fix: default lock directory selection

Signed-off-by: Ali Caglayan <[email protected]>

Author: Alizter

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

@@ -43,12 +43,7 @@ Now change the workspace to use a custom lock directory:
   > EOF
 
   $ dune exec -- bar
-  Error: No rule found for default/.lock/dune.lock (context _private)
-  -> required by lock directory environment for context "default"
-  -> required by base environment for context "default"
-  -> required by loading findlib for context "default"
-  -> required by creating installed environment for "default"
-  [1]
+  hello from bar
 
 Now try with a context stanza that references the custom lock directory with a
 context stanza: