WIP home-manager: avoid profile management during activation

This commit deprecates profile management from the activation script.
The profile management is instead the responsibility of the driving
software, for example, the `home-manager` tool in the case of
standalone installs.

The legacy behavior is still available for backwards compatibility but
may be removed in the future.

The new behavior resolves (or moves us closer to resolving) a number
of long standing open issues:

- `home-manager switch --rollback`, which performs a rollback to the
  previous Home Manager generation before activating. While it was
  previously possible to accomplish this by activating an old
  generation, it did always create a new profile generation.

  This option has been implemented as part of this commit.

- `home-manager switch --test`, which activates the configuration but
  does not create a new profile generation.

  This option has _not_ been implemented here since it relies on the
  current configuration being activated on login, which we do not
  currently do.

- When using the "Home Manager as a NixOS module" installation method
  we previously created an odd `home-manager` per-user "shadow
  profile" for the user. This is no longer necessary.

  This has been implemented as part of this commit.

Fixes #3450

README.md

@@ -53,11 +53,6 @@ Home Manager targets [NixOS][] unstable and NixOS version 24.05 (the current
 stable version), it may or may not work on other Linux distributions and NixOS
 versions.
 
-Also, the `home-manager` tool does not explicitly support rollbacks at the
-moment so if your home directory gets messed up you'll have to fix it yourself.
-See the [rollbacks][] section for instructions on how to manually perform a
-rollback.
-
 Now when your expectations have been built up and you are eager to try all this
 out you can go ahead and read the rest of this text.
 

docs/home-manager.1

@@ -27,6 +27,7 @@
 .Cm | option Ar option.name
 .Cm | packages
 .Cm | remove-generations Ar ID \&...
+.Cm | switch Op Fl -rollback
 .Cm | uninstall
 .Brc
 .Op Fl A Ar attrPath
@@ -155,9 +156,14 @@ sub-command to find suitable generation numbers.
 .RE
 .Pp
 
-.It Cm switch
+.It Cm switch Op Fl -rollback
 .RS 4
 Build and activate the configuration\&.
+.sp
+If the
+.Fl -rollback
+option is given, then the build is not done, instead roll back to and
+activate the configuration prior to the current configuration\&.
 .RE
 .Pp
 

docs/manual/internals.md

@@ -0,0 +1,11 @@
+# Home Manager Internals {#ch-internals}
+
+This chapter collects some documentation about the internal workings
+of Home Manager. The information here is mostly aimed to developers of
+Home Manager and those who do non-trivial integration with Home
+Manager.
+
+
+```{=include=} sections
+internals/activation.md
+```

docs/manual/internals/activation.md

@@ -0,0 +1,41 @@
+# Activation {#sec-internals-activation}
+
+Activating a Home Manager configuration ensures that the built
+configuration is introduced into the user's environment. The
+activation is performed by a suitably named script
+{command}`activate`. This script is generated as part of the
+configuration build and will be placed in the root of the build
+output.
+
+The activation script is implemented in the Bash language and consists
+of initialization code followed by a number of _activation script
+blocks_. These blocks are specified using the
+[home.activation](#opt-home.activation) option. The blocks may have
+dependencies among themselves and the generated activation script will
+contain the blocks serialized such that the dependencies are
+satisfied. A dependency cycle causes a failure when the configuration
+is built.
+
+Historically, the activation script has been responsible for creating
+a new generation of the `home-manager` Nix profile. The more modern
+way, however, is to let the _activation driver_ – that is, the
+software calling the activation script – manage the profile. Indeed,
+in some cases we may not have a `home-manager` profile at all! This is
+the case when Home Manager is used as a NixOS or nix-darwin module, in
+these cases the system profile will contain references to the
+corresponding Home Manager configurations.
+
+Note, to maintain backwards compatibility, the old activation script
+behavior is still the default. To choose the new mode of operation you
+have to call the activation script with the command line option
+`--driver-version 1`. The old behavior is available using
+`--driver-version 0`, or simply omit it entirely.
+
+Unfortunately, driver software need to support both modes of operation
+for the time being since a user may wish to activate an old generation
+that contains an activation script that does not support
+`--driver-version`. To determine whether support is available, check
+the {file}`gen-version` file in the configuration build output root.
+If the file is missing then the activation script does not support
+`--driver-version`. If the file exists and contains the integer 1 or
+higher, then `--driver-version 1` is supported.

docs/manual/manual.md

@@ -13,6 +13,7 @@ usage.md
 nix-flakes.md
 writing-modules.md
 contributing.md
+internals.md
 3rd-party.md
 faq.md
 ```

docs/manual/usage/rollbacks.md

@@ -1,32 +1,45 @@
 # Rollbacks {#sec-usage-rollbacks}
 
-While the `home-manager` tool does not explicitly support rollbacks at
+When you perform a `home-manager switch` and discover a problem then
-the moment it is relatively easy to perform one manually. The steps to
+it is possible to _roll back_ to the previous version of your
-do so are
+configuration using `home-manager switch --rollback`. This will turn
+the previous configuration into the current configuration.
 
-1.  Run `home-manager generations` to determine which generation you
+::: {.example #ex-rollback-scenario}
-    wish to rollback to:
+### Home Manager Rollback
 
-    ``` shell
+Imagine you have just updated Nixpkgs and switched to a new Home
-    $ home-manager generations
+Manager configuration. You discover that a package update included in
-    2018-01-04 11:56 : id 765 -> /nix/store/kahm1rxk77mnvd2l8pfvd4jkkffk5ijk-home-manager-generation
+your new configuration has a bug that was not present in the previous
-    2018-01-03 10:29 : id 764 -> /nix/store/2wsmsliqr5yynqkdyjzb1y57pr5q2lsj-home-manager-generation
+configuration.
-    2018-01-01 12:21 : id 763 -> /nix/store/mv960kl9chn2lal5q8lnqdp1ygxngcd1-home-manager-generation
-    2017-12-29 21:03 : id 762 -> /nix/store/6c0k1r03fxckql4vgqcn9ccb616ynb94-home-manager-generation
-    2017-12-25 18:51 : id 761 -> /nix/store/czc5y6vi1rvnkfv83cs3rn84jarcgsgh-home-manager-generation
-    …
-    ```
 
-2.  Copy the Nix store path of the generation you chose, e.g.,
+You can then run `home-manager switch --rollback` to recover your
+previous configuration, which includes the working version of the
+package.
 
-        /nix/store/mv960kl9chn2lal5q8lnqdp1ygxngcd1-home-manager-generation
+To see what happened above we can observe the list of Home Manager
+generations before and after the rollback:
 
-    for generation 763.
+``` shell
+$ home-manager generations
+2024-01-04 11:56 : id 765 -> /nix/store/kahm1rxk77mnvd2l8pfvd4jkkffk5ijk-home-manager-generation (current)
+2024-01-03 10:29 : id 764 -> /nix/store/2wsmsliqr5yynqkdyjzb1y57pr5q2lsj-home-manager-generation
+2024-01-01 12:21 : id 763 -> /nix/store/mv960kl9chn2lal5q8lnqdp1ygxngcd1-home-manager-generation
+2023-12-29 21:03 : id 762 -> /nix/store/6c0k1r03fxckql4vgqcn9ccb616ynb94-home-manager-generation
+2023-12-25 18:51 : id 761 -> /nix/store/czc5y6vi1rvnkfv83cs3rn84jarcgsgh-home-manager-generation
+…
 
-3.  Run the `activate` script inside the copied store path:
+$ home-manager switch --rollback
+Starting home manager activation
+…
 
-    ``` shell
+$ home-manager generations
-    $ /nix/store/mv960kl9chn2lal5q8lnqdp1ygxngcd1-home-manager-generation/activate
+2024-01-04 11:56 : id 765 -> /nix/store/kahm1rxk77mnvd2l8pfvd4jkkffk5ijk-home-manager-generation
-    Starting home manager activation
+2024-01-03 10:29 : id 764 -> /nix/store/2wsmsliqr5yynqkdyjzb1y57pr5q2lsj-home-manager-generation (current)
-    …
+2024-01-01 12:21 : id 763 -> /nix/store/mv960kl9chn2lal5q8lnqdp1ygxngcd1-home-manager-generation
-    ```
+2023-12-29 21:03 : id 762 -> /nix/store/6c0k1r03fxckql4vgqcn9ccb616ynb94-home-manager-generation
+2023-12-25 18:51 : id 761 -> /nix/store/czc5y6vi1rvnkfv83cs3rn84jarcgsgh-home-manager-generation
+…
+```
+
+:::

docs/release-notes/rl-2411.md

@@ -7,7 +7,29 @@ is therefore not final.
 
 This release has the following notable changes:
 
-- No changes.
+- The `home-manager` Nix profile update that the Home Manager
+  activation script has previously performed is now deprecated. The
+  profile update is instead the responsibility of the software calling
+  the activation script, such as the `home-manager` tool..
+
+  The legacy behavior is the default for backwards compatibility but
+  may be emit a deprecation warning in the future, for eventual
+  removal. If you have developed tooling that directly call the
+  generated activation script, then you are encouraged to adapt to the
+  new behavior. See [Activation](#sec-internals-activation) for
+  details on how to call the activation script.
+
+- The `home-manager switch` command now offers a `--rollback` option.
+  When given, the switch performs a rollback to the Home Manager
+  generation prior to the current before activating. While it was
+  previously possible to accomplish this by manually activating an old
+  generation, it always created a new profile generation. The new
+  behavior mirrors the behavior of `nixos-rebuild switch --rollback`.
+  See the [Rollbacks](#sec-usage-rollbacks) section for more.
+
+- When using Home Manager as a NixOS or nix-darwin module we
+  previously created an unnecessary `home-manager` per-user "shadow
+  profile" for the user. This no longer happens.
 
 ## State Version Changes {#sec-release-24.11-state-version-changes}
 

home-manager/home-manager

@@ -476,7 +476,7 @@ EOF
         _i "Creating initial Home Manager generation..."
         echo
 
-        if doSwitch; then
+        if doSwitch --switch; then
             # translators: The "%s" specifier will be replaced by a file path.
             _i $'All done! The home-manager tool should now be installed and you can edit\n\n    %s\n\nto configure Home Manager. Run \'man home-configuration.nix\' to\nsee all available options.' \
                 "$confFile"
@@ -635,10 +635,42 @@ function doBuild() {
 }
 
 function doSwitch() {
+    setHomeManagerPathVariables
+    setVerboseArg
     setWorkDir
 
+    local action
+
+    while (( $# > 0 )); do
+        local opt="$1"
+        shift
+
+        case $opt in
+            --switch)
+                action='switch'
+                ;;
+            --test)
+                action='test'
+                ;;
+            --rollback)
+                action='rollback'
+                ;;
+            *)
+                errorEcho "home-manager switch: unknown option '%s'" "$opt" >&2
+                return 1
+                ;;
+        esac
+    done
+
+    if [[ ! -v action ]]; then
+        errorEcho "home-manager switch: missing required option" >&2
+        return 1
+    fi
+
     local generation
 
+    case $action in
+        switch|test)
             # Build the generation and run the activate script. Note, we
             # specify an output link so that it is treated as a GC root. This
             # prevents an unfortunately timed GC from removing the generation
@@ -650,16 +682,42 @@ function doSwitch() {
                 doBuildFlake \
                     "$FLAKE_CONFIG_URI.activationPackage" \
                     --out-link "$generation" \
-            ${PRINT_BUILD_LOGS+--print-build-logs} \
+                    ${PRINT_BUILD_LOGS+--print-build-logs}
-            && "$generation/activate" || return
             else
                 doBuildAttr \
                     --out-link "$generation" \
-            --attr activationPackage \
+                    --attr activationPackage
-            && "$generation/activate" || return
+            fi
+            ;;
+        rollback)
+            generation="$HM_PROFILE_DIR/home-manager"
+            ;;
+    esac
+
+    # If we are doing a switch but built a legacy configuration, where the
+    # activation script manages the profile, then we instead perform a test
+    # action.
+    #
+    # The migration away from legacy activation scripts happened when
+    # introducing the gen-version file, hence the existence check.
+    if [[ $action == 'switch' && ! -e "$generation/gen-version" ]]; then
+        action='test'
     fi
 
+    case $action in
+        switch)
+            run nix-env $VERBOSE_ARG --profile "$HM_PROFILE_DIR/home-manager" --set "$generation"
+            ;;
+        rollback)
+            run nix-env $VERBOSE_ARG --profile "$HM_PROFILE_DIR/home-manager" --rollback
+            ;;
+    esac
+
+    "$generation"/activate --driver-version 1 || return
+
+    if [[ $action == 'switch' || $action == 'test' ]]; then
         presentNews
+    fi
 }
 
 function doListGens() {
@@ -672,10 +730,14 @@ function doListGens() {
     fi
 
     pushd "$HM_PROFILE_DIR" > /dev/null
+    local curProfile
+    curProfile=$(readlink home-manager)
+
     # shellcheck disable=2012
     ls --color=$color -gG --time-style=long-iso --sort time home-manager-*-link \
         | cut -d' ' -f 4- \
-        | sed -E 's/home-manager-([[:digit:]]*)-link/: id \1/'
+        | sed -E -e "/$curProfile/ { s/\$/ \(current\)/ }" \
+                 -e 's/home-manager-([[:digit:]]*)-link/: id \1/'
     popd > /dev/null
 }
 
@@ -929,7 +991,11 @@ function doHelp() {
     echo
     echo "  instantiate  Instantiate the configuration and print the resulting derivation"
     echo
-    echo "  switch       Build and activate configuration"
+    echo "  switch [OPTION]"
+    echo "      Build and activate configuration"
+    echo
+    echo "      --rollback    Do not build a new configuration, instead roll back to"
+    echo "                    the configuration prior to the current configuration."
     echo
     echo "  generations  List all home environment generations"
     echo
@@ -960,7 +1026,7 @@ while [[ $# -gt 0 ]]; do
     opt="$1"
     shift
     case $opt in
-        build|init|instantiate|option|edit|expire-generations|generations|help|news|packages|remove-generations|switch|uninstall)
+        build|init|instantiate|option|edit|expire-generations|generations|help|news|packages|remove-generations|rollback|switch|test|uninstall)
             COMMAND="$opt"
             ;;
         -A)
@@ -1025,6 +1091,17 @@ while [[ $# -gt 0 ]]; do
         -n|--dry-run)
             export DRY_RUN=1
             ;;
+        --rollback)
+            case $COMMAND in
+                switch)
+                    COMMAND_ARGS+=("$opt")
+                    ;;
+                *)
+                    _iError 'home-manager: "--rollback" can only be used after "switch"' >&2
+                    exit 1
+                    ;;
+            esac
+            ;;
         --option|--arg|--argstr)
             [[ -v 1 && $1 != -* ]] || errMissingOptArg "$opt"
             [[ -v 2 ]] || errMissingOptArg "$opt $1"
@@ -1083,14 +1160,22 @@ case $COMMAND in
         doInstantiate
         ;;
     switch)
-        doSwitch
+        doSwitch --switch "${COMMAND_ARGS[@]}"
         ;;
+    # TODO: The test functionality is not really sensible until we perform
+    # activation through some form of systemd unit.
+    # test)
+    #     doSwitch --test
+    #     ;;
     generations)
         doListGens
         ;;
     remove-generations)
         doRmGenerations "${COMMAND_ARGS[@]}"
         ;;
+    rollback)
+        doRollback
+        ;;
     expire-generations)
         if [[ ${#COMMAND_ARGS[@]} != 1 ]]; then
             _i 'expire-generations expects one argument, got %d.' "${#COMMAND_ARGS[@]}" >&2

modules/files.nix

@@ -105,10 +105,7 @@ in
     # 1. Remove files from the old generation that are not in the new
     #    generation.
     #
-    # 2. Switch over the Home Manager gcroot and current profile
+    # 2. Symlink files from the new generation into $HOME.
-    #    links.
-    #
-    # 3. Symlink files from the new generation into $HOME.
     #
     # This order is needed to ensure that we always know which links
     # belong to which generation. Specifically, if we're moving from
@@ -215,28 +212,6 @@ in
           }
 
           cleanOldGen
-
-          if [[ ! -v oldGenPath || "$oldGenPath" != "$newGenPath" ]] ; then
-            _i "Creating profile generation %s" $newGenNum
-            if [[ -e "$genProfilePath"/manifest.json ]] ; then
-              # Remove all packages from "$genProfilePath"
-              # `nix profile remove '.*' --profile "$genProfilePath"` was not working, so here is a workaround:
-              nix profile list --profile "$genProfilePath" \
-                | cut -d ' ' -f 4 \
-                | xargs -rt $DRY_RUN_CMD nix profile remove $VERBOSE_ARG --profile "$genProfilePath"
-              run nix profile install $VERBOSE_ARG --profile "$genProfilePath" "$newGenPath"
-            else
-              run nix-env $VERBOSE_ARG --profile "$genProfilePath" --set "$newGenPath"
-            fi
-
-            run --quiet nix-store --realise "$newGenPath" --add-root "$newGenGcPath" --indirect
-            if [[ -e "$legacyGenGcPath" ]]; then
-              run rm $VERBOSE_ARG "$legacyGenGcPath"
-            fi
-          else
-            _i "No change so reusing latest profile generation %s" "$oldGenNum"
-          fi
-
           linkNewGen
         ''
     );

modules/home-environment.nix

@@ -431,6 +431,18 @@ in
       description = "The package containing the complete activation script.";
     };
 
+    home.activationGenerateGcRoot = mkOption {
+      internal = true;
+      type = types.bool;
+      default = true;
+      description = ''
+        Whether the activation script should create a GC root to avoid being
+        garbage collected. Typically you want this but if you know for certain
+        that the Home Manager generation is referenced from some other GC root,
+        then it may be appropriate to not create our own root.
+      '';
+    };
+
     home.extraActivationPath = mkOption {
       internal = true;
       type = types.listOf types.package;
@@ -582,9 +594,21 @@ in
 
     home.packages = [ config.home.sessionVariablesPackage ];
 
-    # A dummy entry acting as a boundary between the activation
+    # The entry acting as a boundary between the activation script's "check" and
-    # script's "check" and the "write" phases.
+    # the "write" phases. This is where we commit to attempting to actually
-    home.activation.writeBoundary = hm.dag.entryAnywhere "";
+    # activate the configuration.
+    #
+    # Note, if we are run by a version 0 driver then we update the profile here.
+    home.activation.writeBoundary = hm.dag.entryAnywhere ''
+      if (( $hmDriverVersion < 1 )); then
+        if [[ ! -v oldGenPath || "$oldGenPath" != "$newGenPath" ]] ; then
+          _i "Creating new profile generation"
+          run nix-env $VERBOSE_ARG --profile "$genProfilePath" --set "$newGenPath"
+        else
+          _i "No change so reusing latest profile generation"
+        fi
+      fi
+    '';
 
     # Install packages to the user environment.
     #
@@ -710,6 +734,38 @@ in
           export PATH="${activationBinPaths}"
           ${config.lib.bash.initHomeManagerLib}
 
+          # The driver version indicates the behavior expected by the caller of
+          # this script.
+          #
+          # - 0 : legacy behavior
+          # - 1 : the script will not attempt to update the Home Manager Nix profile.
+          hmDriverVersion=0
+
+          while (( $# > 0 )); do
+            opt="$1"
+            shift
+
+            case $opt in
+              --driver-version)
+                if (( $# == 0 )); then
+                  errorEcho "$0: no driver version specified" >&2
+                  exit 1
+                elif (( 0 <= $1 && $1 <= 1 )); then
+                  hmDriverVersion=$1
+                else
+                  errorEcho "$0: unexpected driver version $1" >&2
+                  exit 1
+                fi
+                shift
+                ;;
+              *)
+                _iError "%s: unknown option '%s'" "$0" "$opt" >&2
+                exit 1
+                ;;
+            esac
+          done
+          unset opt
+
           ${builtins.readFile ./lib-bash/activation-init.sh}
 
           if [[ ! -v SKIP_SANITY_CHECKS ]]; then
@@ -717,7 +773,22 @@ in
             checkHomeDirectory ${escapeShellArg config.home.homeDirectory}
           fi
 
+          ${optionalString config.home.activationGenerateGcRoot ''
+            # Create a temporary GC root to prevent collection during activation.
+            trap 'run rm -f $VERBOSE_ARG "$newGenGcPath"' EXIT
+            run --silence nix-store --realise "$newGenPath" --add-root "$newGenGcPath"
+          ''}
+
           ${activationCmds}
+
+          ${optionalString (config.home.activationGenerateGcRoot && !config.uninstall) ''
+            # Create the "current generation" GC root.
+            run --silence nix-store --realise "$newGenPath" --add-root "$currentGenGcPath"
+
+            if [[ -e "$legacyGenGcPath" ]]; then
+              run rm $VERBOSE_ARG "$legacyGenGcPath"
+            fi
+          ''}
         '';
       in
         pkgs.runCommand
@@ -730,6 +801,11 @@ in
 
             echo "${config.home.version.full}" > $out/hm-version
 
+            # The gen-version indicates the format of the generation package
+            # itself. It allows us to make backwards incompatible changes in the
+            # package output and have surrounding tooling adapt.
+            echo 1 > $out/gen-version
+
             cp ${activationScript} $out/activate
 
             mkdir $out/bin

modules/lib-bash/activation-init.sh

@@ -59,34 +59,13 @@ function setupVars() {
     declare -gr hmDataPath="${XDG_DATA_HOME:-$HOME/.local/share}/home-manager"
     declare -gr genProfilePath="$profilesDir/home-manager"
     declare -gr newGenPath="@GENERATION_DIR@";
-    declare -gr newGenGcPath="$hmGcrootsDir/current-home"
+    declare -gr newGenGcPath="$hmGcrootsDir/new-home"
+    declare -gr currentGenGcPath="$hmGcrootsDir/current-home"
     declare -gr legacyGenGcPath="$globalGcrootsDir/current-home"
 
-    declare greatestGenNum
+    if [[ -e $currentGenGcPath ]] ; then
-    greatestGenNum=$( \
-        nix-env --list-generations --profile "$genProfilePath" \
-            | tail -1 \
-            | sed -E 's/ *([[:digit:]]+) .*/\1/')
-
-    if [[ -n $greatestGenNum ]] ; then
-        declare -gr oldGenNum=$greatestGenNum
-        declare -gr newGenNum=$((oldGenNum + 1))
-    else
-        declare -gr newGenNum=1
-    fi
-
-    if [[ -e $genProfilePath ]] ; then
         declare -g oldGenPath
-        oldGenPath="$(readlink -e "$genProfilePath")"
+        oldGenPath="$(readlink -e "$currentGenGcPath")"
-    fi
-
-    _iVerbose "Sanity checking oldGenNum and oldGenPath"
-    if [[ -v oldGenNum && ! -v oldGenPath
-            || ! -v oldGenNum && -v oldGenPath ]]; then
-        _i $'The previous generation number and path are in conflict! These\nmust be either both empty or both set but are now set to\n\n    \'%s\' and \'%s\'\n\nIf you don\'t mind losing previous profile generations then\nthe easiest solution is probably to run\n\n   rm %s/home-manager*\n   rm %s/current-home\n\nand trying home-manager switch again. Good luck!' \
-           "${oldGenNum:-}" "${oldGenPath:-}" \
-           "$profilesDir" "$hmGcrootsDir"
-        exit 1
     fi
 }
 
@@ -181,15 +160,13 @@ if [[ -v VERBOSE ]]; then
 fi
 
 _iVerbose "Activation variables:"
-if [[ -v oldGenNum ]] ; then
+if [[ -v oldGenPath ]] ; then
-    verboseEcho "  oldGenNum=$oldGenNum"
     verboseEcho "  oldGenPath=$oldGenPath"
 else
-    verboseEcho "  oldGenNum undefined (first run?)"
     verboseEcho "  oldGenPath undefined (first run?)"
 fi
 verboseEcho "  newGenPath=$newGenPath"
-verboseEcho "  newGenNum=$newGenNum"
 verboseEcho "  genProfilePath=$genProfilePath"
 verboseEcho "  newGenGcPath=$newGenGcPath"
+verboseEcho "  currentGenGcPath=$currentGenGcPath"
 verboseEcho "  legacyGenGcPath=$legacyGenGcPath"

nix-darwin/default.nix

@@ -22,7 +22,7 @@ in {
                 lib.escapeShellArg cfg.backupFileExtension
               }"}
               ${lib.optionalString cfg.verbose "export VERBOSE=1"}
-              exec ${usercfg.home.activationPackage}/activate
+              exec ${usercfg.home.activationPackage}/activate --driver-version 1
             ''
           }
         '') cfg.users);

nixos/default.nix

@@ -13,6 +13,24 @@ let
 in {
   imports = [ ./common.nix ];
 
+  options.home-manager = {
+    enableLegacyProfileManagement = mkOption {
+      type = types.bool;
+      default = versionOlder config.system.stateVersion "24.05";
+      defaultText = lib.literalMD ''
+        - `true` for `system.stateVersion` < 24.05,
+        - `false` otherwise'';
+      description = ''
+        Whether to enable legacy profile (and garbage collection root)
+        management during activation. When enabled, the Home Manager activation
+        will produce a per-user `home-manager` Nix profile as well as a garbage
+        collection root, just like in the standalone installation of Home
+        Manager. Typically, this is not desired when Home Manager is embedded in
+        the system configuration.
+      '';
+    };
+  };
+
   config = mkMerge [
     {
       home-manager = {
@@ -26,12 +44,21 @@ in {
 
           # Inherit glibcLocales setting from NixOS.
           i18n.glibcLocales = lib.mkDefault config.i18n.glibcLocales;
+
+          # Legacy profile management is when the activation script generates GC
+          # root and home-manager profile. The modern way simply relies on the
+          # GC root that the system maintains, which should also protect the
+          # Home Manager activation package outputs.
+          home.activationGenerateGcRoot = cfg.enableLegacyProfileManagement;
         }];
       };
     }
     (mkIf (cfg.users != { }) {
       systemd.services = mapAttrs' (_: usercfg:
-        let username = usercfg.home.username;
+        let
+          username = usercfg.home.username;
+          driverVersion =
+            if cfg.enableLegacyProfileManagement then "0" else "1";
         in nameValuePair ("home-manager-${utils.escapeSystemdPath username}") {
           description = "Home Manager environment for ${username}";
           wantedBy = [ "multi-user.target" ];
@@ -78,7 +105,7 @@ in {
                   | ${sed} -En '/^(${exportedSystemdVariables})=/s/^/export /p'
                 )"
 
-                exec "$1/activate"
+                exec "$1/activate" --driver-version ${driverVersion}
               '';
             in "${setupEnv} ${usercfg.home.activationPackage}";
           };

tests/integration/default.nix

@@ -11,6 +11,8 @@ let
 
   tests = {
     nixos-basics = runTest ./nixos/basics.nix;
+    nixos-legacy-profile-management =
+      runTest ./nixos/legacy-profile-management.nix;
     standalone-flake-basics = runTest ./standalone/flake-basics.nix;
     standalone-standard-basics = runTest ./standalone/standard-basics.nix;
   };

tests/integration/nixos/basics.nix

@@ -7,6 +7,8 @@
   nodes.machine = { ... }: {
     imports = [ ../../../nixos ]; # Import the HM NixOS module.
 
+    system.stateVersion = "24.05";
+
     users.users.alice = {
       isNormalUser = true;
       description = "Alice Foobar";
@@ -14,9 +16,13 @@
       uid = 1000;
     };
 
-    home-manager.users.alice = { ... }: {
+    home-manager = {
+      enableLegacyProfileManagement = false;
+
+      users.alice = { ... }: {
         home.stateVersion = "24.05";
         home.file.test.text = "testfile";
+
         # Enable a light-weight systemd service.
         services.pueue.enable = true;
         # We focus on sd-switch since that hopefully will become the default in
@@ -24,6 +30,7 @@
         systemd.user.startServices = "sd-switch";
       };
     };
+  };
 
   testScript = ''
     def login_as_alice():
@@ -79,17 +86,13 @@
 
       logout_alice()
 
-    with subtest("GC root and profile"):
+    with subtest("no GC root and profile"):
-      # There should be a GC root and Home Manager profile and they should point
+      # There should be no GC root and Home Manager profile since we are not
-      # to the same path in the Nix store.
+      # using legacy profile management.
-      gcroot = "/home/alice/.local/state/home-manager/gcroots/current-home"
+      hmState = "/home/alice/.local/state/home-manager"
-      gcrootTarget = machine.succeed(f"readlink {gcroot}")
+      machine.succeed(f"test ! -e {hmState}")
 
-      profile = "/home/alice/.local/state/nix/profiles"
+      hmProfile = "/home/alice/.local/state/nix/profiles/home-manager"
-      profileTarget = machine.succeed(f"readlink {profile}/home-manager")
+      machine.succeed(f"test ! -e {hmProfile}")
-      profile1Target = machine.succeed(f"readlink {profile}/{profileTarget}")
-
-      assert gcrootTarget == profile1Target, \
-        f"expected GC root and profile to point to same, but pointed to {gcrootTarget} and {profile1Target}"
   '';
 }

tests/integration/nixos/legacy-profile-management.nix

@@ -0,0 +1,46 @@
+{ pkgs, ... }:
+
+{
+  name = "nixos-legacy-profile-management";
+  meta.maintainers = [ pkgs.lib.maintainers.rycee ];
+
+  nodes.machine = { ... }: {
+    imports = [ ../../../nixos ]; # Import the HM NixOS module.
+
+    system.stateVersion = "23.11";
+
+    users.users.alice = { isNormalUser = true; };
+
+    home-manager.users.alice = { ... }: {
+      home.stateVersion = "23.11";
+      home.file.test.text = "testfile";
+    };
+  };
+
+  testScript = ''
+    start_all()
+
+    machine.wait_for_unit("home-manager-alice.service")
+
+    with subtest("Home Manager file"):
+      # The file should be linked with the expected content.
+      path = "/home/alice/test"
+      machine.succeed(f"test -L {path}")
+      actual = machine.succeed(f"cat {path}")
+      expected = "testfile"
+      assert actual == expected, f"expected {path} to contain {expected}, but got {actual}"
+
+    with subtest("GC root and profile"):
+      # There should be a GC root and Home Manager profile and they should point
+      # to the same path in the Nix store.
+      gcroot = "/home/alice/.local/state/home-manager/gcroots/current-home"
+      gcrootTarget = machine.succeed(f"readlink {gcroot}")
+
+      profile = "/home/alice/.local/state/nix/profiles"
+      profileTarget = machine.succeed(f"readlink {profile}/home-manager")
+      profile1Target = machine.succeed(f"readlink {profile}/{profileTarget}")
+
+      assert gcrootTarget == profile1Target, \
+        f"expected GC root and profile to point to same, but pointed to {gcrootTarget} and {profile1Target}"
+  '';
+}