Explicitly configure uv to use hard links (#519)

Since in newer versions of uv (specifically uv 0.10.5+, which we haven't
yet updated to) the default link mode has changed from `hardlink` to
`clone` (aka reflink), and reflinks are slower in some environments
(such as Heroku's build system).

See:
astral-sh/uv#18259

Even if the uv default ends up being changed back to hard links again, it
seems worth us explicitly requesting that link mode, given that we know
it's the best mode for our use-case.

GUS-W-21429394.

Author: edmorley

CHANGELOG.md

@@ -10,6 +10,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 ### Changed
 
 - Updated pip from 25.3 to 26.0.1. ([#508](https://github.com/heroku/buildpacks-python/pull/508))
+- Explicitly configured uv to use hard links to maintain the behaviour of previous versions. ([#519](https://github.com/heroku/buildpacks-python/pull/519))
 
 ## [5.1.0] - 2026-02-26
 

src/layers/uv.rs

@@ -39,9 +39,9 @@ pub(crate) fn install_uv(
         },
     )?;
 
-    // Prevent uv from downloading/using its own Python installation:
-    // https://docs.astral.sh/uv/concepts/python-versions/#disabling-automatic-python-downloads
     let mut layer_env = LayerEnv::new()
+        // Prevent uv from downloading/using its own Python installation:
+        // https://docs.astral.sh/uv/concepts/python-versions/#disabling-automatic-python-downloads
         .chainable_insert(
             Scope::Build,
             ModificationBehavior::Override,
@@ -53,6 +53,14 @@ pub(crate) fn install_uv(
             ModificationBehavior::Override,
             "UV_PYTHON_DOWNLOADS",
             "never",
+        )
+        // Force uv to use hardlinks rather than the new default of reflinks, since the latter are
+        // significantly slower in some environments: https://github.com/astral-sh/uv/issues/18259
+        .chainable_insert(
+            Scope::Build,
+            ModificationBehavior::Override,
+            "UV_LINK_MODE",
+            "hardlink",
         );
 
     match layer.state {

tests/uv_test.rs

@@ -50,6 +50,7 @@ fn uv_basic_install_and_cache_reuse() {
                 PYTHONUNBUFFERED=1
                 SOURCE_DATE_EPOCH=315532801
                 UV_CACHE_DIR=/layers/heroku_python/uv-cache
+                UV_LINK_MODE=hardlink
                 UV_NO_MANAGED_PYTHON=1
                 UV_PROJECT_ENVIRONMENT=/layers/heroku_python/venv
                 UV_PYTHON_DOWNLOADS=never