nixos/postgresql: fix enableJIT

Make sure that JIT is actually available when using

    services.postgresql = {
      enable = true;
      enableJIT = true;
      package = pkgs.postgresql_15;
    };

The current behavior is counter-intuitive because the docs state that
`enableJIT = true;` is sufficient even though it wasn't in that case
because the declared package doesn't have the LLVM dependency.

Fixed by using `package.withJIT` if `enableJIT = true;` and
`package.jitSupport` is `false`.

Also updated the postgresql-jit test to test for that case.

nixos/modules/services/databases/postgresql.nix

@@ -7,9 +7,18 @@ let
   cfg = config.services.postgresql;
 
   postgresql =
+    let
+      # ensure that
+      #   services.postgresql = {
+      #     enableJIT = true;
+      #     package = pkgs.postgresql_<major>;
+      #   };
+      # works.
+      base = if cfg.enableJIT && !cfg.package.jitSupport then cfg.package.withJIT else cfg.package;
+    in
     if cfg.extraPlugins == []
-      then cfg.package
-      else cfg.package.withPackages (_: cfg.extraPlugins);
+      then base
+      else base.withPackages (_: cfg.extraPlugins);
 
   toStr = value:
     if true == value then "yes"

nixos/tests/postgresql-jit.nix

@@ -7,9 +7,7 @@ with import ../lib/testing-python.nix { inherit system pkgs; };
 
 let
   inherit (pkgs) lib;
-  packages = lib.filter
-    (lib.hasSuffix "_jit")
-    (builtins.attrNames (import ../../pkgs/servers/sql/postgresql pkgs));
+  packages = builtins.attrNames (import ../../pkgs/servers/sql/postgresql pkgs);
 
   mkJitTest = packageName: makeTest {
     name = "${packageName}";
@@ -17,8 +15,8 @@ let
     nodes.machine = { pkgs, lib, ... }: {
       services.postgresql = {
         enable = true;
+        enableJIT = true;
         package = pkgs.${packageName};
-        settings.jit = "on";
         initialScript = pkgs.writeText "init.sql" ''
           create table demo (id int);
           insert into demo (id) select generate_series(1, 5);