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Make home mounts WantedBy a new home-specific target #289

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8 changes: 8 additions & 0 deletions README.org
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Original file line number Diff line number Diff line change
Expand Up @@ -426,6 +426,14 @@
All files you want to link or bind to persistent storage. See
[[#files][the NixOS module files section]] for more details.

**** Overriding when persisted paths are mounted

Paths are only mounted when the systemd target for its storage path is brought up. By default these targets are wanted by ~local-fs.target~, meaning that they will be started on boot. For control over when paths are mounted, the targets should be modified to change or remove their ~WantedBy~ lists. To disable automatically mounting the above example:

#+begin_src nix
systemd.targets."home-files-home-bird".wantedBy = lib.mkForce [ ];
#+end_src

** Further reading
The following blog posts provide more information on the concept of ephemeral
roots:
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89 changes: 53 additions & 36 deletions nixos.nix
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Expand Up @@ -109,6 +109,9 @@ let
user = "root";
group = "root";
};

getCleanHomeName = home: builtins.replaceStrings [ "/" ] [ "-" ] home;
getUnitTarget = home: if home != null then "home-files${getCleanHomeName home}.target" else "local-fs.target";
in
{
options = {
Expand Down Expand Up @@ -234,33 +237,34 @@ in
{
systemd.services =
let
mkPersistFileService = { filePath, persistentStoragePath, ... }@args:
mkPersistFileService = { filePath, persistentStoragePath, home, ... }@args:
let
targetFile = concatPaths [ persistentStoragePath filePath ];
mountPoint = escapeShellArg filePath;
in
{
"persist-${escapeSystemdPath targetFile}" = {
description = "Bind mount or link ${targetFile} to ${mountPoint}";
wantedBy = [ "local-fs.target" ];
before = [ "local-fs.target" ];
path = [ pkgs.util-linux ];
unitConfig.DefaultDependencies = false;
serviceConfig = {
Type = "oneshot";
RemainAfterExit = true;
ExecStart = mkPersistFile args;
ExecStop = pkgs.writeShellScript "unbindOrUnlink-${escapeSystemdPath targetFile}" ''
set -eu
if [[ -L ${mountPoint} ]]; then
rm ${mountPoint}
else
umount ${mountPoint}
rm ${mountPoint}
fi
'';
"persist-${escapeSystemdPath targetFile}" =
{
description = "Bind mount or link ${targetFile} to ${mountPoint}";
wantedBy = [ (getUnitTarget home) ];
before = [ (getUnitTarget home) ];
path = [ pkgs.util-linux ];
unitConfig.DefaultDependencies = false;
serviceConfig = {
Type = "oneshot";
RemainAfterExit = true;
ExecStart = mkPersistFile args;
ExecStop = pkgs.writeShellScript "unbindOrUnlink-${escapeSystemdPath targetFile}" ''
set -eu
if [[ -L ${mountPoint} ]]; then
rm ${mountPoint}
else
umount ${mountPoint}
rm ${mountPoint}
fi
'';
};
};
};
};
in
foldl' recursiveUpdate { } (map mkPersistFileService files);
Expand Down Expand Up @@ -288,24 +292,37 @@ in

systemd.mounts =
let
mkBindMount = { dirPath, persistentStoragePath, hideMount, allowTrash, ... }: {
wantedBy = [ "local-fs.target" ];
before = [ "local-fs.target" ];
where = concatPaths [ "/" dirPath ];
what = concatPaths [ persistentStoragePath dirPath ];
unitConfig.DefaultDependencies = false;
type = "none";
options = concatStringsSep "," ([
"bind"
] ++ optionals hideMount [
"x-gvfs-hide"
] ++ optionals allowTrash [
"x-gvfs-trash"
]);
};
mkBindMount = { dirPath, persistentStoragePath, hideMount, allowTrash, home, ... }:
{
wantedBy = [ (getUnitTarget home) ];
before = [ (getUnitTarget home) ];
where = concatPaths [ "/" dirPath ];
what = concatPaths [ persistentStoragePath dirPath ];
unitConfig.DefaultDependencies = false;
type = "none";
options = concatStringsSep "," ([
"bind"
] ++ optionals hideMount [
"x-gvfs-hide"
] ++ optionals allowTrash [
"x-gvfs-trash"
]);
};
in
map mkBindMount directories;

systemd.targets = builtins.listToAttrs (builtins.map
(entry: {
name = "home-files${getCleanHomeName entry.home}";
value = {
description = "Target for persisted directories and files under ${entry.home}";

# Depends on local-fs.target by default, but can be easily overridden
wantedBy = [ "local-fs.target" ];
};
})
(builtins.filter (entry: entry.home != null) (files ++ directories)));

system.activationScripts =
let
# Script to create directories in persistent and ephemeral
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