home-manager: add backup overwrite option

When using the backupFileExtension option, if the backup file exists,
the activation process fails. This adds an option to instead overwrite
the old backup instead of failing.

modules/files.nix

@@ -149,6 +149,9 @@ in
             if [[ -e "$targetPath" && ! -L "$targetPath" && -n "$HOME_MANAGER_BACKUP_EXT" ]] ; then
               # The target exists, back it up
               backup="$targetPath.$HOME_MANAGER_BACKUP_EXT"
+              if [[ -e "$backup" && -n "$HOME_MANAGER_BACKUP_OVERWRITE" ]]; then
+                run rm $VERBOSE_ARG "$backup"
+              fi
               run mv $VERBOSE_ARG "$targetPath" "$backup" || errorEcho "Moving '$targetPath' failed!"
             fi
 

modules/files/check-link-targets.sh

@@ -33,8 +33,10 @@ for sourcePath in "$@" ; do
     elif [[ ! -L "$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
+      if [[ -e "$backup" && -z "$HOME_MANAGER_BACKUP_OVERWRITE" ]] ; then
         collisionErrors+=("Existing file '$backup' would be clobbered by backing up '$targetPath'")
+      elif [[ -e "$backup" && -n "$HOME_MANAGER_BACKUP_OVERWRITE" ]] ; then
+        warnEcho "Existing file '$targetPath' is in the way of '$sourcePath' and '$backup' exists. Backup will be clobbered due to HOME_MANAGER_BACKUP_OVERWRITE=1"
       else
         warnEcho "Existing file '$targetPath' is in the way of '$sourcePath', will be moved to '$backup'"
       fi

nix-darwin/default.nix

@@ -28,6 +28,7 @@ in
               ${lib.optionalString (
                 cfg.backupFileExtension != null
               ) "export HOME_MANAGER_BACKUP_EXT=${lib.escapeShellArg cfg.backupFileExtension}"}
+              ${lib.optionalString cfg.overwriteBackup "export HOME_MANAGER_BACKUP_OVERWRITE=1"}
               ${lib.optionalString cfg.verbose "export VERBOSE=1"}
               exec ${usercfg.home.activationPackage}/activate --driver-version ${driverVersion} >&2
             ''}

nixos/common.nix

@@ -91,6 +91,10 @@ in
       '';
     };
 
+    overwriteBackup = mkEnableOption ''
+      forced overwriting of existing backup files when using `backupFileExtension`
+    '';
+
     extraSpecialArgs = mkOption {
       type = types.attrs;
       default = { };

nixos/default.nix

@@ -7,15 +7,18 @@
 }:
 
 let
-
+  inherit (lib) mkIf;
   cfg = config.home-manager;
 
-  serviceEnvironment =
+  serviceEnvironment = lib.mkMerge [
-    lib.optionalAttrs (cfg.backupFileExtension != null) {
+    (mkIf cfg.verbose { VERBOSE = "1"; })
-      HOME_MANAGER_BACKUP_EXT = cfg.backupFileExtension;
-    }
-    // lib.optionalAttrs cfg.verbose { VERBOSE = "1"; };
 
+    (mkIf (cfg.backupFileExtension != null) {
+      HOME_MANAGER_BACKUP_EXT = cfg.backupFileExtension;
+    })
+
+    (mkIf cfg.overwriteBackup { HOME_MANAGER_BACKUP_OVERWRITE = "1"; })
+  ];
 in
 {
   imports = [ ./common.nix ];
@@ -41,7 +44,7 @@ in
         ];
       };
     }
-    (lib.mkIf (cfg.users != { }) {
+    (mkIf (cfg.users != { }) {
       systemd.services = lib.mapAttrs' (
         _: usercfg:
         let