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November 2018
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2018-11-25 »

Xnest, Xephyr, ChromeOS, synergy, and syncing some clipboards

I recently decided to switch my laptop from a Macbook to a Chromebook, partly because Apple's keyboards are so terrible lately, and partly because ChromeOS is suddenly useful now that they invented Crostini.

(Some people ask why I, a person who actually knows how to use Linux and has debugged wifi drivers and XF86Config files, would want to use a locked-down desktop Linux variant instead of just installing Debian or something. And I do install Debian, on desktop hardware. But on a laptop, hardware support is paramount: external monitors (eg. for presentations), bluetooth audio (for music while travelling), long battery life, and rapid, non-crashy suspend/resume, are all really important to me. ChromeOS actually does all that stuff reliably nowadays, because they design the OS and the hardware at the same time. Debian can't compete with that.)

One showstopper for me when I'm trying to do software development, however, is having a proper window manager, which is to say, one that I can run without resorting to a mouse or touchpad. Because I am old and crusty and unreasonably opinionated, the one I want to run is ion1. I had it working on MacOS, but I wanted it on ChromeOS.

Now, modern ChromeOS uses Wayland as its display manager, not X11, which is of some concern becuse ion1 stopped evolving more than a decade ago (which is how I like it) and therefore only understands X11. Also, ChromeOS provides a Wayland compositor and doesn't let you replace it from Crostini, even though they do let you securely launch Wayland and X11 windows from Crostini (which is pretty cool).

Do we give up? No! The "obvious" "solution" is Xnest, an "X proxy" from before the dawn of time, which puts all your windows inside one big window. So I created one big full-screen Xnest window (managed by Wayland), and then ran ion1 and a bunch of rxvt terminals inside.

This actually worked almost right, except: ion1 doesn't support these fancypants client-rendered fonts. No. It's old and crusty, like me. It expects the X server to render its fonts. And unfortunately, ChromeOS contains only about four fonts in its X server, all of which are hopelessly microscopic on the 200dpi screen in my Chromebook. Oops.

Luckily, about 11 years ago, slightly after the dawn of time, someone else didn't like some Xnest limitations and made Xephyr, which apparently is more of a framebuffer and less of a proxy, the upshot of which is that it renders its own "server side" fonts (which from Wayland's point of view are on the client side, but from ion1's point of view are definitely on the server side). As a bonus, thanks to xrandr, it understands the idea of having its window resized, so I can use the handy ChromeOS "full screen" key and have it do something nice. Or drag it to an external monitor and get decent results.

I didn't try to do anything with 3D, but nominally Xephyr can do that too. But Crostini supposedly can't. I don't know and I don't really care, I'm just trying to run some terminals here.

Xephyr and its DPI calculation

One weird problem I've had with Xephyr is that whatever it's doing to calculate "dots per inch" (as reported by xdpyinfo) is completely insane. It starts off with a value that is definitely not the same as its host display, and then if you resize the window, it just changes the value and gets more and more confused. This is bad for people who want to specify their font size in points so that fonts will be roughly the same size no matter what size the display is.

As far as I can tell, this is just a bug in Xephyr. But if anybody knows what's going on or (especially) how to fix it, I'd love to know. Meanwhile, I learned to specify my font sizes in pixels instead of points.

Cut and paste

An even more annoying problem, which deserves its own section, is the question of how to deal with cut-and-paste between my Xephyr+ion1+rxvt session and the toplevel Wayland session. This is needed for two reasons:

  1. I want to copy URLs and text between my web browser and my terminals.

  2. I want to be able to run multiple Xephyr sessions and share text between them.

By default, Xephyr appears to have no clipboard sync at all between its internal clipboard and its host server's clipboard. That's no fun. (To their credit, ChromeOS does seem to manage to sync the clipboard between Wayland and its toplevel XWayland session, which is essential if we want anything to work. It's just Xnest and Xephyr that break the chain.)

Now, there are a few things you should know about X11 clipboards. The canonical explanation is jwz's X Selections, Cut Buffers, and Kill Rings, which is quite excellent and gives some background on how it's not the X11 clipboard that's crazy, it's all the apps using it.

So anyway, with that background in mind, all we need to do is magically keep the clipboard in sync between :0 (the toplevel XWayland server, which is synced with Wayland and Chrome), and :1 (inside my Xephyr server), and ideally :2 .. :n (inside other nested Xephyr servers). How hard can it be?

Well, apparently it can be hard. The best answers I could find on the Internet (which I won't link to, because they suck) are:

  1. Run a script that uses xclip to periodically grab the clipboard content from each server. If one server has different clipboard content than the currently-expected content, then copy it to the other server. This method has a few problems: first, you have to choose a periodicity for the sync process, which is inevitably either annoyingly long or battery-killingly short. Second, the "content based" sync decision is rather error prone and results in potentially unstable race conditions, especially with multi-way sync. And third, typical implementations are a little too pushy about copying the clipboard data to all screens: jwz's article talks about this problem in refence to the "X cut buffer" support before the new-style support was added. If you highlight/copy text frequently or in large volumes, it's pretty wasteful to copy it to other screens before it's needed for pasting.

  2. Run synergy, a tool that lets you seamlessly extend your mouse/keyboard/clipboard across multiple displays on multiple computers. This was very tempting, despite being severe overkill (I don't want to extend my mouse and keyboard, just my clipboard). Unfortunately, it didn't work. It almost worked. But it didn't.

Luckily(?) for you, I spent quite some time diagnosing why synergy didn't work for me in my use case. The symptom was that it would sync the clipboard in only one direction (say A->B), and only the first time I copied something. If I copied another thing on A, the clipboard on B would not be updated. To make it update, I had to copy something on B (which always fails to sync to A), and then copy something on A (which would work).

How hard can it be? I thought to myself, again, foolishly, and decided to read the source code.

Now, the synergy source code is actually pretty good. It has a nice abstraction layer for the various clipboard types in X, MacOS, and Windows. It's pretty easy to follow. It has a bit too few debug trace messages, but okay, those are easy enough to add as we go.

Unfortunately, synergy's clipboard support has two fatal design flaws:

  1. Like the periodic xclip case above, it grabs a copy of clipboard data right away when the clipboard ownership changes. It's better than a naive xclip script, because it actually gets a notification when the clipboard ownership changes, rather than polling periodically. Unfortunately, those notifications are also its downfall. See, in X11, there is only a clipboard notification when the clipboard owner changes, not when the content changes. If I copy text from rxvt, it will grab the clipboard. Synergy will notice this and read the clipboard. But if I then copy different text in rxvt, the owner doesn't change, so there is no notification, so Synergy doesn't re-copy it. That explains why it only worked the first time. (It also explains why copying on B and then on A causes it to work again exactly once: the clipboard ownership changes.) (This bug may not be visible on all terminals. If rxvt would give up the clipboard, then take it back, every time I made a copy, it would work around this problem.) (I think VNC's clipboard sync has/had the same problem.)

  2. Synergy, because it's mostly a keyboard/mouse sharing app, maintains the concept of a "current screen." That is, it watches which screen currently has the mouse pointer, and only replicates the clipboard to that screen. This is a performance optimization: since it (like the poorly designed "x cut buffer" mentioned by jwz) takes a copy every time the clipboard changes, it doesn't want to replicate this to screens where you're not using it. Unfortunately, since my screens are nested, I had to disable the keyboard/mouse sharing feature, which also leaves the "current screen" incorrect exactly half the time, which is why the clipboard fails to replicate from B->A and only works from A->B.

I was willing to try to fix some minor clipboard bugs in synergy, but I gave up when I realized this design (grab and replicate the content as soon as clipboard owner changes) was never going to work well with rxvt. That's when I gave up and decided to write my own trivial clipboard syncing tool, based on all the otherwise-useless trivia I had acquired while investigating the above.

The result is xclipsync, and, other than omitting non-text clipboard formats, I think I did it right.

  1. It starts up by taking ownership of the clipboard on display A.
  2. When it loses clipboard ownership on A, it takes ownership of the clipboard on display B.
  3. When it loses clipboard ownership on B, it goes back to step 1.
  4. When it receives a request for clipboard contents (which should be someone requesting a paste), it then reads the clipboard content from the display that it doesn't currently own, and forwards it along.

And that's it!

This avoids the problem of a single owner changing their clipboard content (since it grabs content only on demand). It doesn't do extra work if you copy content without pasting. It actually does nothing at all if you do a lot of work on one display: it loses the clipboard content on that display, which means it does nothing at all until you use the clipboard on the other display. It doesn't ever poll anything, so there are no arbitrary delays or race conditions.

And best of all, this algorithm works even for multi-way sync. You can run parallel instances of xclipsync between any two displays, and as long as you don't create any bridging loops, it will do the right thing across all of them. That is, exactly one display will "own" the clipboard, and all the other ones will copy from it. This works because every time you copy something from a new display, exactly one xclipsync instance will lose ownership, which causes it to assert ownership on exactly one display. If another xclipsync is syncing with that display, it will then lose ownership, and assert ownership on exactly one other display, and so on. As long as there are no loops, this process will terminate, and it'll do so very efficiently.

Things that could be better

There's no particular reason xclipsync can't support non-plaintext clip formats. I just didn't implement it, because I didn't need anything but text, since my Xephyr session is just terminals anyway.

xclipsync currently uses tcl/tk to take clipboard ownership (yes!). This would have been unnecessary if xclip had just one more feature: the ability to run a command at paste time, rather than always reading clipboard content from stdin at startup time. Then xclipsync would have been just a couple of (foregrounded) alternating xclip calls in a loop.

Xephyr probably should just implement this exact clipboard syncing protocol internally.

Note that it appears ChromeOS+Wayland is actually implementing some other kind of clipboard sync between Wayland and the toplevel XWayland server. When xclipsync tries to take ownership of the XWayland clipboard, it immediately experiences one "paste" operation and then loses ownership. This might be related to WAyland's inter-process security isolation features. In any case, xclipsync reacts as usual (giving clipboard ownership to XWayland and proxying requests to XWayland from other displays that want to paste) and all is well.

I really wish Alt-Tab would work even when the Xephyr instance is fullscreened in ChromeOS. I understand why they want to let me capture Alt-Tab in my full-screen X apps, but also... I don't want to.

The marketing-driven "Assistant key" on the Pixelbook is a user-hostile disaster in its current form. On the other hand, if they would let me remap it to, say, Meta, it would instantly redeem itself.

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