Experiments

This is a completely experimental proof-of-concept type setup of - using Putter for file management, - removing profile management from activation script, and - add `home-manager test` command, which activates the configuration but does not create a new profile generation.
2024-01-24 22:24:32 +01:00
7 changed files with 440 additions and 285 deletions
--- a/flake.lock
+++ b/flake.lock
@ -1,5 +1,105 @@
 {
  "nodes": {
+    "crane": {
+      "inputs": {
+        "flake-compat": "flake-compat",
+        "flake-utils": [
+          "putter",
+          "flake-utils"
+        ],
+        "nixpkgs": [
+          "putter",
+          "nixpkgs"
+        ],
+        "rust-overlay": "rust-overlay"
+      },
+      "locked": {
+        "lastModified": 1697588719,
+        "narHash": "sha256-n9ALgm3S+ygpzjesBkB9qutEtM4dtIkhn8WnstCPOew=",
+        "owner": "ipetkov",
+        "repo": "crane",
+        "rev": "da6b58e270d339a78a6e95728012ec2eea879612",
+        "type": "github"
+      },
+      "original": {
+        "owner": "ipetkov",
+        "ref": "v0.14.3",
+        "repo": "crane",
+        "type": "github"
+      }
+    },
+    "flake-compat": {
+      "flake": false,
+      "locked": {
+        "lastModified": 1696267196,
+        "narHash": "sha256-AAQ/2sD+0D18bb8hKuEEVpHUYD1GmO2Uh/taFamn6XQ=",
+        "owner": "edolstra",
+        "repo": "flake-compat",
+        "rev": "4f910c9827911b1ec2bf26b5a062cd09f8d89f85",
+        "type": "github"
+      },
+      "original": {
+        "owner": "edolstra",
+        "repo": "flake-compat",
+        "type": "github"
+      }
+    },
+    "flake-compat_2": {
+      "flake": false,
+      "locked": {
+        "lastModified": 1673956053,
+        "narHash": "sha256-4gtG9iQuiKITOjNQQeQIpoIB6b16fm+504Ch3sNKLd8=",
+        "owner": "edolstra",
+        "repo": "flake-compat",
+        "rev": "35bb57c0c8d8b62bbfd284272c928ceb64ddbde9",
+        "type": "github"
+      },
+      "original": {
+        "owner": "edolstra",
+        "repo": "flake-compat",
+        "type": "github"
+      }
+    },
+    "flake-utils": {
+      "inputs": {
+        "systems": "systems"
+      },
+      "locked": {
+        "lastModified": 1694529238,
+        "narHash": "sha256-zsNZZGTGnMOf9YpHKJqMSsa0dXbfmxeoJ7xHlrt+xmY=",
+        "owner": "numtide",
+        "repo": "flake-utils",
+        "rev": "ff7b65b44d01cf9ba6a71320833626af21126384",
+        "type": "github"
+      },
+      "original": {
+        "owner": "numtide",
+        "repo": "flake-utils",
+        "type": "github"
+      }
+    },
+    "gitignore": {
+      "inputs": {
+        "nixpkgs": [
+          "putter",
+          "pre-commit-hooks",
+          "nixpkgs"
+        ]
+      },
+      "locked": {
+        "lastModified": 1660459072,
+        "narHash": "sha256-8DFJjXG8zqoONA1vXtgeKXy68KdJL5UaXR8NtVMUbx8=",
+        "owner": "hercules-ci",
+        "repo": "gitignore.nix",
+        "rev": "a20de23b925fd8264fd7fad6454652e142fd7f73",
+        "type": "github"
+      },
+      "original": {
+        "owner": "hercules-ci",
+        "repo": "gitignore.nix",
+        "type": "github"
+      }
+    },
    "nixpkgs": {
      "locked": {
        "lastModified": 1705677747,
@ -16,9 +116,133 @@
        "type": "github"
      }
    },
+    "nixpkgs-stable": {
+      "locked": {
+        "lastModified": 1685801374,
+        "narHash": "sha256-otaSUoFEMM+LjBI1XL/xGB5ao6IwnZOXc47qhIgJe8U=",
+        "owner": "NixOS",
+        "repo": "nixpkgs",
+        "rev": "c37ca420157f4abc31e26f436c1145f8951ff373",
+        "type": "github"
+      },
+      "original": {
+        "owner": "NixOS",
+        "ref": "nixos-23.05",
+        "repo": "nixpkgs",
+        "type": "github"
+      }
+    },
+    "nixpkgs_2": {
+      "locked": {
+        "lastModified": 1699094435,
+        "narHash": "sha256-YLZ5/KKZ1PyLrm2MO8UxRe4H3M0/oaYqNhSlq6FDeeA=",
+        "owner": "NixOS",
+        "repo": "nixpkgs",
+        "rev": "9d5d25bbfe8c0297ebe85324addcb5020ed1a454",
+        "type": "github"
+      },
+      "original": {
+        "owner": "NixOS",
+        "ref": "nixpkgs-unstable",
+        "repo": "nixpkgs",
+        "type": "github"
+      }
+    },
+    "pre-commit-hooks": {
+      "inputs": {
+        "flake-compat": "flake-compat_2",
+        "flake-utils": [
+          "putter",
+          "flake-utils"
+        ],
+        "gitignore": "gitignore",
+        "nixpkgs": [
+          "putter",
+          "nixpkgs"
+        ],
+        "nixpkgs-stable": "nixpkgs-stable"
+      },
+      "locked": {
+        "lastModified": 1698852633,
+        "narHash": "sha256-Hsc/cCHud8ZXLvmm8pxrXpuaPEeNaaUttaCvtdX/Wug=",
+        "owner": "cachix",
+        "repo": "pre-commit-hooks.nix",
+        "rev": "dec10399e5b56aa95fcd530e0338be72ad6462a0",
+        "type": "github"
+      },
+      "original": {
+        "owner": "cachix",
+        "repo": "pre-commit-hooks.nix",
+        "type": "github"
+      }
+    },
+    "putter": {
+      "inputs": {
+        "crane": "crane",
+        "flake-utils": "flake-utils",
+        "nixpkgs": "nixpkgs_2",
+        "pre-commit-hooks": "pre-commit-hooks"
+      },
+      "locked": {
+        "lastModified": 1704013409,
+        "narHash": "sha256-v7CTHSKcD6vnIwXRPav+3XETf+uNJz3G+RUF/SHZ+vE=",
+        "ref": "refs/heads/master",
+        "rev": "4d773d3aa9feca3af4578dc62cc6f91ebb16b002",
+        "revCount": 33,
+        "type": "git",
+        "url": "file:///home/rycee/devel/putter"
+      },
+      "original": {
+        "type": "git",
+        "url": "file:///home/rycee/devel/putter"
+      }
+    },
    "root": {
      "inputs": {
-        "nixpkgs": "nixpkgs"
+        "nixpkgs": "nixpkgs",
+        "putter": "putter"
+      }
+    },
+    "rust-overlay": {
+      "inputs": {
+        "flake-utils": [
+          "putter",
+          "crane",
+          "flake-utils"
+        ],
+        "nixpkgs": [
+          "putter",
+          "crane",
+          "nixpkgs"
+        ]
+      },
+      "locked": {
+        "lastModified": 1696299134,
+        "narHash": "sha256-RS77cAa0N+Sfj5EmKbm5IdncNXaBCE1BSSQvUE8exvo=",
+        "owner": "oxalica",
+        "repo": "rust-overlay",
+        "rev": "611ccdceed92b4d94ae75328148d84ee4a5b462d",
+        "type": "github"
+      },
+      "original": {
+        "owner": "oxalica",
+        "repo": "rust-overlay",
+        "type": "github"
+      }
+    },
+    "systems": {
+      "locked": {
+        "lastModified": 1681028828,
+        "narHash": "sha256-Vy1rq5AaRuLzOxct8nz4T6wlgyUR7zLU309k9mBC768=",
+        "owner": "nix-systems",
+        "repo": "default",
+        "rev": "da67096a3b9bf56a91d16901293e51ba5b49a27e",
+        "type": "github"
+      },
+      "original": {
+        "owner": "nix-systems",
+        "repo": "default",
+        "type": "github"
      }
    }
  },
--- a/flake.nix
+++ b/flake.nix
@ -2,8 +2,9 @@
  description = "Home Manager for Nix";

  inputs.nixpkgs.url = "github:NixOS/nixpkgs/nixos-unstable";
+  inputs.putter.url = "git+file:///home/rycee/devel/putter";

-  outputs = { self, nixpkgs, ... }:
+  outputs = { self, nixpkgs, putter, ... }:
    {
      nixosModules = rec {
        home-manager = import ./nixos;
@ -76,7 +77,10 @@
              in lib.throwIf (used != [ ]) msg v;

          in throwForRemovedArgs (import ./modules {
-            inherit pkgs lib check extraSpecialArgs;
+            inherit pkgs lib check;
+            extraSpecialArgs = extraSpecialArgs // {
+              putter = putter.packages.${pkgs.system}.default;
+            };
            configuration = { ... }: {
              imports = modules
                ++ [{ programs.home-manager.path = toString ./.; }];
--- a/home-manager/home-manager
+++ b/home-manager/home-manager
@ -421,7 +421,7 @@ EOF
    # Specify the source of Home Manager and Nixpkgs.
    nixpkgs.url = "github:nixos/nixpkgs/nixos-unstable";
    home-manager = {
-      url = "github:nix-community/home-manager";
+      url = "git+file:///home/rycee/devel/home-manager";
      inputs.nixpkgs.follows = "nixpkgs";
    };
  };
@ -452,7 +452,7 @@ 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"
@ -607,48 +607,92 @@ function doBuild() {
 }

 function doSwitch() {
+    # How we should handle the Home Manager profile before running the
+    # activation. Can be
+    #
+    # - "noop" : build a new configuration but do not update the profile.
+    #
+    # - "set" : build a new configuration and set it as the current generation.
+    #
+    # - "rollback" : rollback to the previous generation.
+    #
+    # Here "leave" is the default for the test action and "set" is the default
+    # for switch.
+    local profileAction
+
+    while (( $# > 0 )); do
+        local opt="$1"
+        shift
+
+        case $opt in
+            --switch)
+                profileAction='set'
+                ;;
+            --test)
+                profileAction='noop'
+                ;;
+            --rollback)
+                profileAction='rollback'
+                ;;
+            *)
+                _i 'Unknown argument %s' "$opt"
+                return 1
+                ;;
+        esac
+    done
+
+    setHomeManagerPathVariables
    setWorkDir

    local generation

-    # 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
-    # before activation completes.
-    generation="$WORK_DIR/generation"
+    case $profileAction in
+        set|noop)
+            # Build the generation and run the activate script. Note, we
+            # specify an output link so that it is treated as a GC root. This
+            # prevents an unfortunately timed GC from removing the generation
+            # before activation completes.
+            generation="$WORK_DIR/generation"

-    setFlakeAttribute
-    if [[ -v FLAKE_CONFIG_URI ]]; then
-        doBuildFlake \
-            "$FLAKE_CONFIG_URI.activationPackage" \
-            --out-link "$generation" \
-            ${PRINT_BUILD_LOGS+--print-build-logs} \
-            && "$generation/activate" || return
-    else
-        doBuildAttr \
-            --out-link "$generation" \
-            --attr activationPackage \
-            && "$generation/activate" || return
+            setFlakeAttribute
+            if [[ -v FLAKE_CONFIG_URI ]]; then
+                doBuildFlake \
+                    "$FLAKE_CONFIG_URI.activationPackage" \
+                    --out-link "$generation" \
+                    ${PRINT_BUILD_LOGS+--print-build-logs}
+            else
+                doBuildAttr \
+                    --out-link "$generation" \
+                    --attr activationPackage
+            fi
+            ;;
+        rollback)
+            generation="$HM_PROFILE_DIR/home-manager"
+            ;;
+    esac
+
+    # TODO: Support nix profile.
+    case $profileAction in
+        set)
+            nix-env --profile "$HM_PROFILE_DIR/home-manager" --set "$generation"
+            ;;
+        rollback)
+            nix-env --profile "$HM_PROFILE_DIR/home-manager" --rollback
+            ;;
+    esac
+
+    "$generation/activate" || return
+
+    if [[ $profileAction == 'set' || $profileAction == 'noop' ]]; then
+        presentNews
    fi
-
-    presentNews
 }

 function doListGens() {
    setHomeManagerPathVariables

-    # Whether to colorize the generations output.
-    local color="never"
-    if [[ ! -v NO_COLOR && -t 1 ]]; then
-        color="always"
-    fi
-
-    pushd "$HM_PROFILE_DIR" > /dev/null
-    # shellcheck disable=2012
-    ls --color=$color -gG --time-style=long-iso --sort time home-manager-*-link \
-        | cut -d' ' -f 4- \
-        | sed -E 's/home-manager-([[:digit:]]*)-link/: id \1/'
-    popd > /dev/null
+    # TODO: Support nix profile.
+    nix-env --profile "$HM_PROFILE_DIR/home-manager" --list-generations
 }

 # Removes linked generations. Takes as arguments identifiers of
@ -928,7 +972,7 @@ 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)
@ -983,6 +1027,17 @@ while [[ $# -gt 0 ]]; do
        -n|--dry-run)
            export DRY_RUN=1
            ;;
+        --rollback)
+            case $COMMAND in
+                switch)
+                    COMMAND_ARGS+=("$opt")
+                    ;;
+                *)
+                    _iError '--rollback can only be used with %s' "switch"
+                    exit 1
+                    ;;
+            esac
+            ;;
        --option|--arg|--argstr)
            PASSTHROUGH_OPTS+=("$opt" "$1" "$2")
            shift 2
@ -1038,7 +1093,10 @@ case $COMMAND in
        doInstantiate
        ;;
    switch)
-        doSwitch
+        doSwitch --switch "${COMMAND_ARGS[@]}"
+        ;;
+    test)
+        doSwitch --test
        ;;
    generations)
        doListGens
@ -1046,6 +1104,9 @@ case $COMMAND in
    remove-generations)
        doRmGenerations "${COMMAND_ARGS[@]}"
        ;;
+    rollback)
+        doRollback
+        ;;
    expire-generations)
        if [[ ${#COMMAND_ARGS[@]} != 1 ]]; then
            _i 'expire-generations expects one argument, got %d.' "${#COMMAND_ARGS[@]}" >&2
--- a/modules/files.nix
+++ b/modules/files.nix
@ -1,4 +1,4 @@
-{ pkgs, config, lib, ... }:
+{ pkgs, config, lib, putter, ... }:

 with lib;

@ -21,6 +21,8 @@ let
      then file.source
      else builtins.path { path = file.source; name = sourceName; };

+  putterStatePath = "${config.xdg.stateHome}/home-manager/putter-state.json";
+
 in

 {
@ -31,6 +33,14 @@ in
      type = fileType "home.file" "{env}`HOME`" homeDirectory;
    };

+    home.internal = {
+      filePutterConfig = mkOption {
+        type = types.package;
+        internal = true;
+        description = "Putter configuration.";
+      };
+    };
+
    home-files = mkOption {
      type = types.package;
      internal = true;
@ -70,221 +80,17 @@ in

    # This verifies that the links we are about to create will not
    # overwrite an existing file.
-    home.activation.checkLinkTargets = hm.dag.entryBefore ["writeBoundary"] (
-      let
-        # Paths that should be forcibly overwritten by Home Manager.
-        # Caveat emptor!
-        forcedPaths =
-          concatMapStringsSep " " (p: ''"$HOME"/${escapeShellArg p}'')
-            (mapAttrsToList (n: v: v.target)
-            (filterAttrs (n: v: v.force) cfg));
+    home.activation.checkLinkTargets = hm.dag.entryBefore ["writeBoundary"] ''
+      ${getExe putter} check -v \
+        --state-file "${putterStatePath}" \
+        ${config.home.internal.filePutterConfig}
+    '';

-        check = pkgs.writeText "check" ''
-          ${config.lib.bash.initHomeManagerLib}
-
-          # A symbolic link whose target path matches this pattern will be
-          # considered part of a Home Manager generation.
-          homeFilePattern="$(readlink -e ${escapeShellArg builtins.storeDir})/*-home-manager-files/*"
-
-          forcedPaths=(${forcedPaths})
-
-          newGenFiles="$1"
-          shift
-          for sourcePath in "$@" ; do
-            relativePath="''${sourcePath#$newGenFiles/}"
-            targetPath="$HOME/$relativePath"
-
-            forced=""
-            for forcedPath in "''${forcedPaths[@]}"; do
-              if [[ $targetPath == $forcedPath* ]]; then
-                forced="yeah"
-                break
-              fi
-            done
-
-            if [[ -n $forced ]]; then
-              verboseEcho "Skipping collision check for $targetPath"
-            elif [[ -e "$targetPath" \
-                && ! "$(readlink "$targetPath")" == $homeFilePattern ]] ; then
-              # The target file already exists and it isn't a symlink owned by Home Manager.
-              if cmp -s "$sourcePath" "$targetPath"; then
-                # First compare the files' content. If they're equal, we're fine.
-                warnEcho "Existing file '$targetPath' is in the way of '$sourcePath', will be skipped since they are the same"
-              elif [[ ! -L "$targetPath" && -n "$HOME_MANAGER_BACKUP_EXT" ]] ; then
-                # Next, try to move the file to a backup location if configured and possible
-                backup="$targetPath.$HOME_MANAGER_BACKUP_EXT"
-                if [[ -e "$backup" ]]; then
-                  errorEcho "Existing file '$backup' would be clobbered by backing up '$targetPath'"
-                  collision=1
-                else
-                  warnEcho "Existing file '$targetPath' is in the way of '$sourcePath', will be moved to '$backup'"
-                fi
-              else
-                # Fail if nothing else works
-                errorEcho "Existing file '$targetPath' is in the way of '$sourcePath'"
-                collision=1
-              fi
-            fi
-          done
-
-          if [[ -v collision ]] ; then
-            errorEcho "Please move the above files and try again or use 'home-manager switch -b backup' to back up existing files automatically."
-            exit 1
-          fi
-        '';
-      in
-      ''
-        function checkNewGenCollision() {
-          local newGenFiles
-          newGenFiles="$(readlink -e "$newGenPath/home-files")"
-          find "$newGenFiles" \( -type f -or -type l \) \
-              -exec bash ${check} "$newGenFiles" {} +
-        }
-
-        checkNewGenCollision || exit 1
-      ''
-    );
-
-    # This activation script will
-    #
-    # 1. Remove files from the old generation that are not in the new
-    #    generation.
-    #
-    # 2. Switch over the Home Manager gcroot and current profile
-    #    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
-    # generation A to generation B having sets of home file links FA
-    # and FB, respectively then cleaning before linking produces state
-    # transitions similar to
-    #
-    #      FA   →   FA ∩ FB   →   (FA ∩ FB) ∪ FB = FB
-    #
-    # and a failure during the intermediate state FA ∩ FB will not
-    # result in lost links because this set of links are in both the
-    # source and target generation.
-    home.activation.linkGeneration = hm.dag.entryAfter ["writeBoundary"] (
-      let
-        link = pkgs.writeShellScript "link" ''
-          ${config.lib.bash.initHomeManagerLib}
-
-          newGenFiles="$1"
-          shift
-          for sourcePath in "$@" ; do
-            relativePath="''${sourcePath#$newGenFiles/}"
-            targetPath="$HOME/$relativePath"
-            if [[ -e "$targetPath" && ! -L "$targetPath" && -n "$HOME_MANAGER_BACKUP_EXT" ]] ; then
-              # The target exists, back it up
-              backup="$targetPath.$HOME_MANAGER_BACKUP_EXT"
-              run mv $VERBOSE_ARG "$targetPath" "$backup" || errorEcho "Moving '$targetPath' failed!"
-            fi
-
-            if [[ -e "$targetPath" && ! -L "$targetPath" ]] && cmp -s "$sourcePath" "$targetPath" ; then
-              # The target exists but is identical – don't do anything.
-              verboseEcho "Skipping '$targetPath' as it is identical to '$sourcePath'"
-            else
-              # Place that symlink, --force
-              # This can still fail if the target is a directory, in which case we bail out.
-              run mkdir -p $VERBOSE_ARG "$(dirname "$targetPath")"
-              run ln -Tsf $VERBOSE_ARG "$sourcePath" "$targetPath" || exit 1
-            fi
-          done
-        '';
-
-        cleanup = pkgs.writeShellScript "cleanup" ''
-          ${config.lib.bash.initHomeManagerLib}
-
-          # A symbolic link whose target path matches this pattern will be
-          # considered part of a Home Manager generation.
-          homeFilePattern="$(readlink -e ${escapeShellArg builtins.storeDir})/*-home-manager-files/*"
-
-          newGenFiles="$1"
-          shift 1
-          for relativePath in "$@" ; do
-            targetPath="$HOME/$relativePath"
-            if [[ -e "$newGenFiles/$relativePath" ]] ; then
-              verboseEcho "Checking $targetPath: exists"
-            elif [[ ! "$(readlink "$targetPath")" == $homeFilePattern ]] ; then
-              warnEcho "Path '$targetPath' does not link into a Home Manager generation. Skipping delete."
-            else
-              verboseEcho "Checking $targetPath: gone (deleting)"
-              run rm $VERBOSE_ARG "$targetPath"
-
-              # Recursively delete empty parent directories.
-              targetDir="$(dirname "$relativePath")"
-              if [[ "$targetDir" != "." ]] ; then
-                pushd "$HOME" > /dev/null
-
-                # Call rmdir with a relative path excluding $HOME.
-                # Otherwise, it might try to delete $HOME and exit
-                # with a permission error.
-                run rmdir $VERBOSE_ARG \
-                    -p --ignore-fail-on-non-empty \
-                    "$targetDir"
-
-                popd > /dev/null
-              fi
-            fi
-          done
-        '';
-      in
-        ''
-          function linkNewGen() {
-            _i "Creating home file links in %s" "$HOME"
-
-            local newGenFiles
-            newGenFiles="$(readlink -e "$newGenPath/home-files")"
-            find "$newGenFiles" \( -type f -or -type l \) \
-              -exec bash ${link} "$newGenFiles" {} +
-          }
-
-          function cleanOldGen() {
-            if [[ ! -v oldGenPath || ! -e "$oldGenPath/home-files" ]] ; then
-              return
-            fi
-
-            _i "Cleaning up orphan links from %s" "$HOME"
-
-            local newGenFiles oldGenFiles
-            newGenFiles="$(readlink -e "$newGenPath/home-files")"
-            oldGenFiles="$(readlink -e "$oldGenPath/home-files")"
-
-            # Apply the cleanup script on each leaf in the old
-            # generation. The find command below will print the
-            # relative path of the entry.
-            find "$oldGenFiles" '(' -type f -or -type l ')' -printf '%P\0' \
-              | xargs -0 bash ${cleanup} "$newGenFiles"
-          }
-
-          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 "$newGenPath"
-            fi
-
-            run --silence nix-store --realise "$newGenPath" --add-root "$newGenGcPath"
-            if [[ -e "$legacyGenGcPath" ]]; then
-              run rm $VERBOSE_ARG "$legacyGenGcPath"
-            fi
-          else
-            _i "No change so reusing latest profile generation %s" "$oldGenNum"
-          fi
-
-          linkNewGen
-        ''
-    );
+    home.activation.linkGeneration = hm.dag.entryAfter ["writeBoundary"] ''
+      ${getExe putter} apply $VERBOSE_ARG -v ''${DRY_RUN:+--dry-run} \
+        --state-file "${putterStatePath}" \
+        ${config.home.internal.filePutterConfig}
+    '';

    home.activation.checkFilesChanged = hm.dag.entryBefore ["linkGeneration"] (
      let
@ -325,6 +131,16 @@ in
      '') (filter (v: v.onChange != "") (attrValues cfg))
    );

+    home.internal.filePutterConfig =
+      let putter = import ./lib/putter.nix { inherit lib; };
+          manifest = putter.mkPutterManifest {
+            inherit putterStatePath;
+            sourceBaseDirectory = config.home-files;
+            targetBaseDirectory = config.home.homeDirectory;
+            fileEntries = attrValues cfg;
+          };
+      in pkgs.writeText "hm-putter.json" manifest;
+
    # Symlink directories and files that have the right execute bit.
    # Copy files that need their execute bit changed.
    home-files = pkgs.runCommandLocal
--- a/modules/home-environment.nix
+++ b/modules/home-environment.nix
@ -570,9 +570,13 @@ in

    home.packages = [ config.home.sessionVariablesPackage ];

-    # A dummy entry acting as a boundary between the activation
-    # script's "check" and the "write" phases.
-    home.activation.writeBoundary = hm.dag.entryAnywhere "";
+    # The entry acting as a boundary between the activation script's "check" and
+    # the "write" phases. This is where we commit to attempting to actually
+    # activate the configuration. We do this by creating a GC root for the new
+    # generation so that we guard against it disappearing before we complete.
+    home.activation.writeBoundary = hm.dag.entryAnywhere ''
+      run --silence nix-store --realise "$newGenPath" --add-root "$newGenGcPath"
+    '';

    # Install packages to the user environment.
    #
@ -706,6 +710,11 @@ in
          fi

          ${activationCmds}
+
+          # Create the "current generation" GC root and remove the temporary
+          # "activation in-progress" GC root.
+          run --silence nix-store --realise "$newGenPath" --add-root "$currentGenGcPath"
+          run rm $VERBOSE_ARG "$newGenGcPath"
        '';
      in
        pkgs.runCommand
@ -726,6 +735,7 @@ in
            substituteInPlace $out/activate \
              --subst-var-by GENERATION_DIR $out

+            ln -s ${config.home.internal.filePutterConfig} $out/putter.json
            ln -s ${config.home-files} $out/home-files
            ln -s ${cfg.path} $out/home-path

--- a/modules/lib-bash/activation-init.sh
+++ b/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
-    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
+    if [[ -e $currentGenGcPath ]] ; then
        declare -g oldGenPath
-        oldGenPath="$(readlink -e "$genProfilePath")"
-    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" "$hmGcrootsDir"
-        exit 1
+        oldGenPath="$(readlink -e "$currentGenGcPath")"
    fi
 }

@ -181,15 +160,13 @@ if [[ -v VERBOSE ]]; then
 fi

 _iVerbose "Activation variables:"
-if [[ -v oldGenNum ]] ; then
-    verboseEcho "  oldGenNum=$oldGenNum"
+if [[ -v oldGenPath ]] ; then
    verboseEcho "  oldGenPath=$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"
--- a/modules/lib/putter.nix
+++ b/modules/lib/putter.nix
@ -0,0 +1,63 @@
+# Contains some handy functions for generating Putter file manifests.
+
+{ lib }:
+
+let
+
+  inherit (lib)
+    concatMap concatLists mapAttrsToList hasPrefix removePrefix filter;
+
+in {
+  # Converts a Home Manager style list of file specifications into a Putter
+  # configuration.
+  #
+  # Note, the interface of this function is not considered stable, it may change
+  # as the needs of Home Manager change.
+  mkPutterManifest =
+    { putterStatePath, sourceBaseDirectory, targetBaseDirectory, fileEntries }:
+    let
+      # Convert a directory to a Putter configuration. Basically, this will
+      # create a file entry for each file in the directory. Any sub-directories
+      # will be handled recursively.
+      mkDirEntry = f:
+        concatLists (mapAttrsToList (n: v:
+          let
+            f' = f // {
+              source = "${f.source}/${n}";
+              target = "${f.target}/${n}";
+            };
+          in mkEntriesForType f' v) (builtins.readDir f.source));
+
+      mkEntriesForType = f: t:
+        if t == "regular" || t == "symlink" then
+          mkFileEntry f
+        else if t == "directory" then
+          mkDirEntry f
+        else
+          throw "unexpected file type ${t}";
+
+      # Create a file entry for the given file.
+      mkFileEntry = f: [{
+        collision.resolution = if f.force then "force" else "abort";
+        action.type = "symlink";
+        source = "${sourceBaseDirectory}/${f.target}";
+        target =
+          (if hasPrefix "/" f.target then "" else "${targetBaseDirectory}/")
+          + f.target;
+      }];
+
+      # Given a Home Manager file entry, produce a list of Putter entries. For
+      # recursive HM file entries, we recursively traverse the source directory
+      # and generate a Putter entry for each file we encounter.
+      mkEntries = f:
+        if f.recursive then mkEntriesForType f "directory" else mkFileEntry f;
+
+      putterJson = {
+        version = "1";
+        state = putterStatePath;
+        files = concatMap mkEntries (filter (f: f.enable) fileEntries);
+      };
+
+      putterJsonText = builtins.toJSON putterJson;
+    in putterJsonText;
+}