From 4bffc92b7a93e499eb98c6d118b22d17acbc50d2 Mon Sep 17 00:00:00 2001
From: Daiderd Jordan <daiderd@gmail.com>
Date: Mon, 1 Jul 2019 21:34:37 +0200
Subject: [PATCH] nix: check nix.useDaemon value

This case will cause problems when existing files in /etc are replaced
instead of skipped with a warning, potentially breaking the nix install
in the process.
---
 modules/system/checks.nix | 16 ++++++++++++++++
 1 file changed, 16 insertions(+)

diff --git a/modules/system/checks.nix b/modules/system/checks.nix
index 0ff08930..8b438450 100644
--- a/modules/system/checks.nix
+++ b/modules/system/checks.nix
@@ -47,6 +47,21 @@ let
     fi
   '';
 
+  singleUser = ''
+    if grep -q 'build-users-group =' /etc/nix/nix.conf; then
+        echo "error: The daemon is not enabled but this is a multi-user install, aborting activation" >&2
+        echo "Enable the nix-daemon service:" >&2
+        echo >&2
+        echo "    services.nix-daemon.enable = true;" >&2
+        echo >&2
+        echo "or set" >&2
+        echo >&2
+        echo "    nix.useDaemon = true;" >&2
+        echo >&2
+        exit 2
+    fi
+  '';
+
   nixChannels = ''
     channelsLink=$(readlink "$HOME/.nix-defexpr/channels") || true
     case "$channelsLink" in
@@ -170,6 +185,7 @@ in
       darwinChanges
       runLink
       (mkIf config.nix.useDaemon buildUsers)
+      (mkIf (!config.nix.useDaemon) singleUser)
       nixStore
       (mkIf (config.nix.gc.automatic && config.nix.gc.user == null) nixGarbageCollector)
       nixChannels