tests/acme: check consistent account hash for legacy settings

To allow migration from 23.11 to 24.05 without triggering re-registrations,
the account hashing behaviour of the previous release can be retained by setting
`security.acme.defaults.server` to `null`.

We better also check for hash consistency with that setting to avoid unexpected
account hash changes again.

nixos/tests/acme.nix

@@ -200,6 +200,14 @@ in {
         # Tests HTTP-01 verification using Lego's built-in web server
         http01lego.configuration = simpleConfig;
 
+        # account hash generation with default server from <= 23.11
+        http01lego_legacyAccountHash.configuration = lib.mkMerge [
+          simpleConfig
+          {
+            security.acme.defaults.server = lib.mkForce null;
+          }
+        ];
+
         renew.configuration = lib.mkMerge [
           simpleConfig
           {
@@ -424,7 +432,7 @@ in {
       backoff = BackoffTracker()
 
 
-      def switch_to(node, name):
+      def switch_to(node, name, allow_fail=False):
           # On first switch, this will create a symlink to the current system so that we can
           # quickly switch between derivations
           root_specs = "/tmp/specialisation"
@@ -438,9 +446,14 @@ in {
           if rc > 0:
               switcher_path = f"/tmp/specialisation/{name}/bin/switch-to-configuration"
 
+          if not allow_fail:
             node.succeed(
                 f"{switcher_path} test"
             )
+          else:
+            node.execute(
+                f"{switcher_path} test"
+            )
 
 
       # Ensures the issuer of our cert matches the chain
@@ -544,7 +557,7 @@ in {
           check_issuer(webserver, "http.example.test", "pebble")
 
       # Perform account hash test
-      with subtest("Assert that account hash didn't unexpected change"):
+      with subtest("Assert that account hash didn't unexpectedly change"):
           hash = webserver.succeed("ls /var/lib/acme/.lego/accounts/")
           print("Account hash: " + hash)
           assert hash.strip() == "d590213ed52603e9128d"
@@ -727,5 +740,23 @@ in {
               webserver.wait_for_unit(f"acme-finished-{test_domain}.target")
               wait_for_server()
               check_connection_key_bits(client, test_domain, "384")
+
+      # Perform http-01 w/ lego test again, but using the pre-24.05 account hashing
+      # (see https://github.com/NixOS/nixpkgs/pull/317257)
+      with subtest("Check account hashing compatibility with pre-24.05 settings"):
+          webserver.succeed("rm -rf /var/lib/acme/.lego/accounts/*")
+          switch_to(webserver, "http01lego_legacyAccountHash", allow_fail=True)
+          # unit is failed, but in a way that this throws no exception:
+          try:
+            webserver.wait_for_unit("acme-finished-http.example.test.target")
+          except Exception:
+            # The unit is allowed – or even expected – to fail due to not being able to
+            # reach the actual letsencrypt server. We only use it for serialising the
+            # test execution, such that the account check is done after the service run
+            # involving the account creation has been executed at least once.
+            pass
+          hash = webserver.succeed("ls /var/lib/acme/.lego/accounts/")
+          print("Account hash: " + hash)
+          assert hash.strip() == "1ccf607d9aa280e9af00"
     '';
 }