From 15f5d97457e4aa59838c5018f3f964045e97ecdf Mon Sep 17 00:00:00 2001
From: Tobias Happ <tobias.happ@gmx.de>
Date: Sat, 30 Nov 2019 01:30:33 +0100
Subject: [PATCH] Add environment.etcBackupExtension option

---
 .../initial-build/nix-on-droid.nix.default    |  3 ++
 modules/environment/etc/default.nix           | 47 +++++++++++++------
 modules/environment/etc/setup-etc.sh          | 12 ++++-
 3 files changed, 45 insertions(+), 17 deletions(-)

diff --git a/modules/build/initial-build/nix-on-droid.nix.default b/modules/build/initial-build/nix-on-droid.nix.default
index 85756c6..d6f6d76 100644
--- a/modules/build/initial-build/nix-on-droid.nix.default
+++ b/modules/build/initial-build/nix-on-droid.nix.default
@@ -24,6 +24,9 @@
     #unzip
   ];
 
+  # Backup etc files instead of failing to activate generation if a file already exists in /etc
+  environment.etcBackupExtension = ".bak";
+
   # Read the changelog before changing this value
   system.stateVersion = "19.09";
 
diff --git a/modules/environment/etc/default.nix b/modules/environment/etc/default.nix
index 7e460a2..6a120d7 100644
--- a/modules/environment/etc/default.nix
+++ b/modules/environment/etc/default.nix
@@ -72,20 +72,37 @@ in
 
   options = {
 
-    environment.etc = mkOption {
-      type = types.loaOf fileType;
-      default = {};
-      example = literalExample ''
-        {
-          example-configuration-file = {
-            source = "/nix/store/.../etc/dir/file.conf.example";
-          };
-          "default/useradd".text = "GROUP=100 ...";
-        }
-      '';
-      description = ''
-        Set of files that have to be linked in <filename>/etc</filename>.
-      '';
+    environment = {
+      etc = mkOption {
+        type = types.loaOf fileType;
+        default = {};
+        example = literalExample ''
+          {
+            example-configuration-file = {
+              source = "/nix/store/.../etc/dir/file.conf.example";
+            };
+            "default/useradd".text = "GROUP=100 ...";
+          }
+        '';
+        description = ''
+          Set of files that have to be linked in <filename>/etc</filename>.
+        '';
+      };
+
+      etcBackupExtension = mkOption {
+        type = types.nullOr types.str;
+        default = null;
+        example = ".bak";
+        description = ''
+          Backup file extension.
+          </para><para>
+          If a file in <filename>/etc</filename> already exists and is not managed
+          by nix-on-droid, the activation fails because we do not overwrite unknown
+          files. When an extension is provided through this option, the original
+          file will be moved in respect of the backup extension and the activation
+          executes successfully.
+        '';
+      };
     };
 
   };
@@ -99,7 +116,7 @@ in
       inherit etc;
 
       activation.setUpEtc = ''
-        $DRY_RUN_CMD bash ${./setup-etc.sh} /etc ${etc}/etc
+        $DRY_RUN_CMD bash ${./setup-etc.sh} /etc ${etc}/etc ${config.environment.etcBackupExtension}
       '';
     };
 
diff --git a/modules/environment/etc/setup-etc.sh b/modules/environment/etc/setup-etc.sh
index c9cfa7c..bafdfaf 100644
--- a/modules/environment/etc/setup-etc.sh
+++ b/modules/environment/etc/setup-etc.sh
@@ -6,6 +6,7 @@
 etc="${1}"
 static="/etc/static"
 new_etc="${2}"
+backup_extension="${3:-}"
 
 function atomic_symlink() {
     local source="${1}"
@@ -25,7 +26,7 @@ function cleanup() {
     for file in $(find "${etc}" -xtype l); do
         local target="$(readlink "${file}")"
         if [[ ! -L "${target}" ]]; then
-            echo "removing obsolete symlink '${file}'..."
+            echo "Removing obsolete symlink '${file}'..."
             rm "${file}"
         fi
     done
@@ -64,7 +65,14 @@ function link() {
         mkdir -p "$(dirname "${target}")"
 
         if [[ -e "${target}" ]] && ! is_static "${target}"; then
-            echo "Linking of ${target} failed. Please remove this file."
+            if [[ -n "${backup_extension}" ]]; then
+                echo "Backing up '${target}' to '${target}${backup_extension}'..."
+                cp "${target}" "${target}${backup_extension}"
+
+                atomic_symlink "${static}/${name}" "${target}"
+            else
+                echo "Linking of ${target} failed. Please remove this file."
+            fi
         else
             atomic_symlink "${static}/${name}" "${target}"
         fi