Add module 'versioning'

examples/configuration.nix

@@ -209,4 +209,9 @@
   # servers. You should change this only after NixOS release notes say you
   # should.
   system.stateVersion = "18.09"; # Did you read the comment?
+
+  # The nix-bitcoin release version that your config is compatible with.
+  # When upgrading to a backwards-incompatible release, nix-bitcoin will display an
+  # an error and provide hints for migrating your config to the new release.
+  nix-bitcoin.configVersion = "0.0.18";
 }

modules/modules.nix

@@ -22,6 +22,7 @@
     ./recurring-donations.nix
 
     # Support features
+    ./versioning.nix
     ./security.nix
     ./netns-isolation.nix
     ./backups.nix

modules/versioning.nix

@@ -0,0 +1,59 @@
+{ config, pkgs, lib, ... }:
+
+with lib;
+let
+  version = config.nix-bitcoin.configVersion;
+
+  # Sorted by increasing version numbers
+  changes = [
+    # None yet
+    # {
+    #   version = "0.1";
+    #   condition = config.services.foo.enabled;
+    #   message = ''
+    #     demo message
+    #   '';
+    # }
+  ];
+
+  incompatibleChanges = optionals
+    (version != null && versionOlder lastChange)
+    (builtins.filter (change: versionOlder change && (change.condition or true)) changes);
+
+  errorMsg = ''
+
+    This version of nix-bitcoin contains the following changes
+    that are incompatible with your config (version ${version}):
+
+    ${concatMapStringsSep "\n" (change: ''
+      - ${change.message}(This change was introduced in version ${change.version})
+    '') incompatibleChanges}
+    After addressing the above changes, set nix-bitcoin.configVersion = "${lastChange.version}";
+    in your nix-bitcoin configuration.
+  '';
+
+  versionOlder = change: (builtins.compareVersions change.version version) > 0;
+  lastChange = builtins.elemAt changes (builtins.length changes - 1);
+in
+{
+  options = {
+    nix-bitcoin.configVersion = mkOption {
+      type = with types; nullOr str;
+      default = null;
+      description = ''
+        Set this option to the nix-bitcoin release version that your config is
+        compatible with.
+
+        When upgrading to a backwards-incompatible release, nix-bitcoin will throw an
+        error during evaluation and provide hints for migrating your config to the
+        new release.
+      '';
+    };
+  };
+
+  ## No config because there are no backwards incompatible releases yet
+  # config = {
+  #   # Force evaluation. An actual option value is never assigned
+  #   system.extraDependencies = optional (builtins.length incompatibleChanges > 0) (builtins.throw errorMsg);
+  # };
+}