Two misc cleanups

src/libstore/dummy-store.cc

@@ -258,18 +258,6 @@ struct DummyStoreImpl : DummyStore
         });
     }
 
-    void narFromPath(const StorePath & path, Sink & sink) override
-    {
-        bool visited = contents.cvisit(path, [&](const auto & kv) {
-            const auto & [info, accessor] = kv.second;
-            SourcePath sourcePath(accessor);
-            dumpPath(sourcePath, sink, FileSerialisationMethod::NixArchive);
-        });
-
-        if (!visited)
-            throw Error("path '%s' is not valid", printStorePath(path));
-    }
-
     void queryRealisationUncached(
         const DrvOutput & drvOutput, Callback<std::shared_ptr<const UnkeyedRealisation>> callback) noexcept override
     {

src/libstore/include/nix/store/local-fs-store.hh

@@ -78,7 +78,6 @@ struct LocalFSStore : virtual Store, virtual GcStore, virtual LogStore
 
     LocalFSStore(const Config & params);
 
-    void narFromPath(const StorePath & path, Sink & sink) override;
     ref<SourceAccessor> getFSAccessor(bool requireValidPath = true) override;
     std::shared_ptr<SourceAccessor> getFSAccessor(const StorePath & path, bool requireValidPath = true) override;
 

src/libstore/include/nix/store/store-api.hh

@@ -609,7 +609,7 @@ public:
     /**
      * Write a NAR dump of a store path.
      */
-    virtual void narFromPath(const StorePath & path, Sink & sink) = 0;
+    virtual void narFromPath(const StorePath & path, Sink & sink);
 
     /**
      * For each path, if it's a derivation, build it.  Building a

src/libstore/include/nix/store/uds-remote-store.hh

@@ -68,7 +68,7 @@ struct UDSRemoteStore : virtual IndirectRootStore, virtual RemoteStore
 
     void narFromPath(const StorePath & path, Sink & sink) override
     {
-        LocalFSStore::narFromPath(path, sink);
+        Store::narFromPath(path, sink);
     }
 
     /**

src/libstore/local-fs-store.cc

@@ -112,13 +112,6 @@ std::shared_ptr<SourceAccessor> LocalFSStore::getFSAccessor(const StorePath & pa
     return std::make_shared<PosixSourceAccessor>(std::move(absPath));
 }
 
-void LocalFSStore::narFromPath(const StorePath & path, Sink & sink)
-{
-    if (!isValidPath(path))
-        throw Error("path '%s' is not valid", printStorePath(path));
-    dumpPath(getRealStoreDir() + std::string(printStorePath(path), storeDir.size()), sink);
-}
-
 const std::string LocalFSStore::drvsLogDir = "drvs";
 
 std::optional<std::string> LocalFSStore::getBuildLogExact(const StorePath & path)

src/libstore/restricted-store.cc

@@ -226,7 +226,7 @@ void RestrictedStore::narFromPath(const StorePath & path, Sink & sink)
 {
     if (!goal.isAllowed(path))
         throw InvalidPath("cannot dump unknown path '%s' in recursive Nix", printStorePath(path));
-    LocalFSStore::narFromPath(path, sink);
+    Store::narFromPath(path, sink);
 }
 
 void RestrictedStore::ensurePath(const StorePath & path)

src/libstore/ssh-store.cc

@@ -143,7 +143,7 @@ struct MountedSSHStore : virtual SSHStore, virtual LocalFSStore
 
     void narFromPath(const StorePath & path, Sink & sink) override
     {
-        return LocalFSStore::narFromPath(path, sink);
+        return Store::narFromPath(path, sink);
     }
 
     ref<SourceAccessor> getFSAccessor(bool requireValidPath) override

src/libstore/store-api.cc

@@ -300,6 +300,13 @@ ValidPathInfo Store::addToStoreSlow(
     return info;
 }
 
+void Store::narFromPath(const StorePath & path, Sink & sink)
+{
+    auto accessor = requireStoreObjectAccessor(path);
+    SourcePath sourcePath{accessor};
+    dumpPath(sourcePath, sink, FileSerialisationMethod::NixArchive);
+}
+
 StringSet Store::Config::getDefaultSystemFeatures()
 {
     auto res = settings.systemFeatures.get();

src/libutil/include/nix/util/ref.hh

@@ -17,22 +17,32 @@ private:
 
     std::shared_ptr<T> p;
 
+    void assertNonNull()
+    {
+        if (!p)
+            throw std::invalid_argument("null pointer cast to ref");
+    }
+
 public:
 
     using element_type = T;
 
     explicit ref(const std::shared_ptr<T> & p)
         : p(p)
     {
-        if (!p)
-            throw std::invalid_argument("null pointer cast to ref");
+        assertNonNull();
+    }
+
+    explicit ref(std::shared_ptr<T> && p)
+        : p(std::move(p))
+    {
+        assertNonNull();
     }
 
     explicit ref(T * p)
         : p(p)
     {
-        if (!p)
-            throw std::invalid_argument("null pointer cast to ref");
+        assertNonNull();
     }
 
     T * operator->() const
@@ -45,14 +55,22 @@ public:
         return *p;
     }
 
-    operator std::shared_ptr<T>() const
+    std::shared_ptr<T> get_ptr() const &
     {
         return p;
     }
 
-    std::shared_ptr<T> get_ptr() const
+    std::shared_ptr<T> get_ptr() &&
     {
-        return p;
+        return std::move(p);
+    }
+
+    /**
+     * Convenience to avoid explicit `get_ptr()` call in some cases.
+     */
+    operator std::shared_ptr<T>(this auto && self)
+    {
+        return std::forward<decltype(self)>(self).get_ptr();
     }
 
     template<typename T2>