fetchToStore(): Avoid duplicate copying if the input is already a sto…

…re path

This is needed for the path:// input scheme (until it returns a
FSInputAccessor to the original path, but that's blocked by NixOS#10089)
and the Mercurial input scheme.

src/libfetchers/fetch-to-store.cc

@@ -1,6 +1,7 @@
 #include "fetch-to-store.hh"
 #include "fetchers.hh"
 #include "cache.hh"
+#include "posix-source-accessor.hh"
 
 namespace nix {
 
@@ -13,8 +14,16 @@ StorePath fetchToStore(
     PathFilter * filter,
     RepairFlag repair)
 {
-    // FIXME: add an optimisation for the case where the accessor is
-    // an FSInputAccessor pointing to a store path.
+    if (path.accessor->isStorePath
+        && path.path.isRoot()
+        && method == FileIngestionMethod::Recursive
+        && !filter)
+    {
+        if (auto accessor = path.accessor.dynamic_pointer_cast<PosixSourceAccessor>())
+            if (auto storePath = store.maybeParseStorePath(accessor->root.string()))
+                if (storePath->name() == name)
+                    return *storePath;
+    }
 
     std::optional<fetchers::Attrs> cacheKey;
 
@@ -52,5 +61,4 @@ StorePath fetchToStore(
     return storePath;
 }
 
-
 }

src/libfetchers/path.cc

@@ -155,7 +155,12 @@ struct PathInputScheme : InputScheme
         }
         input.attrs.insert_or_assign("lastModified", uint64_t(mtime));
 
-        return {makeStorePathAccessor(store, *storePath), std::move(input)};
+        auto accessor = makeStorePathAccessor(store, *storePath);
+
+        /* Optimize fetchToStore() calls on this path. */
+        accessor->isStorePath = true;
+
+        return {accessor, std::move(input)};
     }
 
     std::optional<std::string> getFingerprint(ref<Store> store, const Input & input) const override

src/libfetchers/unix/mercurial.cc

@@ -356,6 +356,9 @@ struct MercurialInputScheme : InputScheme
 
         accessor->setPathDisplay("«" + input.to_string() + "»");
 
+        /* Optimize fetchToStore() calls on this path. */
+        accessor->isStorePath = true;
+
         return {accessor, input};
     }
 

src/libutil/input-accessor.hh

@@ -13,6 +13,13 @@ struct InputAccessor : virtual SourceAccessor, std::enable_shared_from_this<Inpu
 {
     std::optional<std::string> fingerprint;
 
+    /**
+     * Whether this is a store path using
+     * FileIngestionMethod::Recursive. This is used to optimize
+     * `fetchToStore()`.
+     */
+    bool isStorePath = false;
+
     /**
      * Return the maximum last-modified time of the files in this
      * tree, if available.