From 9d87ab1dc8b3e200c01e04e5fb6c8381b9a04301 Mon Sep 17 00:00:00 2001
From: Eelco Dolstra <edolstra@gmail.com>
Date: Thu, 24 Apr 2025 16:03:49 +0200
Subject: [PATCH] Add a setting to enable lazy trees

---
 src/libexpr/include/nix/expr/eval-settings.hh |  5 +++++
 src/libexpr/paths.cc                          | 11 +++++------
 tests/functional/flakes/flakes.sh             |  3 ++-
 tests/functional/flakes/unlocked-override.sh  |  3 ++-
 4 files changed, 14 insertions(+), 8 deletions(-)

diff --git a/src/libexpr/include/nix/expr/eval-settings.hh b/src/libexpr/include/nix/expr/eval-settings.hh
index fb482568a..6e5bbca20 100644
--- a/src/libexpr/include/nix/expr/eval-settings.hh
+++ b/src/libexpr/include/nix/expr/eval-settings.hh
@@ -247,6 +247,11 @@ struct EvalSettings : Config
 
           This option can be enabled by setting `NIX_ABORT_ON_WARN=1` in the environment.
         )"};
+
+    Setting<bool> lazyTrees{this, false, "lazy-trees",
+        R"(
+          If set to true, flakes and trees fetched by [`builtins.fetchTree`](@docroot@/language/builtins.md#builtins-fetchTree) are only copied to the Nix store when they're used as a dependency of a derivation. This avoids copying (potentially large) source trees unnecessarily.
+        )"};
 };
 
 /**
diff --git a/src/libexpr/paths.cc b/src/libexpr/paths.cc
index 8e1c68e9a..451962636 100644
--- a/src/libexpr/paths.cc
+++ b/src/libexpr/paths.cc
@@ -70,7 +70,8 @@ std::string EvalState::computeBaseName(const SourcePath & path)
 StorePath EvalState::mountInput(
     fetchers::Input & input, const fetchers::Input & originalInput, ref<SourceAccessor> accessor, bool requireLockable)
 {
-    auto storePath = StorePath::random(input.getName());
+    auto storePath = settings.lazyTrees ? StorePath::random(input.getName())
+                                        : fetchToStore(*store, accessor, FetchMode::Copy, input.getName());
 
     allowPath(storePath); // FIXME: should just whitelist the entire virtual store
 
@@ -81,11 +82,9 @@ StorePath EvalState::mountInput(
         input.attrs.insert_or_assign("narHash", narHash.to_string(HashFormat::SRI, true));
     }
 
-    // FIXME: check NAR hash
-
-#if 0
-    assert(!originalInput.getNarHash() || storePath == originalInput.computeStorePath(*store));
-#endif
+    // FIXME: what to do with the NAR hash in lazy mode?
+    if (!settings.lazyTrees)
+        assert(!originalInput.getNarHash() || storePath == originalInput.computeStorePath(*store));
 
     return storePath;
 }
diff --git a/tests/functional/flakes/flakes.sh b/tests/functional/flakes/flakes.sh
index 0fcdf0b30..c8cd5f138 100755
--- a/tests/functional/flakes/flakes.sh
+++ b/tests/functional/flakes/flakes.sh
@@ -75,7 +75,8 @@ hash1=$(echo "$json" | jq -r .revision)
 
 echo foo > "$flake1Dir/foo"
 git -C "$flake1Dir" add $flake1Dir/foo
-[[ $(_NIX_TEST_FAIL_ON_LARGE_PATH=1 nix flake metadata flake1 --json --refresh --warn-large-path-threshold 1 | jq -r .dirtyRevision) == "$hash1-dirty" ]]
+[[ $(nix flake metadata flake1 --json --refresh | jq -r .dirtyRevision) == "$hash1-dirty" ]]
+#[[ $(_NIX_TEST_FAIL_ON_LARGE_PATH=1 nix flake metadata flake1 --json --refresh --warn-large-path-threshold 1 | jq -r .dirtyRevision) == "$hash1-dirty" ]]
 [[ "$(nix flake metadata flake1 --json | jq -r .fingerprint)" != null ]]
 
 echo -n '# foo' >> "$flake1Dir/flake.nix"
diff --git a/tests/functional/flakes/unlocked-override.sh b/tests/functional/flakes/unlocked-override.sh
index 9d8d569f1..73784b4e8 100755
--- a/tests/functional/flakes/unlocked-override.sh
+++ b/tests/functional/flakes/unlocked-override.sh
@@ -35,7 +35,8 @@ echo 456 > "$flake1Dir"/x.nix
 expectStderr 1 nix flake lock "$flake2Dir" --override-input flake1 "$TEST_ROOT/flake1" |
   grepQuiet "Will not write lock file.*because it has an unlocked input"
 
-_NIX_TEST_FAIL_ON_LARGE_PATH=1 nix flake lock "$flake2Dir" --override-input flake1 "$TEST_ROOT/flake1" --allow-dirty-locks --warn-large-path-threshold 1
+nix flake lock "$flake2Dir" --override-input flake1 "$TEST_ROOT/flake1" --allow-dirty-locks
+#_NIX_TEST_FAIL_ON_LARGE_PATH=1 nix flake lock "$flake2Dir" --override-input flake1 "$TEST_ROOT/flake1" --allow-dirty-locks --warn-large-path-threshold 1
 
 # Using a lock file with a dirty lock does not require --allow-dirty-locks, but should print a warning.
 expectStderr 0 nix eval "$flake2Dir#x" |