create cache entry for paths already in the nix store

This allows path:/nix/store/* paths to not be copied twice to the nix
store.

(cherry picked from commit 61c6210dbf)

src/libfetchers/fetch-to-store.cc

@@ -4,6 +4,21 @@
 
 namespace nix {
 
+fetchers::Cache::Key makeFetchToStoreCacheKey(
+    const std::string &name,
+    const std::string &fingerprint,
+    ContentAddressMethod method,
+    const std::string &path)
+{
+    return fetchers::Cache::Key{"fetchToStore", {
+        {"name", name},
+        {"fingerprint", fingerprint},
+        {"method", std::string{method.render()}},
+        {"path", path}
+    }};
+
+}
+
 StorePath fetchToStore(
     Store & store,
     const SourcePath & path,
@@ -19,12 +34,7 @@ StorePath fetchToStore(
     std::optional<fetchers::Cache::Key> cacheKey;
 
     if (!filter && path.accessor->fingerprint) {
-        cacheKey = fetchers::Cache::Key{"fetchToStore", {
-            {"name", std::string{name}},
-            {"fingerprint", *path.accessor->fingerprint},
-            {"method", std::string{method.render()}},
-            {"path", path.path.abs()}
-        }};
+        cacheKey = makeFetchToStoreCacheKey(std::string{name}, *path.accessor->fingerprint, method, path.path.abs());
         if (auto res = fetchers::getCache()->lookupStorePath(*cacheKey, store)) {
             debug("store path cache hit for '%s'", path);
             return res->storePath;

src/libfetchers/fetch-to-store.hh

@@ -5,6 +5,7 @@
 #include "file-system.hh"
 #include "repair-flag.hh"
 #include "file-content-address.hh"
+#include "cache.hh"
 
 namespace nix {
 
@@ -22,4 +23,7 @@ StorePath fetchToStore(
     PathFilter * filter = nullptr,
     RepairFlag repair = NoRepair);
 
+fetchers::Cache::Key makeFetchToStoreCacheKey(
+    const std::string & name, const std::string & fingerprint, ContentAddressMethod method, const std::string & path);
+
 }

src/libfetchers/path.cc

@@ -2,6 +2,7 @@
 #include "store-api.hh"
 #include "archive.hh"
 #include "store-path-accessor.hh"
+#include "fetch-to-store.hh"
 
 namespace nix::fetchers {
 
@@ -157,6 +158,15 @@ struct PathInputScheme : InputScheme
             });
             storePath = store->addToStoreFromDump(*src, "source");
         }
+
+        // To avoid copying the path again to the /nix/store, we need to add a cache entry.
+        ContentAddressMethod method = ContentAddressMethod::Raw::NixArchive;
+        auto fp = getFingerprint(store, input);
+        if (fp) {
+            auto cacheKey = makeFetchToStoreCacheKey(input.getName(), *fp, method, "/");
+            fetchers::getCache()->upsert(cacheKey, *store, {}, *storePath);
+        }
+
         input.attrs.insert_or_assign("lastModified", uint64_t(mtime));
 
         return {makeStorePathAccessor(store, *storePath), std::move(input)};