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Prompt

Prompt is a terminal that marries the best of GNOME Builder's
seamless container support, the beauty of GNOME Text Editor, and the
robustness of VTE. I like to think of it as a companion terminal to
Builder.

Though it's also useful for immutable/container-oriented desktops
like Fedora Silverblue or Project Bluefin where containers are
front-and-center.

A screenshot of Prompt with a menu open showing a list of available
containers to spawn a new terminal shell within.

This came out of a prototype I made for the GNOME Builder IDE nearly
a decade ago. We already had all the container abstractions so why
not expose them as a Terminal Workspace?

Prompt extracts the container portion of the code-foundry into a
standalone program.

My prototype didn't go anywhere in recent years because I was
conflicted. I have a high performance bar for software I ship and VTE
wasn't there yet on Wayland-based compositors which I use. But if you
frequent this blog you already know that I reached out to the
meticulous VTE maintainers and helped them pick the right battles to
nearly double performance this GNOME cycle. I also ported
gnome-terminal to GTK 4 which provided me ample opportunity to see
where and how containers would integrate from an application
perspective.

I designed Prompt to be Flatpak-first. That has design implications
if you want a robust feature-set. Typically an application is
restricted to the PID and PTY namespace within the Flatpak sandbox
even if you're capable of executing processes on the host. That means
using TTY API like tcgetpgrp() becomes utterly useless when the
kernel ioctl() returns you a PID of 0 (as it's in a different
namespace). Notably, 0 is the one value tcgetpgrp() is not documented
to return. How fun!

To give Prompt the best chance at tracking containers and foreground
processes a prompt-agent runs from the host system. It is restricted
to very old versions of GLib/GObject/GIO and JSON-GLib because we
know that /usr/bin/flatpak will already require them. Using those
libraries instead of certain GLibc API helps us in situations where
GLibc is only forward-compatible and not backwards-compatible.
Combined with point-to-point D-Bus serialization on top of a
socketpair() we have a robust way to pass file-descriptors between
the agent and the UI process and we'll use that a bunch.

There are a lot of little tricks in here to keep things fast and
avoid energy-drain. For example, process tracking is done with a
combination of exponential-backoff and re-triggering based on either
new content arriving or certain key-presses. It gives a very
low-latency feeling to the sudo/ssh feature I love from Console,
albeit with less overhead.

One thing I find helpful with Builder is that when I come back to it
my project session is right there. So this has session support too.
It will restore your tabs/containers how they were before. So if you
have a similar workflow you might find that useful. If not? Just turn
it off in Preferences.

I want to have a bit of fun because right now I'm stuck indoors
caring for my young, paraplegic dog. So it's packed full of palettes
you can play with. Who doesn't like a little color!

A screenshot of Prompt with the preferences window open allowing the
selection of numerous palettes with diverse color sets. The terminal
window is recolored using colors from the palette.

There are some subtle performance choices that make for a better
experience in Prompt. For example, I do like having a small amount of
padding around the terminal so that rounded corners look nice and
also avoids an extra framebuffer when rendering on the GPU. However,
that looks odd with scrollback. So Prompt rewrites the snapshot from
VTE to remove the background and extraneous clipping. We already have
a background from window recoloring anyway. It's a small detail that
just feels good when using it.

Another subtle improvement is detecting when we are entering the tab
overview. Currently, libadwaita uses a GtkWidgetPaintable to
represent the tab contents. This works great for the likes of
Epiphany where the contents are backed by GL textures. But for a
terminal application we have a lot of text and we don't want to
redraw it scaled as would happen in this case. That puts a lot of
pressure on the glyph cache. So instead, we create a single texture
upfront and scale that texture. Much smoother.

For people writing terminal applications there is a little inspector
you can popup to help you out. It can be difficult to know if you're
doing the right thing or getting the right behavior so this might be
something we can extend going forward to make that easier for you.
GTK's inspector already does so much so this is just an extension of
what you could do there.

A terminal window open with a secondary "inspector" window open. The
inspector shows what column and row the mouse is positioned as well
as the cursor and what non-visible OSC hyperlink is under the
pointer.

Creating Prompt has elevated problems we should fix.

  * Podman creates an additional PTY which sort of breaks the whole
    notion of foreground processes. Filed an issue upstream and it
    seems likely we can get that addressed for personal containers.
    That will improve what happens when you close your terminal tab
    with something running or if you SSH'd into another host from the
    container.
  * Container tracking is currently limited to Fedora hosts because
    toolbox only emits the container-tracking escape sequences when
    the host is Fedora. The current goal I've discussed with VTE
    maintainers is that we'll use a new "termprop" feature in VTE
    that will be silently dropped on terminal applications not
    interested in it. That way toolbox and the likes can safely emit
    the escape sequence.
  * Currently podman will exit if you pass a --user or --workdir that
    does not exist in the container. That isn't a problem with
    toolbox as it is always your user and fails gracefully for
    directories. So we need a good strategy to see if both of those
    are available to inherit when creating new tabs.
  * This does have transparency support, but it's hidden in a
    GSetting for now. Once we have libadwaita with CSS variable
    support we can probably make this look better during transitions
    which is where it falls down now. We also need some work on how
    AdwTabOverview does snapshots of tabs and inserts a background.
  * I have a .bashrc snippet to treat jhbuild as a container which is
    helpful for those of us GNOME developers still using it.
  * Accessibility is an extremely important piece of our
    infrastructure in GNOME. So part of this work will inevitably tie
    into making sure the a11y portion of VTE works with the
    soon-to-land a11y improvements in GTK. That has always been
    missing on GTK 4-based VTE and therefore every terminal based
    upon it.

$ flatpak install --user --from https://nightly.gnome.org/repo/appstream/org.gnome.Prompt.Devel.flatpakref

If you like software that I write, consider donating to a pet shelter
near you this holiday season. We're so lucky to have great pet care
in Oregon but not everywhere is so lucky.

Happy Holiday Hacking!

Posted on December 14, 2023December 15, 2023Author chergertCategories
Uncategorized

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