home-manager: Add --flake option to home-manager (#1856)

Implements a --flake options for build and switch, along with the usual
flake related optons (for lock-files etc).

Configurations in the flake are automatically discovered in the
following order:
1. `outputs.homeConfigurations."$flake-uri"` (the `--flake parameter`)
2. `outputs.homeConfigurations."$USERNAME@$HOSTNAME"`
3. `outputs.homeConfigurations."$USERNAME"`

Make home-manager use default configuration from
~/.config/nixpkgs/flake.nix, if it exists and nothing else is
specified.

Co-authored-by: Nicolas Berbiche <nicolas@normie.dev>

doc/man-home-manager.xml

@@ -66,6 +66,10 @@
     -I <replaceable>path</replaceable>
    </arg>
     
+   <arg>
+    --flake <replaceable>flake-uri</replaceable>
+   </arg>
+    
    <arg>
     -b <replaceable>ext</replaceable>
    </arg>
@@ -160,7 +164,7 @@
   <title>Description</title>
   <para>
    This command updates the user environment so that it corresponds to the
-   configuration specified in <filename>~/.config/nixpkgs/home.nix</filename>.
+      configuration specified in <filename>~/.config/nixpkgs/home.nix</filename> or <filename>~/.config/nixpkgs/flake.nix</filename>.
   </para>
   <para>
    All operations using this tool expects a sub-command that indicates the
@@ -343,6 +347,18 @@
      </para>
     </listitem>
    </varlistentry>
+   <varlistentry>
+    <term>
+     <option>--flake <replaceable>flake-uri[#name]</replaceable></option>
+    </term>
+    <listitem>
+     <para>
+      Build Home Manager configuration from the flake, which must contain the
+      output homeConfigurations.name. If no name is specified it will first try
+      username@hostname and then username.
+     </para>
+    </listitem>
+   </varlistentry>
    <varlistentry>
     <term>
      <option>-b <replaceable>extension</replaceable></option>

home-manager/home-manager

@@ -74,7 +74,35 @@ function setHomeManagerNixPath() {
     done
 }
 
+function setFlakeAttribute() {
+    local configFlake="${XDG_CONFIG_HOME:-$HOME/.config}/nixpkgs/flake.nix"
+    if [[ -z $FLAKE_ARG && ! -v HOME_MANAGER_CONFIG && -e "$configFlake" ]]; then
+        FLAKE_ARG="$(dirname "$(readlink -f "$configFlake")")"
+    fi
+
+    if [[ -n "$FLAKE_ARG" ]]; then
+        local flake="${FLAKE_ARG%#*}"
+        case $FLAKE_ARG in
+            *#*)
+                local name="${FLAKE_ARG#*#}"
+            ;;
+            *)
+                local name="$USER@$(hostname)"
+                if [ "$(nix eval "$flake#homeConfigurations" --apply "x: x ? \"$name\"")" = "false" ]; then
+                    name="$USER"
+                fi
+            ;;
+        esac
+        export FLAKE_CONFIG_URI="$flake#homeConfigurations.\"$name\""
+    fi
+}
+
 function doInstantiate() {
+    setFlakeAttribute
+    if [[ -v FLAKE_CONFIG_URI ]]; then
+        errorEcho "Can't instantiate a flake configuration"
+        exit 1
+    fi
     setConfigFile
     setHomeManagerNixPath
 
@@ -178,6 +206,17 @@ function doBuild() {
         return 1
     fi
 
+    setFlakeAttribute
+    if [[ -v FLAKE_CONFIG_URI ]]; then
+        local exitCode=0
+        nix build \
+            "${PASSTHROUGH_OPTS[@]}" \
+             ${DRY_RUN+--dry-run} \
+            "$FLAKE_CONFIG_URI.activationPackage" \
+            || exitCode=1
+        return $exitCode
+    fi
+
     setWorkDir
 
     local newsInfo
@@ -194,6 +233,16 @@ function doBuild() {
 }
 
 function doSwitch() {
+    setFlakeAttribute
+    if [[ -v FLAKE_CONFIG_URI ]]; then
+        local exitCode=0
+        nix run \
+            "${PASSTHROUGH_OPTS[@]}" \
+            "$FLAKE_CONFIG_URI.activationPackage" \
+            || exitCode=1
+        return $exitCode
+    fi
+
     setWorkDir
 
     local newsInfo
@@ -414,6 +463,7 @@ function doHelp() {
     echo "  -A ATTRIBUTE      Optional attribute that selects a configuration"
     echo "                    expression in the configuration file."
     echo "  -I PATH           Add a path to the Nix expression search path."
+    echo "  --flake flake-uri Use home-manager configuration at flake-uri"
     echo "  -b EXT            Move existing files to new path rather than fail."
     echo "  -v                Verbose output"
     echo "  -n                Do a dry run, only prints what actions would be taken"
@@ -467,6 +517,7 @@ HOME_MANAGER_CONFIG_ATTRIBUTE=""
 PASSTHROUGH_OPTS=()
 COMMAND=""
 COMMAND_ARGS=()
+FLAKE_ARG=""
 
 while [[ $# -gt 0 ]]; do
     opt="$1"
@@ -491,6 +542,21 @@ while [[ $# -gt 0 ]]; do
             HOME_MANAGER_CONFIG="$1"
             shift
             ;;
+        --flake)
+            FLAKE_ARG="$1"
+            shift
+            ;;
+        --recreate-lock-file|--no-update-lock-file|--no-write-lock-file|--no-registries|--commit-lock-file)
+            PASSTHROUGH_OPTS+=("$opt")
+            ;;
+        --update-input)
+            PASSTHROUGH_OPTS+=("$opt" "$1")
+            shift
+            ;;
+        --override-input)
+            PASSTHROUGH_OPTS+=("$opt" "$1" "$2")
+            shift 2
+            ;;
         -h|--help)
             doHelp
             exit 0

modules/home-environment.nix

@@ -552,6 +552,9 @@ in
 
             cp ${activationScript} $out/activate
 
+            mkdir $out/bin
+            ln -s $out/activate $out/bin/home-manager-generation
+
             substituteInPlace $out/activate \
               --subst-var-by GENERATION_DIR $out