copyPaths(): Don't query path info for a path the target already has

For example, this cuts "nix-copy-closure --from" on a NixOS system
closure from 15.9s to 0.5s.

1 files changed, 1 insertions, 0 deletions

diff --git a/src/libstore/store-api.cc b/src/libstore/store-api.cc
index 9c755965e45b..9ab3f36329b4 100644
--- a/src/libstore/store-api.cc
+++ b/src/libstore/store-api.cc
@@ -822,6 +822,7 @@ void copyPaths(ref<Store> from, ref<Store> to, const Paths & storePaths, bool su
         PathSet(storePaths.begin(), storePaths.end()),
 
         [&](const Path & storePath) {
+            if (to->isValidPath(storePath)) return PathSet();
             return from->queryPathInfo(storePath)->references;
         },