fix: switching to custom lock dir

src/dune_rules/lock_dir.ml

@@ -177,11 +177,23 @@ let get_source_path_for_context ctx_name =
       | false -> None
       | true -> Some ctx)
   with
-  | None | Some (Default { lock_dir = None; _ }) -> Memo.return (Some default_source_path)
   | Some (Default { lock_dir = Some lock_dir_selection; _ }) ->
     let+ source_lock_dir = select_lock_dir lock_dir_selection in
     Some source_lock_dir
   | Some (Opam _) -> Memo.return None
+  | None | Some (Default { lock_dir = None; _ }) ->
+    (* CR-someday Alizter: The logic here is hard to follow. Improve the lock
+       directory selection logic so that it is harder to make a mistake. This
+       can be done by enforcing invariants about contexts and their lock
+       directories slightly earlier. *)
+    Memo.return
+    @@
+    (* Context doesn't specify a lock_dir, check if there's a top-level one *)
+    (match workspace.lock_dirs with
+      | [ lock_dir ] -> Some lock_dir.path
+      | [] | _ :: _ :: _ ->
+        (* No clear choice, fall back to default *)
+        Some default_source_path)
 ;;
 
 let get_path ctx_name =

test/blackbox-tests/test-cases/pkg/lockdir-switch.t

@@ -0,0 +1,67 @@
+This test checks what happens when you lock with default directory then change
+workspace to use a custom lock directory.
+
+  $ . ./helpers.sh
+  $ mkrepo
+  $ mkpkg foo
+  $ add_mock_repo_if_needed
+
+  $ cat > dune-project <<EOF
+  > (lang dune 3.21)
+  > (package
+  >  (name myproject)
+  >  (depends foo))
+  > EOF
+
+Lock with default directory name:
+
+  $ dune pkg lock
+  Solution for dune.lock
+
+  Dependencies common to all supported platforms:
+  - foo.0.0.1
+
+Now change the workspace to use a custom lock directory:
+
+  $ cat > dune-workspace <<EOF
+  > (lang dune 3.21)
+  > (lock_dir
+  >  (path custom.lock)
+  >  (repositories mock))
+  > (repository
+  >  (name mock)
+  >  (url "file://$PWD/mock-opam-repository"))
+  > EOF
+
+  $ cat > dune <<EOF
+  > (executable
+  >  (public_name bar))
+  > EOF
+
+  $ cat > bar.ml <<EOF
+  > let () = print_endline "hello from bar" ;;
+  > EOF
+
+  $ dune exec -- bar
+  hello from bar
+
+Now try with a context stanza that references the custom lock directory with a
+context stanza:
+
+  $ cat > dune-workspace <<EOF
+  > (lang dune 3.21)
+  > (context
+  >  (default
+  >   (lock_dir custom.lock)))
+  > (lock_dir
+  >  (path custom.lock)
+  >  (repositories mock))
+  > (repository
+  >  (name mock)
+  >  (url "file://$PWD/mock-opam-repository"))
+  > EOF
+
+This is working as intended. The error is not relevant here.
+
+  $ dune exec -- bar 
+  hello from bar