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nixos/lightdm: do not lock up with plymouth #71061

Merged
merged 1 commit into from Oct 13, 2019
Merged

nixos/lightdm: do not lock up with plymouth #71061

merged 1 commit into from Oct 13, 2019

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hedning
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@hedning hedning commented Oct 12, 2019

Motivation for this change

Having display-manager conflict with plymouth causes this lock up:

  • plymouth-quit-wait starts up, waiting for plymouth-quit to run
  • lightdm starts up
  • plymouth-quit can't start, it conflicts with lightdm
  • plymouth-quit-wait keeps waiting on plymouth-quit to kill plymouthd

This config was borrowed from GDM, but doesn't work with LightDM as GDM fixes
the lock by killing plymouth itself.

fixes #71034

Things done

Confirmed in a VM that lightdm + plymouth works.

  • Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS)
  • Built on platform(s)
    • NixOS
    • macOS
    • other Linux distributions
  • Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
  • Tested compilation of all pkgs that depend on this change using nix-shell -p nix-review --run "nix-review wip"
  • Tested execution of all binary files (usually in ./result/bin/)
  • Determined the impact on package closure size (by running nix path-info -S before and after)
  • Ensured that relevant documentation is up to date
  • Fits CONTRIBUTING.md.
Notify maintainers

cc @

@worldofpeace
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I wonder how fedora doesn't have this issue.
Is this patch relevant canonical/lightdm#3. They added it when using this.

@hedning
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hedning commented Oct 12, 2019 •
edited

At a quick glance it looks like we aren't patching the path to plymouth:
https://github.com/canonical/lightdm/blob/ff1c38c0616b3df66a5002ed7f531d53f218dfab/src/plymouth.c#L25-L35

@worldofpeace
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Hmm, @hedning I do think it runs plymouth --quit. I typically see in the logs that it can't find the path to the executable. (even when enabled).

@hedning
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hedning commented Oct 12, 2019

Right, plymouth is in systemPackages. Then I'm really not sure, but it sure looks like it doesn't kill it for some reason 😕

@worldofpeace
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worldofpeace commented Oct 12, 2019 •
edited

This probably never happens because plymouth --ping doesn't work https://github.com/canonical/lightdm/blob/95e48f409295bc3ff9dbf3d1affd0fcd7a6d1698/src/seat-local.c#L119

@worldofpeace
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It's weird, shouldn't the display manager service have /run/current-system/sw/bin in PATH?
And for sure now there's the PAM env. Perhaps hardcoding it is an option. Hopefully harmless when it's not enabled as a service.

@worldofpeace worldofpeace mentioned this pull request Oct 13, 2019
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@worldofpeace
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worldofpeace commented Oct 13, 2019 •
edited

@hedning with #71064 I think it's fine to merge this.

Can you also edit your commit message to clarify this issue was triggered because plymouth handling in lightdm was already broken in nixos?

@hedning
nixos/lightdm: do not conflict with plymouth
1cf6000 
Having `display-manager` conflict with plymouth causes this lock up:

 - `plymouth-quit-wait` starts up, waiting for plymouth-quit to run
 - `lightdm` starts up
 - `plymouth-quit` can't start, it conflicts with lightdm
 - `plymouth-quit-wait` keeps waiting on plymouth-quit to kill plymouthd

The idea is having LightDM control when plymouth quits, but communication with
plymouth was broken: NixOS#71064

Unfortunately having the conflict breaks switching to configurations with
plymouth enabled. So we still need to remove the conflict.

fixes NixOS#71034
@hedning
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hedning commented Oct 13, 2019

Right, did a quick test of #71064 to see if lightdm had the same switch configuration issue as gdm, which was the case, so we really need to remove the conflict.

@hedning hedning merged commit d15e5b0 into NixOS:master Oct 13, 2019
@worldofpeace
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@hedning I actually also tested using tty1 with lightdm and there wasn't issues. So perhaps there's something up in GDM's source or the service config.

@hedning
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hedning commented Oct 13, 2019

@worldofpeace hmm, could have something to do with the lightdm greeter sticking around while gdm's greeter gets reaped while not in use? Just spitballing though, I mean the display-manager service sticks around in both cases so...

@worldofpeace worldofpeace mentioned this pull request Feb 29, 2020
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@hedning hedning deleted the fix-lightdm-plymouth branch March 1, 2020 11:50
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plymouthd consumes 60% CPU on one core
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