From 6477d7c2ca3ff84f9d1b502d4d62bddfe3b77b0f Mon Sep 17 00:00:00 2001
From: Eelco Dolstra <edolstra@gmail.com>
Date: Thu, 12 Jun 2025 16:11:54 +0200
Subject: [PATCH] mountInput(): Optimize getting the NAR hash for real store
 paths

---
 src/libexpr/paths.cc | 10 +++++++---
 1 file changed, 7 insertions(+), 3 deletions(-)

diff --git a/src/libexpr/paths.cc b/src/libexpr/paths.cc
index d85f00470..bdf817e3b 100644
--- a/src/libexpr/paths.cc
+++ b/src/libexpr/paths.cc
@@ -81,9 +81,13 @@ StorePath EvalState::mountInput(
 
     auto getNarHash = [&]()
     {
-        if (!_narHash)
-            // FIXME: use fetchToStore to make it cache this
-            _narHash = accessor->hashPath(CanonPath::root);
+        if (!_narHash) {
+            if (store->isValidPath(storePath))
+                _narHash = store->queryPathInfo(storePath)->narHash;
+            else
+                // FIXME: use fetchToStore to make it cache this
+                _narHash = accessor->hashPath(CanonPath::root);
+        }
         return _narHash;
     };