From 2f5a545997034c531f136e794d0f55eaa0269c07 Mon Sep 17 00:00:00 2001
From: Eelco Dolstra <edolstra@gmail.com>
Date: Wed, 11 Jun 2025 16:13:40 +0200
Subject: [PATCH] fetchToStore(): Improve caching in dry-run mode

In dry-run mode, we don't need to require a valid path.
---
 src/libfetchers/cache.cc                      | 7 ++++---
 src/libfetchers/fetch-to-store.cc             | 2 +-
 src/libfetchers/include/nix/fetchers/cache.hh | 5 +++--
 3 files changed, 8 insertions(+), 6 deletions(-)

diff --git a/src/libfetchers/cache.cc b/src/libfetchers/cache.cc
index d369d213f..9e339134b 100644
--- a/src/libfetchers/cache.cc
+++ b/src/libfetchers/cache.cc
@@ -122,7 +122,8 @@ struct CacheImpl : Cache
 
     std::optional<ResultWithStorePath> lookupStorePath(
         Key key,
-        Store & store) override
+        Store & store,
+        bool allowInvalid) override
     {
         key.second.insert_or_assign("store", store.storeDir);
 
@@ -135,7 +136,7 @@ struct CacheImpl : Cache
         ResultWithStorePath res2(*res, StorePath(storePathS));
 
         store.addTempRoot(res2.storePath);
-        if (!store.isValidPath(res2.storePath)) {
+        if (!allowInvalid && !store.isValidPath(res2.storePath)) {
             // FIXME: we could try to substitute 'storePath'.
             debug("ignoring disappeared cache entry '%s:%s' -> '%s'",
                 key.first,
@@ -157,7 +158,7 @@ struct CacheImpl : Cache
         Key key,
         Store & store) override
     {
-        auto res = lookupStorePath(std::move(key), store);
+        auto res = lookupStorePath(std::move(key), store, false);
         return res && !res->expired ? res : std::nullopt;
     }
 };
diff --git a/src/libfetchers/fetch-to-store.cc b/src/libfetchers/fetch-to-store.cc
index d00c2edfb..9a861a11d 100644
--- a/src/libfetchers/fetch-to-store.cc
+++ b/src/libfetchers/fetch-to-store.cc
@@ -35,7 +35,7 @@ StorePath fetchToStore(
 
     if (!filter && (fingerprint = path.accessor->getFingerprint(path.path))) {
         cacheKey = makeFetchToStoreCacheKey(std::string{name}, *fingerprint, method, path.path.abs());
-        if (auto res = fetchers::getCache()->lookupStorePath(*cacheKey, store)) {
+        if (auto res = fetchers::getCache()->lookupStorePath(*cacheKey, store, mode == FetchMode::DryRun)) {
             debug("store path cache hit for '%s'", path);
             return res->storePath;
         }
diff --git a/src/libfetchers/include/nix/fetchers/cache.hh b/src/libfetchers/include/nix/fetchers/cache.hh
index 5b9319d77..4be6b2095 100644
--- a/src/libfetchers/include/nix/fetchers/cache.hh
+++ b/src/libfetchers/include/nix/fetchers/cache.hh
@@ -76,11 +76,12 @@ struct Cache
 
     /**
      * Look up a store path in the cache. The returned store path will
-     * be valid, but it may be expired.
+     * be valid (unless `allowInvalid` is true), but it may be expired.
      */
     virtual std::optional<ResultWithStorePath> lookupStorePath(
         Key key,
-        Store & store) = 0;
+        Store & store,
+        bool allowInvalid = false) = 0;
 
     /**
      * Look up a store path in the cache. Return nothing if its TTL