files: manage static files in state directory

This improves atomicity since all files managed by Home Manager will
immediately switch over when the link

  $XDG_DATA_HOME/home-manager/files

is swapped over to the new generation. Removing obsolete files and
adding new ones must still done non-atomically but this improves the
situation significantly.

doc/release-notes/rl-2003.adoc

@@ -74,6 +74,26 @@ new module `services.picom` should be used. This is because Nixpkgs no
 longer packages compton, and instead packages the (mostly) compatible
 fork called picom.
 
+* Entries in <<opt-home.file>> are now linked into place via an
+intermediate location, `$XDG_DATA_HOME/home-manager/files`, much like
+`/etc/static` in NixOS. This change was made to makes the switch
+process more atomic.
++
+In general this should not be noticeable but if you manually activate
+an older generation it may (harmlessly) exit with an error along the
+lines of
++
+....
+Existing file '/home/jane/foo' is in the way
+Please move the above files and try again or use -b <ext> to move automatically.
+....
++
+where `/home/jane/foo` is a file managed through the <<opt-home.file>>
+option. This is due to the older activation script not recognizing the
+new XDG data home path as belonging to Home Manager. You can resolve
+this by manually removing the `/home/jane/foo` symbolic link and
+running the activate script again.
+
 [[sec-release-20.03-state-version-changes]]
 === State Version Changes
 

modules/files.nix

@@ -50,12 +50,15 @@ in
             (mapAttrsToList (n: v: v.target)
             (filterAttrs (n: v: v.force) cfg));
 
+        filesDir = "${config.xdg.dataHome}/home-manager/files";
+
         check = pkgs.writeText "check" ''
           . ${./lib-bash/color-echo.sh}
 
           # A symbolic link whose target path matches this pattern will be
           # considered part of a Home Manager generation.
-          homeFilePattern="$(readlink -e "${builtins.storeDir}")/*-home-manager-files/*"
+          homeFilePatternOld="$(readlink -e "${builtins.storeDir}")/*-home-manager-files/*"
+          homeFilePattern="${filesDir}/*"
 
           forcedPaths=(${forcedPaths})
 
@@ -76,6 +79,7 @@ in
             if [[ -n $forced ]]; then
               $VERBOSE_ECHO "Skipping collision check for $targetPath"
             elif [[ -e "$targetPath" \
+                && ! "$(readlink "$targetPath")" == $homeFilePatternOld \
                 && ! "$(readlink "$targetPath")" == $homeFilePattern ]] ; then
               if [[ ! -L "$targetPath" && -n "$HOME_MANAGER_BACKUP_EXT" ]] ; then
                 backup="$targetPath.$HOME_MANAGER_BACKUP_EXT"
@@ -133,6 +137,8 @@ in
     # source and target generation.
     home.activation.linkGeneration = hm.dag.entryAfter ["writeBoundary"] (
       let
+        filesDir = "${config.xdg.dataHome}/home-manager/files";
+
         link = pkgs.writeText "link" ''
           newGenFiles="$1"
           shift
@@ -151,9 +157,10 @@ in
         cleanup = pkgs.writeText "cleanup" ''
           . ${./lib-bash/color-echo.sh}
 
-          # A symbolic link whose target path matches this pattern will be
+          # A symbolic link whose target path matches these patterns will be
           # considered part of a Home Manager generation.
-          homeFilePattern="$(readlink -e "${builtins.storeDir}")/*-home-manager-files/*"
+          homeFilePatternOld="$(readlink -e "${builtins.storeDir}")/*-home-manager-files/*"
+          homeFilePattern="${filesDir}/*"
 
           newGenFiles="$1"
           shift 1
@@ -161,7 +168,8 @@ in
             targetPath="$HOME/$relativePath"
             if [[ -e "$newGenFiles/$relativePath" ]] ; then
               $VERBOSE_ECHO "Checking $targetPath: exists"
-            elif [[ ! "$(readlink "$targetPath")" == $homeFilePattern ]] ; then
+            elif [[ ! "$(readlink "$targetPath")" == $homeFilePatternOld \
+                && ! "$(readlink "$targetPath")" == $homeFilePattern ]] ; then
               warnEcho "Path '$targetPath' not link into Home Manager generation. Skipping delete."
             else
               $VERBOSE_ECHO "Checking $targetPath: gone (deleting)"
@@ -189,9 +197,8 @@ in
           function linkNewGen() {
             echo "Creating home file links in $HOME"
 
-            local newGenFiles
+            local newGenFiles="${filesDir}"
-            newGenFiles="$(readlink -e "$newGenPath/home-files")"
+            find "$newGenFiles/" \( -type f -or -type l \) \
-            find "$newGenFiles" \( -type f -or -type l \) \
               -exec bash ${link} "$newGenFiles" {} +
           }
 
@@ -215,6 +222,13 @@ in
 
           cleanOldGen
 
+          if [[ ! -e "${filesDir}" \
+              || "${config.home-files}" != "$(readlink "${filesDir}")" ]] ; then
+            $DRY_RUN_CMD ln -Tsf $VERBOSE_ARG "${config.home-files}" "${filesDir}"
+          else
+            $VERBOSE_ECHO "No change in static files, skipping linking process"
+          fi
+
           if [[ ! -v oldGenPath || "$oldGenPath" != "$newGenPath" ]] ; then
             echo "Creating profile generation $newGenNum"
             $DRY_RUN_CMD nix-env $VERBOSE_ARG --profile "$genProfilePath" --set "$newGenPath"

modules/home-environment.nix

@@ -431,6 +431,11 @@ in
 
           ${builtins.readFile ./lib-bash/activation-init.sh}
 
+          # Some activation blocks may need to maintain some state, which
+          # should be kept in this directory. We consider creation of this
+          # directory exempt from the write boundary.
+          $DRY_RUN_CMD mkdir $VERBOSE_ARG -p "${config.xdg.dataHome}/home-manager"
+
           ${activationCmds}
         '';
       in