fix: default lock directory selection

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

Author: Alizter

src/dune_rules/lock_dir.ml

@@ -177,11 +177,25 @@ 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
+      | [] -> Some default_source_path
+      | [ lock_dir ] -> Some lock_dir.path
+      | _ :: _ :: _ ->
+        (* Multiple lock dirs but context doesn't specify which one to use,
+          fall back to default *)
+        Some default_source_path)
 ;;
 
 let get_path ctx_name =

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

@@ -32,7 +32,12 @@ Now change the workspace to use a custom lock directory:
   > EOF
 
   $ dune build
-  Error: No rule found for default/.lock/dune.lock (context _private)
+  Error: The package myproject does not have any user defined stanzas attached
+  to it. If this is intentional, add (allow_empty) to the package definition in
+  the dune-project file
+  -> required by _build/default/myproject.install
+  -> required by alias all
+  -> required by alias default
   [1]
 
 Now try with a context stanza that references the custom lock directory with a