Create default Store::narFromPath implementation in terms of getFSAccessor

This is a good default (the methods that allow for an arbitrary choice
of source accessor are generally preferable both to implement and to
use). And it also pays its way by allowing us to delete *both* the
`DummyStore` and `LocalStore` implementations.

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();