Keep a log over time with the config commits

m/common/main.nix

@@ -11,6 +11,7 @@
     ./ssh.nix
     ./users.nix
     ./watchdog.nix
+    ./rev.nix
   ];
 
   nixpkgs.overlays = [
@@ -18,6 +19,11 @@
     (import ../../pkgs/overlay.nix)
   ];
 
+  system.configurationRevision =
+    if theFlake ? rev
+    then theFlake.rev
+    else throw ("Refusing to build from a dirty Git tree!");
+
   nix.nixPath = [
     "nixpkgs=${nixpkgs}"
     "bscpkgs=${bscpkgs}"
@@ -28,14 +34,6 @@
   nix.registry.bscpkgs.flake = bscpkgs;
   nix.registry.jungle.flake = theFlake;
 
-  system.configurationRevision =
-    if theFlake ? rev
-    then theFlake.rev
-    else throw ("Refusing to build from a dirty Git tree!");
-
-  # Save the commit of the config in /etc/nixos/config.rev
-  environment.etc."nixos/config.rev".text = system.configurationRevision;
-
   environment.systemPackages = with pkgs; [
     vim wget git htop tmux pciutils tcpdump ripgrep nix-index nixos-option
     nix-diff ipmitool freeipmi ethtool lm_sensors ix cmake gnumake file tree

m/common/rev.nix

@@ -0,0 +1,18 @@
+{ theFlake, ... }:
+
+let
+  rev = if theFlake ? rev then theFlake.rev
+    else throw ("Refusing to build from a dirty Git tree!");
+in {
+  # Save the commit of the config in /etc/configrev
+  environment.etc.configrev.text = rev + "\n";
+
+  # Keep a log with the config over time
+  system.activationScripts.configRevLog.text = ''
+    BOOTED=$(cat /run/booted-system/etc/configrev 2>/dev/null || echo unknown)
+    CURRENT=$(cat /run/current-system/etc/configrev 2>/dev/null || echo unknown)
+    NEXT=${rev}
+    DATENOW=$(date --iso-8601=seconds)
+    echo "$DATENOW booted=$BOOTED current=$CURRENT next=$NEXT" >> /var/configrev.log
+  '';
+}