From 3b930bb653dd9ed7da5fe86c147bbc67492efe2c Mon Sep 17 00:00:00 2001
From: Damien Cassou <damien@cassou.me>
Date: Wed, 14 May 2025 21:45:20 +0200
Subject: [PATCH] services.nix-gc: improve error message when nix.gc.frequency
 is invalid on darwin

Previously, if an invalid value was passed, the build would fail with:

       error: attribute '"00:02:03"' missing
       at /nix/store/sz92b5gqi0ma61d18fwbihi8p37mkvir-source/modules/services/nix-gc.nix:69:5:
           68|     in
           69|     freq.${frequency};
             |     ^
           70|

There was an assertion that should have prevented this from happening
but the crash would happen before the assertion gets a chance to stop
the build with a nice error message.

This commit both gives the assertion a chance to trigger and improves
the assertion's error message.
---
 modules/services/nix-gc.nix                            |  6 +++---
 .../nix-gc-darwin/darwin-nix-gc-interval-assertion.nix | 10 ++++++++++
 tests/modules/services/nix-gc-darwin/default.nix       |  5 ++++-
 3 files changed, 17 insertions(+), 4 deletions(-)
 create mode 100644 tests/modules/services/nix-gc-darwin/darwin-nix-gc-interval-assertion.nix

diff --git a/modules/services/nix-gc.nix b/modules/services/nix-gc.nix
index d95d6e94c..a49220990 100644
--- a/modules/services/nix-gc.nix
+++ b/modules/services/nix-gc.nix
@@ -66,7 +66,7 @@ let
         ];
       };
     in
-    freq.${frequency};
+    freq.${frequency} or null;
 
   nixPackage =
     if config.nix.enable && config.nix.package != null then config.nix.package else pkgs.nix;
@@ -95,7 +95,7 @@ in
 
           On Linux this is a string as defined by {manpage}`systemd.time(7)`.
 
-          On Darwin it must be one of: ${toString darwinIntervals}, which are
+          On Darwin it must be one of: ${lib.concatStringsSep ", " darwinIntervals}, which are
           implemented as defined in the manual page above.
         '';
       };
@@ -172,7 +172,7 @@ in
         assertions = [
           {
             assertion = lib.elem cfg.frequency darwinIntervals;
-            message = "On Darwin nix.gc.frequency must be one of: ${toString darwinIntervals}.";
+            message = "On Darwin nix.gc.frequency must be one of: ${lib.concatStringsSep ", " darwinIntervals}.";
           }
         ];
 
diff --git a/tests/modules/services/nix-gc-darwin/darwin-nix-gc-interval-assertion.nix b/tests/modules/services/nix-gc-darwin/darwin-nix-gc-interval-assertion.nix
new file mode 100644
index 000000000..6dbf70e83
--- /dev/null
+++ b/tests/modules/services/nix-gc-darwin/darwin-nix-gc-interval-assertion.nix
@@ -0,0 +1,10 @@
+{
+  nix.gc = {
+    automatic = true;
+    frequency = "00:02:03";
+  };
+
+  test.asserts.assertions.expected = [
+    "On Darwin nix.gc.frequency must be one of: hourly, daily, weekly, monthly, semiannually, annually."
+  ];
+}
diff --git a/tests/modules/services/nix-gc-darwin/default.nix b/tests/modules/services/nix-gc-darwin/default.nix
index f2fc20aa7..251e0d23a 100644
--- a/tests/modules/services/nix-gc-darwin/default.nix
+++ b/tests/modules/services/nix-gc-darwin/default.nix
@@ -1 +1,4 @@
-{ nix-gc = ./basic.nix; }
+{
+  nix-gc = ./basic.nix;
+  darwin-nix-gc-interval-assertion = ./darwin-nix-gc-interval-assertion.nix;
+}