Allow non fs stores as lower layer of overlay stores.

The local-overlay:// documentation states that any store can be
specified as lower, e.g. another daemon store, but the code currently
downcasts the store to LocalFSStore unconditionally, which makes it
impossible to use any other store as lower.

The only reason I found to do the downcast is to perform an optional
check-mount action, which checks if the lower and upper stores are
indeed overlayed correctly.

This change relaxes that undocumented constraint and unlocks non-trivial
store compositions.

src/libstore/local-overlay-store.cc

@@ -26,9 +26,13 @@ LocalOverlayStore::LocalOverlayStore(std::string_view scheme, PathView path, con
     , Store(params)
     , LocalFSStore(params)
     , LocalStore(params)
-    , lowerStore(openStore(percentDecode(lowerStoreUri.get())).dynamic_pointer_cast<LocalFSStore>())
+    , lowerStore(openStore(percentDecode(lowerStoreUri.get())))
 {
+    auto fsStore = lowerStore.dynamic_pointer_cast<LocalFSStore>();
     if (checkMount.get()) {
+        if (!fsStore) {
+            throw Error("check-mount is only supported for local filesystem stores, but '%s' store is specified", lowerStore->getUri());
+        }
         std::smatch match;
         std::string mountInfo;
         auto mounts = readFile(std::filesystem::path{"/proc/self/mounts"});
@@ -45,7 +49,7 @@ LocalOverlayStore::LocalOverlayStore(std::string_view scheme, PathView path, con
             return std::regex_search(mountInfo, std::regex("\\b" + option + "=" + value + "( |,)"));
         };
 
-        auto expectedLowerDir = lowerStore->realStoreDir.get();
+        auto expectedLowerDir = fsStore->realStoreDir.get();
         if (!checkOption("lowerdir", expectedLowerDir) || !checkOption("upperdir", upperLayer)) {
             debug("expected lowerdir: %s", expectedLowerDir);
             debug("expected upperdir: %s", upperLayer);

src/libstore/local-overlay-store.hh

@@ -97,7 +97,7 @@ class LocalOverlayStore : public virtual LocalOverlayStoreConfig, public virtual
      * is that store's store dir, and the upper layer is some
      * scratch storage just for us.
      */
-    ref<LocalFSStore> lowerStore;
+    ref<Store> lowerStore;
 
 public:
     LocalOverlayStore(const Params & params)