh-ups are central to the vision of the Internet Computer! But what happens if the services now evolve their types in different ways, e.g. towards

type CUser = record   login : text; name : text; age : opt nat  

and

type DUser = record   login : text; name : text; age : opt variant   under_age; adult  

Individually, these are type changes that we want to allow. But now the call from D to C may transmit a value of record ; age = opt variant under_age when C expects a natural number! This is precisely what we want to prevent by introducing types, and it seems we have lost soundness. The best solution we could find is to make the opt type somewhat special, and apply these extra rules when decoding at an expected type of opt t.

• If this was a record field, and the provided record value doesn t even have a field of that name, don t fail but instead treat this as null. This handles the first-order example above.
• If there is a value, look at its type (which, in Candid, is part of the message).
  • If it is opt t' and t' <: t, then decode as normal.
  • If it is opt t' but t' <: t does not hold, then treat that as null. This should only happen in these relatively obscure higher-order cases where services evolved differently and incompatibly, and makes sure that the calls that worked before the upgrades will continue to work afterwards. It is not advisable to actually use that rule when changing your service s interface. Tools that assist the developer with an upgrade check should prevent or warn about the use of this rule.
• Not strictly required here, but since we are making opt special anyways: If its type t' is not of the form opt , pretend it was opt t', and also pretend that the given value was wrapped in opt. This allows services to make record field in arguments that were required in the old version optional in a new version, e.g. as a way to deprecated them. So it is mildly useful, although I can report that it makes the meta-theory and implementation rather complex, in particular together with equirecursion and beasts like type O = opt O. See this discussion for a glimpse of that.

In the above I stress that we look at the type of the provided value, and not just the value. For example, if the sender sends the value opt vec at type opt vec nat, and the recipient expects opt vec bool, then this will decode as null, even though one could argue that the value opt vec could easily be understood at type opt vec bool. We initially had that, but then noticed that this still breaks soundness when there are references inside, and we have to do a full subtyping check in the decoder. This is very unfortunate, because it makes writing a Candid decoder a noticeably harder task that requires complicated graph algorithms with memoization (which I must admit Motoko has not implemented yet), but it s the least bad solution we could find so far.

Isn t that just dynamic typing? You might have noticed that with these rules, t <: opt t' holds for all types t and t'. In other words, every opt is a top type (like reserved), thus all optional types are equivalent. One could argue that we are throwing all of the benefits of static typing over board this way. But it s not quite as bad: It s true that decoding Candid values now involves a dynamic check inserting null values in certain situations, but as long as everyone plays by the rules (i.e. upgrades their services according to the Candid safe upgrade rules, and heeds the warning alluded to above), these certain situations will only happen in the obscurest of cases. It is, by the way, not material to this solution that the special subtyping behavior is applied to the opt type, and a neighboring point in the design space would be a dedicated upgraded t type constructor. That would allow developers to use the canonical opt type with the usual subtyping rules and communicate their intent ( clients should consider this field optional vs. old clients don t know about this field, but new clients should use it ) more cleanly at the expense of having more non-canonical types in the type system. To see why it is convenient if Candid has mostly just the normal types, read the next post, which will describe how Candid can be integrated into a host language.

• [1] https://tinyurl.com/ydrgflz4

The Debian Janitor is an automated system that commits fixes for (minor) issues in Debian packages that can be fixed by software. It gradually started proposing merges in early December. The first set of changes sent out ran lintian-brush on sid packages maintained in Git. This post is part of a series about the progress of the Janitor. As covered in my post from last week, the Janitor now regularly tries to import new upstream git snapshots or upstream releases into packages in Sid.

webhooks for upstream repositories At the moment there is no service in Debian (yet - perhaps this is something that vcswatch or a sibling service could also do?) that scans upstream repositories for changes. However, if you maintain an upstream package, you can use a webhook to notify the janitor that commits have been made to your repository, and it will create a new package in fresh-snapshots. Webhooks from the following hosting site software are currently supported:

• GitHub
• Gitlab
• Launchpad
• Gitea
• Gogs

You can simply use the URL https://janitor.debian.net/ as the target for hooks. There is no need to specify a secret, and the hook can either use a JSON or form encoding payload. The endpoint should tell you whether it understood a webhook request, and whether it took any action. It s fine to submit webhooks for repositories that the janitor does not (yet) know about.

GitHub For GitHub, you can do so in the Webhooks section of the Settings tab. Fill the form as shown below and click on Add webhook:

GitLab On GitLab instances, you can find the Webhooks tab under the Settings menu for each repository (under the gear symbol). Fill the form in as shown below and click Add Webhook:

Launchpad For Launchpad, go to the repository (for Git) web view and click Manage Webhooks. From there, you can add a new webhook; fill the form in as shown below and click Add Webhook:

Dual screen desktop running i3, Emacs, some terminals, including a Quake console, Firefox, Polybar as the status bar, and Dunst as the notification daemon.

i3: the window manager i3 aims to be a minimal tiling window manager. Its documentation can be read from top to bottom in less than an hour. i3 organize windows in a tree. Each non-leaf node contains one or several windows and has an orientation and a layout. This information arbitrates the window positions. i3 features three layouts: split, stacking, and tabbed. They are demonstrated in the below screenshot:

Demonstration of the layouts available in i3. The main container is split horizontally. The first child is split vertically. The second one is tabbed. The last one is stacking.

Tree representation of the previous screenshot.

Most of the other tiling window managers, including the awesome window manager, use predefined layouts. They usually feature a large area for the main window and another area divided among the remaining windows. These layouts can be tuned a bit, but you mostly stick to a couple of them. When a new window is added, the behavior is quite predictable. Moreover, you can cycle through the various windows without thinking too much as they are ordered. i3 is more flexible with its ability to build any layout on the fly, it can feel quite overwhelming as you need to visualize the tree in your head. At first, it is not unusual to find yourself with a complex tree with many useless nested containers. Moreover, you have to navigate windows using directions. It takes some time to get used to. I set up a split layout for Emacs and a few terminals, but most of the other workspaces are using a tabbed layout. I don t use the stacking layout. You can find many scripts trying to emulate other tiling window managers but I did try to get my setup pristine of these tentatives and get a chance to familiarize myself. i3 can also save and restore layouts, which is quite a powerful feature. My configuration is quite similar to the default one and has less than 200 lines.

i3 companion: the missing bits i3 philosophy is to keep a minimal core and let the user implements missing features using the IPC protocol:

Do not add further complexity when it can be avoided. We are generally happy with the feature set of i3 and instead focus on fixing bugs and maintaining it for stability. New features will therefore only be considered if the benefit outweighs the additional complexity, and we encourage users to implement features using the IPC whenever possible. Introduction to the i3 window manager

While this is not as powerful as an embedded language, it is enough for many cases. Moreover, as high-level features may be opinionated, delegating them to small, loosely coupled pieces of code keeps them more maintainable. Libraries exist for this purpose in several languages. Users have published many scripts to extend i3: automatic layout and window promotion to mimic the behavior of other tiling window managers, window swallowing to put a new app on top of the terminal launching it, and cycling between windows with Alt+Tab. Instead of maintaining a script for each feature, I have centralized everything into a single Python process, i3-companion using asyncio and the i3ipc-python library. Each feature is self-contained into a function. It implements the following components:

make a workspace exclusive to an application

When a workspace contains Emacs or Firefox, I would like other applications to move to another workspace, except for the terminal which is allowed to intrude into any workspace. The workspace_exclusive() function monitors new windows and moves them if needed to an empty workspace or to one with the same application already running.

implement a Quake console

The quake_console() function implements a drop-down console available from any workspace. It can be toggled with Mod+ . This is implemented as a scratchpad window.

back and forth workspace switching on the same output

With the workspace back_and_forth command, we can ask i3 to switch to the previous workspace. However, this feature is not restricted to the current output. I prefer to have one keybinding to switch to the workspace on the next output and one keybinding to switch to the previous workspace on the same output. This behavior is implemented in the previous_workspace() function by keeping a per-output history of the focused workspaces.

create a new empty workspace or move a window to an empty workspace

To create a new empty workspace or move a window to an empty workspace, you have to locate a free slot and use workspace number 4 or move container to workspace number 4. The new_workspace() function finds a free number and use it as the target workspace.

restart some services on output change

When adding or removing an output, some actions need to be executed: refresh the wallpaper, restart some components unable to adapt their configuration on their own, etc. i3 triggers an event for this purpose. The output_update() function also takes an extra step to coalesce multiple consecutive events and to check if there is a real change with the low-level library xcffib.

I will detail the other features as this post goes on. On the technical side, each function is decorated with the events it should react to:

@on(CommandEvent("previous-workspace"), I3Event.WORKSPACE_FOCUS)
async def previous_workspace(i3, event):
    """Go to previous workspace on the same output."""

The CommandEvent() event class is my way to send a command to the companion, using either i3-msg -t send_tick or binding a key to a nop command. The latter is used to avoid spawning a shell and a i3-msg process just to send a message. The companion listens to binding events and checks if this is a nop command.

bindsym $mod+Tab nop "previous-workspace"

There are other decorators to avoid code duplication: @debounce() to coalesce multiple consecutive calls, @static() to define a static variable, and @retry() to retry a function on failure. The whole script is a bit more than 1000 lines. I think this is worth a read as I am quite happy with the result.

dunst: the notification daemon Unlike the awesome window manager, i3 does not come with a built-in notification system. Dunst is a lightweight notification daemon. I am running a modified version with HiDPI support for X11 and recursive icon lookup. The i3 companion has a helper function, notify(), to send notifications using DBus. container_info() and workspace_info() uses it to display information about the container or the tree for a workspace.

Notification showing i3 s tree for a workspace

polybar: the status bar i3 bundles i3bar, a versatile status bar, but I have opted for Polybar. A wrapper script runs one instance for each monitor. The first module is the built-in support for i3 workspaces. To not have to remember which application is running in a workspace, the i3 companion renames workspaces to include an icon for each application. This is done in the workspace_rename() function. The icons are from the Font Awesome project. I maintain a mapping between applications and icons. This is a bit cumbersome but it looks great.

i3 workspaces in Polybar

For CPU, memory, brightness, battery, disk, and audio volume, I am relying on the built-in modules. Polybar s wrapper script generates the list of filesystems to monitor and they get only displayed when available space is low. The battery widget turns red and blinks slowly when running out of power. Check my Polybar configuration for more details.

Polybar displaying various information: CPU usage, memory usage, screen brightness, battery status, Bluetooth status (with a connected headset), network status (connected to a wireless network and to a VPN), notification status, and speaker volume.

For Bluetooh, network, and notification statuses, I am using Polybar s ipc module: the next version of Polybar can receive an arbitrary text on an IPC socket. The module is defined with a single hook to be executed at the start to restore the latest status.

[module/network]
type = custom/ipc
hook-0 = cat $XDG_RUNTIME_DIR/i3/network.txt 2> /dev/null
initial = 1

It can be updated with polybar-msg action "#network.send.XXXX". In the i3 companion, the @polybar() decorator takes the string returned by a function and pushes the update through the IPC socket. The i3 companion reacts to DBus signals to update the Bluetooth and network icons. The @on() decorator accepts a DBusSignal() object:

@on(
    StartEvent,
    DBusSignal(
        path="/org/bluez",
        interface="org.freedesktop.DBus.Properties",
        member="PropertiesChanged",
        signature="sa sv as",
        onlyif=lambda args: (
            args[0] == "org.bluez.Device1"
            and "Connected" in args[1]
            or args[0] == "org.bluez.Adapter1"
            and "Powered" in args[1]
        ),
    ),
)
@retry(2)
@debounce(0.2)
@polybar("bluetooth")
async def bluetooth_status(i3, event, *args):
    """Update bluetooth status for Polybar."""

The middle of the bar is occupied by the date and a weather forecast. The latest also uses the IPC mechanism, but the source is a Python script triggered by a timer.

Current date and weather forecast for the day in Polybar. The data is retrieved with the OpenWeather API.

I don t use the system tray integrated with Polybar. The embedded icons usually look horrible and they all behave differently. A few years back, Gnome has removed the system tray. Most of the problems are fixed by the DBus-based Status Notifier Item protocol also known as Application Indicators or Ayatana Indicators for GNOME. However, Polybar does not support this protocol. In the i3 companion, The implementation of Bluetooth and network icons, including displaying notifications on change, takes about 200 lines. I got to learn a bit about how DBus works and I get exactly the info I want.

picom: the compositor I like having slightly transparent backgrounds for terminals and to reduce the opacity of unfocused windows. This requires a compositor.¹ picom is a lightweight compositor. It works well for me, but it may need some tweaking depending on your graphic card.² Unlike the awesome window manager, i3 does not handle transparency, so the compositor needs to decide by itself the opacity of each window. Check my configuration for details.

systemd: the service manager I use systemd to start i3 and the various services around it. My xsession script only sets some environment variables and lets systemd handles everything else. Have a look at this article from Micha G ral for the rationale. Notably, each component can be easily restarted and their logs are not mangled inside the ~/.xsession-errors file.³ I am using a two-stage setup: i3.service depends on xsession.target to start services before i3:

[Unit]
Description=X session
BindsTo=graphical-session.target
Wants=autorandr.service
Wants=dunst.socket
Wants=inputplug.service
Wants=picom.service
Wants=pulseaudio.socket
Wants=policykit-agent.service
Wants=redshift.service
Wants=spotify-clean.timer
Wants=ssh-agent.service
Wants=xiccd.service
Wants=xsettingsd.service
Wants=xss-lock.service

Then, i3 executes the second stage by invoking the i3-session.target:

[Unit]
Description=i3 session
BindsTo=graphical-session.target
Wants=wallpaper.service
Wants=wallpaper.timer
Wants=polybar-weather.service
Wants=polybar-weather.timer
Wants=polybar.service
Wants=i3-companion.service
Wants=misc-x.service

Have a look on my configuration files for more details.

rofi: the application launcher Rofi is an application launcher. Its appearance can be customized through a CSS-like language and it comes with several themes. Have a look at my configuration for mine.

Rofi as an application launcher

It can also act as a generic menu application. I have a script to control a media player and another one to select the wifi network. It is quite a flexible application.

Rofi to select a wireless network

xss-lock and i3lock: the screen locker i3lock is a simple screen locker. xss-lock invokes it reliably on inactivity or before a system suspend. For inactivity, it uses the XScreenSaver events. The delay is configured using the xset s command. The locker can be invoked immediately with xset s activate. X11 applications know how to prevent the screen saver from running. I have also developed a small dimmer application that is executed 20 seconds before the locker to give me a chance to move the mouse if I am not away.⁴ Have a look at my configuration script.

Demonstration of xss-lock, xss-dimmer and i3lock with a 4 speedup.

The remaining components

• autorandr is a tool to detect the connected display, match them against a set of profiles, and configure them with xrandr.
• inputplug executes a script for each new mouse and keyboard plugged. This is quite useful to load the appropriate the keyboard map. See my configuration.
• xsettingsd provides settings to X11 applications, not unlike xrdb but it notifies applications for changes. The main use is to configure the Gtk and DPI settings. See my article on HiDPI support on Linux with X11.
• Redshift adjusts the color temperature of the screen according to the time of day.
• maim is a utility to take screenshots. I use Prt Scn to trigger a screenshot of a window or a specific area and Mod+Prt Scn to capture the whole desktop to a file. Check the helper script for details.
• I have a collection of wallpapers I rotate every hour. A script selects them using advanced machine learning algorithms and stitches them together on multi-screen setups. The selected wallpaper is reused by i3lock.

---

1. Apart from the eye candy, a compositor also helps to get tear-free video playbacks.
2. My configuration works with both Haswell (2014) and Whiskey Lake (2018) Intel GPUs. It also works with AMD GPU based on the Polaris chipset (2017).
3. You cannot manage two different displays this way e.g. :0 and :1. In the first implementation, I did try to parametrize each service with the associated display, but this is useless: there is only one DBus user session and many services rely on it. For example, you cannot run two notification daemons.
4. I have only discovered later that XSecureLock ships such a dimmer with a similar implementation. But mine has a cool countdown!

Background I have been maintaining detailed upgrade guides, on my wiki, starting with the jessie release, but I have actually written such guides for Koumbit.org as far back as Debian squeeze in 2011 (another worker wrote the older Debian lenny upgrade guide in 2009). Koumbit, since then, has kept maintaining those guides all the way to the latest bullseye upgrade, through 7 major releases! Over the years, those guides evolved from a quick "cheat-sheet" format copied from the release notes into a more or less "scripted" form that I currently use. Each guide has a procedure made of a few steps that can be basically copy-pasted to batch-upgrade a host (or multiple hosts in parallel) as quickly as possible. There is also the predict-os script which allows you to keep track of progress of the upgrades in a Puppet cluster.

Limitations of the official procedure In comparison with my procedure, the official upgrade guide is mostly designed to upgrade a single machine, typically a workstation, with a rather slow and exhaustive process. The PDF version of the upgrade guide is 14 pages long! This, obviously, does not work when you have tens or hundreds of machines to upgrade. Debian upgrades are notorious for being extremely reliable, but we have a lot of packages, and there are always corner cases where the upgrade will just fail because of a bug specific to your environment. Those will only be fixed after some back and forth in the community (and that's assuming users report those bugs, which is not always the case). There's no obvious way to deploy "hot fixes" in this context, at least not without fixing the package and publishing it on an unofficial Debian archive while the official ones catch up. This is slow and difficult. Or some packages require manual labor. Examples of this are the PostgreSQL or Ganeti packages which require you to upgrade your clusters by hand, while the old and new packages live side by side. Debian packages bring you far in the upgrade process, but sometimes not all the way. Which means every Debian install needs to be manually upgraded and inspected when a new release comes out. That's slow and error prone and we can do better.

How to automate major upgrades I have a proposal to automate this. It's been mostly dormant in the Debian wiki, for 5 years now. Fundamentally, this is a hard problem: Debian gets installed in so many different environments, from workstations to physical servers to virtual machines, embedded systems and so on, that it's extremely hard to come up with a "one size fits all" system. The (manual) procedure I'm using is mostly targeting servers, but I'm also using it on workstations. And I'll note that it's specific to my home setup: I have a different procedure at work, although it has a lot of common code. To automate this, I would factor out that common code with hooks where you could easily inject special code like "you need to upgrade ferm first", "you need an extra reboot here", or "this is how you finish the PostgreSQL upgrade". With Debian getting closer to a 2 year release cycle, with the previous release being supported basically only one year after the new stable comes out, I feel more and more strongly that this needs better automation. So I'm thinking that I should write a prototype for this. Ubuntu has do-release-upgrade that is too Ubuntu-specific to be reused. An attempt at collaborating on this has been mostly met with silence from Ubuntu's side as well. I'm thinking that using something like Fabric, Mitogen, or Transilience: anything that will allow me to write simple, portable Python code that can run transparently on a local machine (for single systems upgrades, possibly with a GUI frontend) to remote servers (for large clusters of servers, maybe with canaries and grouping using Cumin). I'll note that Koumbit started experimenting with Puppet Bolt in the bullseye upgrade process, but that feels too site-specific to be useful more broadly.

Trade-offs I am not sure where this stands in the XKCD time trade-off evaluation, because the table doesn't actually cover the time frequency of Debian release (which is basically "biennial") and the amount of time the upgrade would take across a cluster (which varies a lot, but that I estimate to be between one to 6 hours per machine). Assuming I have 80 machines to upgrade, that is 80 to 480 hours (between ~3 to 20 days) of work! It's unclear how much work such an automated system would shave off, however. Assuming things are an order of magnitude faster (say I upgrade 10 machines at a time), I would shave off between 3 and 18 days of work, which implies I might allow myself to spend a minimum of 5 days working on such a project.

The other option: never upgrade Before people mention those: I am aware of containers, Kubernetes, and other deployment mechanisms. Indeed, those may be a long-term solution, we currently can't afford to migrate everything over to containers right now: that is a huge migration and a total paradigm shift. At that point, whatever is left might not even be Debian in the first place. And besides, if you run Kubernetes, you still need to run some OS underneath and upgrade that, so that problem never completely disappears. Still, maybe that's the final answer: never upgrade. For some stateless machines like DNS replicas or load balancers, that might make a lot of sense as there's no or little data to carry to the new host. But this implies a seamless and fast provisioning process, and we don't have that either: at my work, installing a machine takes about as long as upgrading it, and that's after a significant amount of work automating that process, partly writing my own Debian installer with Fabric (!).

What is your process? I'm curious to hear what people think of those ideas. It strikes me as really odd that no one has really tackled that problem yet, considering how many clusters of Debian machines are out there. Surely people are upgrading those, and not following that slow step by step guide, right? I suspect everyone is doing the same thing: we all have our little copy-paste script we batch onto multiple machines, sometimes in parallel. That is what the Debian.org sysadmins are doing as well. There must be a better way. What is yours?

My upgrades so far So far, I have upgraded 2 out of my 3 home machines running buster -- others have been installed directly in bullseye -- with only my main, old, messy server left. Upgrades have been pretty painless so far (see another report, for example), much better than the previous buster upgrade. Obviously, for me personal use, automating this is pointless. Work-side, however, is another story: we have over 80 boxes to upgrade there and that will take a while. The last stretch to buster cycle took about two years to complete, so we might be done by the time the next release (12, "bookworm") is released, but that's actually a full year after "buster" becomes EOL, so it's actually too late... At least I fixed the installers so that new the machines we create all ship with bullseye, so we stopped accumulating new buster hosts...

Thanks to lelutin and pabs for reviewing a draft of this post.

• Firstly, Holger Levsen gave a talk titled Reproducible Buster, Bullseye & Bookworm - where we come from and where we are going (slides) which provided a high-level update on the status of reproducible builds within Debian, summing up the status in Debian Buster, Bullseye and then spoke about the outlook for Bookworm. This talk was also given at BornHack 2021 on the Danish island of Funen (schedule entry, video)
• Secondly, Vagrant Cascadian gave a talk titled Looking Forward to Reproducible Builds which mentions some historic blockers within Debian that have been solved, worked around or which are still in progress. It also touched on some recent developments, with an eye to what might happen as Debian embarks upon the new Bookworm development cycle after the release of Bullseye.
• Lastly, Johannes Schauer Marin Rodrigues & Fr d ric Pierret gave a joint talk on Making use of snapshot.debian.org for fun and profit about various tools they have developed to interact with the snapshot.debian.org wayback machine for Debian packages. In particular, they mention how they are using the service to reproduce and validate builds as well as touch on an alternative snapshot service that has been mentioned in previous reports.

Software development kpcyrd announced an interesting new project this month called I probably didn t backdoor this which is an attempt to be:

a practical attempt at shipping a program and having reasonably solid evidence there s probably no backdoor. All source code is annotated and there are instructions explaining how to use reproducible builds to rebuild the artifacts distributed in this repository from source. The idea is shifting the burden of proof from you need to prove there s a backdoor to we need to prove there s probably no backdoor . This repository is less about code (we re going to try to keep code at a minimum actually) and instead contains technical writing that explains why these controls are effective and how to verify them. You are very welcome to adopt the techniques used here in your projects. ( )

As the project s README goes on the mention: the techniques used to rebuild the binary artifacts are only possible because the builds for this project are reproducible . This was also announced on our mailing list this month in a thread titled i-probably-didnt-backdoor-this: Reproducible Builds for upstreams. kpcyrd also wrote a detailed blog post about the problems surrounding Linux distributions (such as Alpine and Arch Linux) that distribute compiled Python bytecode in the form of .pyc files generated during the build process.

diffoscope diffoscope is our in-depth and content-aware diff utility. Not only can it locate and diagnose reproducibility issues, it can provide human-readable diffs from many kinds of binary formats. This month, Chris Lamb made a number of changes, including releasing version 180), version 181) and version 182) as well as the following changes:

• New features:
  • Add support for extracting the signing block from Android APKs. [ ]
  • If we specify a suffix for a temporary file or directory within the code, ensure it starts with an underscore (ie. _ ) to make the generated filenames more human-readable. [ ]
  • Don t include short GCC lines that differ on a single prefix byte either. These are distracting, not very useful and are simply the strings(1) command s idea of the build ID, which is displayed elsewhere in the diff. [ ][ ]
  • Don t include specific .debug-like lines in the ELF-related output, as it is invariably a duplicate of the debug ID that exists better in the readelf(1) differences for this file. [ ]
• Bug fixes:
  • Add a special case to SquashFS image extraction to not fail if we aren t the superuser. [ ]
  • Only use java -jar /path/to/apksigner.jar if we have an apksigner.jar as newer versions of apksigner in Debian use a shell wrapper script which will be rejected if passed directly to the JVM. [ ]
  • Reduce the maximum line length for calculating Wagner-Fischer, improving the speed of output generation a lot. [ ]
  • Don t require apksigner in order to compare .apk files using apktool. [ ]
  • Update calls (and tests) for the new version of odt2txt. [ ]
• Output improvements:
  • Mention in the output if the apksigner tool is missing. [ ]
  • Profile diffoscope.diff.linediff and specialize. [ ][ ]
• Logging improvements:
  • Format debug-level messages related to ELF sections using the diffoscope.utils.format_class. [ ]
  • Print the size of generated reports in the logs (if possible). [ ]
  • Include profiling information in --debug output if --profile is not set. [ ]
• Codebase improvements:
  • Clarify a comment about the HUGE_TOOLS Python dictionary. [ ]
  • We can pass -f to apktool to avoid creating a strangely-named subdirectory. [ ]
  • Drop an unused File import. [ ]
  • Update the supported & minimum version of Black. [ ]
  • We don t use the logging variable in a specific place, so alias it to an underscore (ie. _ ) instead. [ ]
  • Update some various copyright years. [ ]
  • Clarify a comment. [ ]
• Test improvements:
  • Update a test to check specific contents of SquashFS listings, otherwise it fails depending on the test systems user ID to username passwd(5) mapping. [ ]
  • Assign seen and expected values to local variables to improve contextual information in failed tests. [ ]
  • Don t print an orphan newline when the source code formatting test passes. [ ]

In addition Santiago Torres Arias added support for Squashfs version 4.5 [ ] and Felix C. Stegerman suggested a number of small improvements to the output of the new APK signing block [ ]. Lastly, Chris Lamb uploaded python-libarchive-c version 3.1-1 to Debian experimental for the new 3.x branch python-libarchive-c is used by diffoscope.

Distribution work In Debian, 68 reviews of packages were added, 33 were updated and 10 were removed this month, adding to our knowledge about identified issues. Two new issue types have been identified too: nondeterministic_ordering_in_todo_items_collected_by_doxygen and kodi_package_captures_build_path_in_source_filename_hash. kpcyrd published another monthly report on their work on reproducible builds within the Alpine and Arch Linux distributions, specifically mentioning rebuilderd, one of the components powering reproducible.archlinux.org. The report also touches on binary transparency, an important component for supply chain security. The @GuixHPC account on Twitter posted an infographic on what fraction of GNU Guix packages are bit-for-bit reproducible: Finally, Bernhard M. Wiedemann posted his monthly reproducible builds status report for openSUSE.

Upstream patches The Reproducible Builds project detects, dissects and attempts to fix as many currently-unreproducible packages as possible. We endeavour to send all of our patches upstream where appropriate. This month, we wrote a large number of such patches, including:

• Bernhard M. Wiedemann:
  • awkward (timestamp issue)
  • ck (build fails in single-CPU mode)
  • cri-o (build ID issue related to Go parallelism)
  • kernel-obs-build (cpio metadata issue)
  • python-PyQt6 (.pyc-related issue)
  • python-dulwich (fails to build in 2023)
  • python-xkbgroup (.pyc-related issue)
  • rnp (fails to build in 2024)
• Bj rn Forsman:
  • dosfstools (from December 2018) was eventually merged.
• Chris Lamb:
  • #992039 filed against mapcache.
  • #992059 filed against spatialindex.
  • #992060 filed against pytsk.
  • #992061 filed against surgescript.
  • #992126 filed against rust-coreutils.
  • #992772 filed against translate.
  • #992773 filed against spirv-cross.
  • #992804 filed against numcodecs.
  • #993279 filed against tty-solitaire.
  • #993304 filed against samtools.
• Simon McVittie:
  • #992620 filed against pkg-config.
  • #992622 filed against pkgconf.
  • #992645 filed against ncftp.
  • #992647 filed against backuppc.
  • #992651 filed against sharutils.
  • #992662 filed against cfengine3.
  • #992702 filed against nbdkit.
  • #992722 filed against nbdkit.
  • #992775 filed against python3.9.
  • #992781 filed against supermin.
  • #992782 filed against virt-p2v.
  • #993249 filed against gnunet.
  • #993250 filed against mpb.
  • #993275 filed against ng.
• Vagrant Cascadian:
  • vlc (libvlccore.so.* embeds the build username and hostname).
  • #991926, #991927 and #991928 filed against grub2.

Elsewhere, it was discovered that when supporting various new language features and APIs for Android apps, the resulting APK files that are generated now vary wildly from build to build (example diffoscope output). Happily, it appears that a patch has been committed to the relevant source tree. This was also discussed on our mailing list this month in a thread titled Android desugaring and reproducible builds started by Marcus Hoffmann.

Website and documentation There were quite a few changes to the Reproducible Builds website and documentation this month, including:

• Felix C. Stegerman:
  • Update the website self-build process to not use the buster-backports suite now that Debian Bullseye is the stable release. [ ]
• Holger Levsen:
  • Add a new page documenting various package rebuilder solutions. [ ]
  • Add some historical talks and slides from DebConf20. [ ][ ]
  • Various improvements to the history page. [ ][ ][ ]
  • Rename the Comparison protocol documentation category to Verification . [ ]
  • Update links to F-Droid documentation. [ ]
• Ian Muchina:
  • Increase the font size of titles and de-emphasize event details on the talk page. [ ]
  • Rename the README file to README.md to improve the user experience when browsing the Git repository in a web browser. [ ]
• Mattia Rizzolo:
  • Drop a position:fixed CSS statement that is negatively affecting with some width settings. [ ]
  • Fix the sizing of the elements inside the side navigation bar. [ ]
  • Show gold level sponsors and above in the sidebar. [ ]
  • Updated the documentation within reprotest to mention how ldconfig conflicts with the kernel variation. [ ]
• Roland Clobus:
  • Added a ticket number for the issue with the live Cinnamon image and diffoscope. [ ]

Testing framework The Reproducible Builds project runs a testing framework at tests.reproducible-builds.org, to check packages and other artifacts for reproducibility. This month, the following changes were made:

• Holger Levsen:
  • Debian-related changes:
    • Make a large number of changes to support the new Debian bookworm release, including adding it to the dashboard [ ], start scheduling tests [ ], adding suitable Apache redirects [ ] etc. [ ][ ][ ][ ][ ]
    • Make the first build use LANG=C.UTF-8 to match the official Debian build servers. [ ]
    • Only test Debian Live images once a week. [ ]
    • Upgrade all nodes to use Debian Bullseye [ ] [ ]
    • Update README documentation for the Debian Bullseye release. [ ]
  • Other changes:
    • Only include rsync output if the $DEBUG variable is enabled. [ ]
    • Don t try to install mock, a tool used to build Fedora packages some time ago. [ ]
    • Drop an unused function. [ ]
    • Various documentation improvements. [ ][ ]
    • Improve the node health check to detect zombie jobs. [ ]
• Jessica Clarke (FreeBSD-related changes):
  • Update the location and branch name for the main FreeBSD Git repository. [ ]
  • Correctly ignore the source tarball when comparing build results. [ ]
  • Drop an outdated version number from the documentation. [ ]
• Mattia Rizzolo:
  • Block F-Droid jobs from running whilst the setup is running. [ ]
  • Enable debugging for the rsync job related to Debian Live images. [ ]
  • Pass BUILD_TAG and BUILD_URL environment for the Debian Live jobs. [ ]
  • Refactor the master_wrapper script to use a Bash array for the parameters. [ ]
  • Prefer YAML s safe_load() function over the unsafe variant. [ ]
  • Use the correct variable in the Apache config to match possible existing files on disk. [ ]
  • Stop issuing HTTP 301 redirects for things that not actually permanent. [ ]
• Roland Clobus (Debian live image generation):
  • Increase the diffoscope timeout from 120 to 240 minutes; the Cinnamon image should now be able to finish. [ ]
  • Use the new snapshot service. [ ]
  • Make a number of improvements to artifact handling, such as moving the artifacts to the Jenkins host [ ] and correctly cleaning them up at the right time. [ ][ ][ ]
  • Where possible, link to the Jenkins build URL that created the artifacts. [ ][ ]
  • Only allow only one job to run at the same time. [ ]
• Vagrant Cascadian:
  • Temporarily disable armhf nodes for DebConf21. [ ][ ]

Lastly, if you are interested in contributing to the Reproducible Builds project, please visit the Contribute page on our website. You can get in touch with us via:

• IRC: #reproducible-builds on irc.oftc.net.
• Twitter (@ReproBuilds) and Mastodon (@reproducible_builds@fosstodon.org).
• Reddit: /r/ReproducibleBuilds
• Mailing list: rb-general@lists.reproducible-builds.org

error: symbol grub_is_lockdown not found

update-grub2

dpkg-reconfigure -plow grub-efi-amd64

Force extra installation to the EFI removable media path?

Some EFI-based systems are buggy and do not handle new bootloaders correctly. If you force an extra installation of GRUB to the EFI removable media path, this should ensure that this system will boot Debian correctly despite such a problem. However, it may remove the ability to boot any other operating systems that also depend on this path. If so, you will need to make sure that GRUB is configured successfully to be able to boot any other OS installations correctly.

Update NVRAM variables to automatically boot into Debian?

GRUB can configure your platform's NVRAM variables so that it boots into Debian automatically when powered on. However, you may prefer to disable this behavior and avoid changes to your boot configuration. For example, if your NVRAM variables have been set up such that your system contacts a PXE server on every boot, this would preserve that behavior.

eddy@aptonia:/ $ cat /etc/modules
# /etc/modules: kernel modules to load at boot time.
#
# This file contains the names of kernel modules that should be loaded
# at boot time, one per line. Lines beginning with "#" are ignored.
r8169

DRV_NAME=rtl8821ce
DRV_VERSION=v5.5.2_34066.20200325

cp -r . /usr/src/$ DRV_NAME -$ DRV_VERSION

dkms add -m $ DRV_NAME -v $ DRV_VERSION

dkms build -m $ DRV_NAME -v $ DRV_VERSION -k 5.10.0-0.bpo.8-amd64
dkms install -m $ DRV_NAME -v $ DRV_VERSION -k 5.10.0-0.bpo.8-amd64

dkms build rtl8821ce/v5.5.2_34066.20200325 -k 4.19.0-17-amd64
dkms install rtl8821ce/v5.5.2_34066.20200325 -k 4.19.0-17-amd64

• https://wiki.debian.org/DebianEvents/de/2021/DebianReunionHamburg#Dates_and_format_of_the_event
• https://wiki.debian.org/DebianEvents/de/2021/DebianReunionHamburg#Covid_things
• https://wiki.debian.org/DebianEvents/de/2021/DebianReunionHamburg#Call_for_papers
• https://wiki.debian.org/DebianEvents/de/2021/DebianReunionHamburg#Registration - registration is mandatory for this event!

Debian

This was my 32nd month of actively contributing to Debian. I became a DM in late March 2019 and a DD on Christmas 19! \o/ Tough month but I mostly spent on it churning through the immense backlog. But that somewhat backfired and I have even more backlog than ever. :D Anyway, I did the following stuff in Debian:

Uploads and bug fixes:

• ruby3.0 (3.0.0-2) - Upload to unstable! \o/

Other $things:

• Mentoring for newcomers.
• Moderation of -project mailing list.

---

Ubuntu

This was my 7th month of actively contributing to Ubuntu. Now that I ve joined Canonical to work on Ubuntu full-time, there s a bunch of things I do! \o/ I mostly worked on different things, I guess. But mostly on packaging keylime and some Google Agents upload(s) and SRU(s). Also did a lot of reviewing, et al. I was too lazy to maintain a list of things I worked on so there s no concrete list atm. Maybe I ll get back to this section later or will start to list stuff from next month onward, as I ve been doing before. :D

---

Debian (E)LTS

Debian Long Term Support (LTS) is a project to extend the lifetime of all Debian stable releases to (at least) 5 years. Debian LTS is not handled by the Debian security team, but by a separate group of volunteers and companies interested in making it a success. And Debian Extended LTS (ELTS) is its sister project, extending support to the Jessie release (+2 years after LTS support). This was my twenty-third month as a Debian LTS and eleventh month as a Debian ELTS paid contributor.
I was assigned 23.75 hours for LTS and 40.00 hours for ELTS and worked on the following things:
(however, I only worked for 23.75h on ELTS work, thereby, carrying the rest to next month)

LTS CVE Fixes and Announcements:

• Issued DLA 2743-1, fixing CVE-2017-5715, for amd64-microcode.
  For Debian 9 stretch, these problems have been fixed in version 3.20181128.1~deb9u1.
• Issued DLA 2744-1, fixing the versioning issue, for usermode.
  For Debian 9 stretch, these problems have been fixed in version 1.109-1+deb9u1.
• Issued DLA 2750-1, fixing CVE-2019-20421, CVE-2021-3482, CVE-2021-29457, CVE-2021-29473, CVE-2021-31291, and CVE-2021-31292, for exiv2.
  For Debian 9 stretch, these problems have been fixed in version 0.25-3.1+deb9u3.

ELTS CVE Fixes and Announcements:

• Issued ELA 479-1, fixing CVE-2019-20421, CVE-2021-3482, CVE-2021-29457, CVE-2021-29473, CVE-2021-31291, and CVE-2021-31292, for exiv2.
  For Debian 8 jessie, these problems have been fixed in version 0.24-4.1+deb8u6.
• Noticed that there s a fallout of CVE-2021-3185, where an update was issued for gst-plugins-bad1.0, however, not for gst-plugins-bad0.10.
  Thanks to Sylvain s script, this came up and I prepped an update for that.
• Started to work on libjdom1-java s regression.

Other (E)LTS Work:

• Front-desk duty from 26-07 until 01-08 and from 30-08 until 05-09 for both LTS and ELTS.
• Triaged haproxy, ntfs-3g, and cyrus-imapd, and exiv2, ffmpeg, git, gpac, inetutils, mc, modsecurity-crs, node-object-path, php-pear, systemd-cron, node-tar, ruby2.3, gst-plugins-bad0.10, jsoup, libxstream-java, qemu, tomcat7, ruby2.1, prototypejs, pillow, cpio, and qtbase-opensource-src, and amd64-microcode.
• Mark CVE-2021-39240/haproxy as not-affected for stretch and jessie.
• Mark CVE-2021-39241/haproxy as not-affected for stretch and jessie.
• Mark CVE-2021-39242/haproxy as not-affected for stretch and jessie.
• Mark CVE-2021-33582/cyrus-imapd as no-dsa for stretch.
• Mark CVE-2020-18771/exiv2 as no-dsa for exiv2 for stretch.
• Mark CVE-2020-18899/exiv2 as no-dsa for exiv2 for stretch.
• Mark CVE-2021-38171/ffmpeg as postponed for stretch.
• Mark CVE-2021-40330/git as no-dsa for stretch and jessie.
• Mark CVE-2020-19481/gpac as ignored for stretch.
• Mark CVE-2021-40491/inetutils as no-dsa for stretch.
• Mark CVE-2021-36370/mc as no-dsa for stretch and jessie.
• Mark CVE-2021-35368/modsecurity-crs as no-dsa for stretch.
• Mark CVE-2021-23434/node-object-path as end-of-life for stretch.
• Mark CVE-2021-32610/php-pear as no-dsa for stretch.
• Mark CVE-2017-9525/systemd-cron as no-dsa for stretch.
• Mark CVE-2021-37701/node-tar as end-of-life for stretch.
• Mark CVE-2021-37712/node-tar as end-of-life in stretch.
• Mark CVE-2021-3750/qemu as postponsed for jessie.
• Mark CVE-2021-27511/prototypejs as postponsed for jessie.
• Mark CVE-2021-23437/pillow as postponed for stretch and jessie.
• Auto EOL ed gpac, cacti, openscad, cgal, cyrus-imapd-2.4, libsolv, mosquitto, atomicparsley, gtkpod, node-tar, libapache2-mod-auth-openidc, neutron, inetutils and linux for jessie.
• Drop cpio from ela-needed; open issues don t warrant an ELA.
• Attended monthly Debian LTS meeting.
• Answered questions (& discussions) on IRC (#debian-lts and #debian-elts).
• General and other discussions on LTS private and public mailing list.

---

Until next time.
:wq for today.

Safe upgrades A central idea behind Candid is that services evolve over time, and so also their interfaces evolve. As they do, it is desirable to keep the interface usable by clients who have not been updated. In particular on a blockchainy platform like the Internet Computer, where some programs are immutable and cannot be changed to accommodate changes in the interface of the services they use, this is of importance. Therefore, Candid defines which changes to an interface are guaranteed to be backward compatible. Of course it s compatible to add new methods to a service, but some changes to a method signature can also be ok. For example, changing

service A1 :  
  get_value : (variant   current; previous : nat  )
    -> (record   value : int; last_change : nat  )
 

to

service A2 :  
  get_value : (variant   current; previous : nat; default  )
    -> (record   value : int; last_change : nat; committed : bool  )
 

is fine: It doesn t matter that clients that still use the old interface don t know about the new constructor of the argument variant. And the extra field in the result record will silently be ignored by old clients. In the Candid spec, this relation is written as A2 <: A1 (note the order!), and the formal footing this builds on is subtyping. We thus say that it is safe to upgrade a service to a subtype , and that A2 s interface is a subtype of A1 s. In small examples, I often use nat and int, because every nat is also a int, but some int values are not not nat values, namely the negative ones. We say nat is a subtype of int, nat <: int. So a function that in the first returns a int can in the new version return a nat without breaking old clients. But the other direction doesn t work: If the old version s method had a return type of nat, then changing that to int would be a breaking change, as old clients would not be prepared to handle negative numbers. Note that arguments of function types follow different rules. In fact, the rules are simply turned around, and in argument position (also called negative position ), we can evolve the interface to accept supertypes. Concretely, a function that originally expected an nat can be changed to expect an int, but the other direction doesn t work, because there might still be old clients around that send negative numbers. This reversing-of-the-rules is called contravariance. The vision is that the developer s tooling will warn the developer before they upgrade the code of a running service to a type that is not a subtype of the old interface. Let me stress, though, that all this is about not breaking existing clients that use the old interface. It does not mean that a client developer who fetches the new interface for your service won t have to change their code to make his programs compile again.

Higher order composition So far, we considered the simple case of one service with a bunch of clients, i.e. the first order situation. Things get much more interesting if we have multiple services that are composed, and that pass around references to each other, and we still want everything to be nicely typed and never fail even as we upgrade services. Therefore, Candid s type system can express that a service s method expects or a returns a reference to another service or method, and the type that this service or method should have. For example

service B :   add_listener : (text, func (int) -> ()) -> ()  

says that the service with interface B has a method called add_listener which takes two arguments, a plain value of type text and a reference to a function that itself expects a int-typed argument. The contravariance of the subtyping rules explained above also applies to the types of these function reference. And because the int in the above type is on the left of two function arrows, the subtyping rule direction flips twice, and it is therefore safe to upgrade the service to accept the following interface:

service B :   add_listener : (text, func (nat) -> ()) -> ()  

Soundness theorem and Coq proof The combination of these higher order features and the safe upgrade mechanism is maybe the unique selling point of Candid, but also what made its design quite tricky sometimes. And although the conventional subtyping rules work well, we wanted to do some less conventional things (more on that below), and more than once thought we had a good solution, only to notice that it did not hold water in complicated higher-order cases. But what does it mean to hold water? I felt the need to precisely define a soundness criteria for higher order interface description languages, which you can find in the document An IDL Soundness Proposition. This is a general framework which you can instantiate with your concrete IDL language and rules, and then it tells you what you have to prove to consider your language to be sound. Intuitively, it simulates all possible ways in which services can be defined and upgraded, and in which they can pass around references to each other, and the soundness property is that then that for all messages sent between services, they can always be understood. The document also shows, in general, that any system that builds on canonical subtyping in sound, as expected. That is still a generic theorem that you can instantiate with a concrete system, but now there is less to do. Because these proofs can get tricky in corner cases, it is valuable to mechanize them in an interactive theorem prover and have the computer check the proofs. So I have created a Coq formalization that defines the soundness criteria, including the reduction to canonical subtyping. It also defines MiniCandid, a greatly simplified variant of Candid, proves various properties about it (transitivity etc.) and finally instantiates the soundness theorem. I am particularly fond of the use of coinduction to model the equirecursive types, and the use of named cases, as we know them from Isabelle, to make the proofs a bit more readable and maintainable. I am less fond of how much work it seems to be to extend MiniCandid with more of Candid s types. Already adding vec was more work than it s worth it, and I defensively blame Coq s not-so-great support for nested recursion. The soundness relies on subtyping, and that is all fine and well as long as we stick to canonical rules. Unfortunately, practical considerations force us to deviate from these a bit, as we see in the next post of this series.

• [1] https://tinyurl.com/yhrq5887
• [2] https://tinyurl.com/yhmog5ex
• [3] https://tinyurl.com/ygnmjm9j
• [4] https://www.bitsnbites.eu/three-fundamental-flaws-of-simd/
• [5] https://www.dailydot.com/debug/ghostezra-telegram-fued-qanon/
• [6] https://tinyurl.com/yhvganlc
• [7] https://tinyurl.com/yfljnlq5
• [8] https://tinyurl.com/yft5qwbc
• [9] https://tinyurl.com/yjjnnyfu
• [10] https://tinyurl.com/yjxq8c2v
• [11] https://tinyurl.com/y7hhrxuz
• [12] https://tinyurl.com/yj33txpj

• a language to describe the interface of a service, with named methods, arguments and results,
• a type system for these arguments and results that maps to many possible implementation languages, thus enabling interoperability between these languages,
• a wire format (encoding) of values of these types,
• a notion of safe upgrade that allows service developers to evolve their interface with the assurance that no clients break, even in the presence of higher-order composition,
• a meta-theory of safe interface description languages and a formal proof that Candid has these properties, and
• a bunch of tools and libraries.

Blog post series overview

1. The rough idea (this post) Announcing the blog post series, and very briefly outlining what an Interface Description Language even is.
2. Safe higher-order upgrades Every can do first-order interface description languages. What makes Candid special is its support for higher-order service composition. This post also contains some general meta-theory and a Coq proof!
3. Opt is special Extending records in argument position is surprisingly tricky. Lean why, and how we solved it.
4. Language integration The point of Candid is inter-op, so let s look the various patterns of connecting Candid to your favorite programming language.
5. Quirks The maybe most interesting post in this series: Candid has a bunch of quirks (hashed field names, no tuples but sequnces, references that nobody used, and more). I ll explain some of them, why they are there, and what else we could have done. Beware: This post will be opinionated.

The rough idea The idea of an interface description language is easy to begin with. Something like

service A :  
  add : (int) -> ();
  get : () -> (int);
 

clearly communicates that a service called A provides two methods add and get, that add takes an integer as an argument, and that get returns such a number. Of course, this only captures the outer shape of the service, but not what it actually does we might assume it implements a counter of sorts, but that is not part of the Candid interface description. In order to now use the service, we also need a (low-level) transport mechanism. Candid itself does not specify that, but merely assumes that it exists, and is able to transport the name of the invoked method and a raw sequence of bytes to the service, and a response (again a raw sequence of bytes) back. Already on the Internet Computer we have two distinct transport mechanisms used are external calls via an HTTP-based RPC interface and on-chain inter-canister calls. Both are handled by the service in the same way. Here we can see that Candid succeeds in abstracting over differences in the low-level transport mechanism. Because of this abstraction, it is possible to use Candid over other transportation mechanisms as well (conventional HTTPS, E-Mail, avian carrier). I think Candid has potential there as well, so even if you are not interested in the Internet Computer, this post may be interesting to you. The translation of argument and result values (e.g. numbers of type int) to the raw sequence of bytes is specified by Candid, defining a wire format. For example, the type int denotes whole numbers of arbitrary size, and they are encoded using the LEB128 scheme. The Candid wire format also contains a magic number (DIDL, which is 0x4449444c in hex) and a type description (more on that later). So when passing the value 2342 to the add, the raw bytes transported will be 0x4449444c00017da612. Of course we want to transfer more data than just integers, and thus Candid supports a fairly complete set of common basic types (nat, int, nat8, nat16, nat32, nat64, int8, int16, int32, int64, float32, float64, bool, text, blob) and composite types (vectors, optional values , records and variants). The design goal is to provide a canonical set of types enough to express most data that you might want to pass, but no more than needed, so that different host languages can support these types easily. The Candid type system is structural. This means that two types are the same when they are defined the same way. Imagine two different services defining the same

type User = record   name : text; user_id : nat  

then although User is defined in two places, it s still the same type. In other words, these name type definitions are always just simple aliases, and what matters is their right-hand side. Because Candid types can be recursive (e.g. type Peano = opt Peano), this means we have an equirecursive type system, which makes some things relatively hard. So far, nothing too special about Candid compared to other interface definition languages (although a structural equirecursive type system is already something). In the next post we look at reference types, and how such higher order features make Candid an interesting technology.