Fix deletion test

Lower layer references are ignored for deleting just in the upper layer.

src/libstore/gc.cc

@@ -741,7 +741,7 @@ void LocalStore::collectGarbage(const GCOptions & options, GCResults & results)
                 auto i = referrersCache.find(*path);
                 if (i == referrersCache.end()) {
                     StorePathSet referrers;
-                    queryReferrers(*path, referrers);
+                    queryGCReferrers(*path, referrers);
                     referrersCache.emplace(*path, std::move(referrers));
                     i = referrersCache.find(*path);
                 }

src/libstore/local-overlay-store.cc

@@ -138,6 +138,12 @@ void LocalOverlayStore::queryReferrers(const StorePath & path, StorePathSet & re
 }
 
 
+void LocalOverlayStore::queryGCReferrers(const StorePath & path, StorePathSet & referrers)
+{
+    LocalStore::queryReferrers(path, referrers);
+}
+
+
 StorePathSet LocalOverlayStore::queryValidDerivers(const StorePath & path)
 {
     auto res = LocalStore::queryValidDerivers(path);
@@ -188,6 +194,7 @@ void LocalOverlayStore::deleteGCPath(const Path & path, uint64_t & bytesFreed)
     }
 }
 
+
 void LocalOverlayStore::optimiseStore()
 {
     Activity act(*logger, actOptimiseStore);
@@ -216,6 +223,7 @@ Path LocalOverlayStore::toRealPathForHardLink(const StorePath & path)
         : Store::toRealPath(path);
 }
 
+
 static RegisterStoreImplementation<LocalOverlayStore, LocalOverlayStoreConfig> regLocalOverlayStore;
 
 }

src/libstore/local-overlay-store.hh

@@ -115,7 +115,16 @@ private:
 
     void optimiseStore() override;
 
+    /**
+     * For lower-store paths, we used the lower store location. This avoids the
+     * wasteful "copying up" that would otherwise happen.
+     */
     Path toRealPathForHardLink(const StorePath & storePath) override;
+
+    /**
+     * Deletion only effects the upper layer, so we ignore lower-layer referrers.
+     */
+    void queryGCReferrers(const StorePath & path, StorePathSet & referrers) override;
 };
 
 }

src/libstore/local-store.hh

@@ -230,6 +230,18 @@ public:
 
     void collectGarbage(const GCOptions & options, GCResults & results) override;
 
+    /**
+     * Called by `collectGarbage` to trace in reverse.
+     *
+     * Using this rather than `queryReferrers` directly allows us to
+     * fine-tune which referrers we consider for garbage collection;
+     * some store implementations take advantage of this.
+     */
+    virtual void queryGCReferrers(const StorePath & path, StorePathSet & referrers)
+    {
+        return queryReferrers(path, referrers);
+    }
+
     /**
      * Called by `collectGarbage` to recursively delete a path.
      * The default implementation simply calls `deletePath`, but it can be