Always add the Pip dependencies layer bin directory to PATH (#232)

Previously the `bin` directory would only be added to `PATH` if it
existed (due to it being set by `lifecycle`), which is only the case
if one of the installed dependencies has an entry point script, which
causes Pip to create the `bin` directory and place a wrapper script
inside it for that package's CLI command.

This meant:
1. We had to suppress Pip's warning during install (since Pip isn't
  to know that the `PATH` will be correctly set later)
2. If an app has no dependencies with an entry-point (so the app
  image doesn't have a `bin` directory), then at run-time tries to
  pip install a new package that does have an entry point (eg when
  debugging), then that new script won't be on `PATH`.

As such, we now add the `bin` directory to PATH explicitly, instead
of relying on `lifecycle` automatic addition.

CHANGELOG.md

@@ -11,6 +11,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 - Stopped manually creating a `src` directory inside the Pip dependencies layer. Pip will create the directory itself if needed (when there are editable VCS dependencies). ([#228](https://github.com/heroku/buildpacks-python/pull/228))
 - Stopped setting `CPATH` and `PKG_CONFIG_PATH` at launch time. ([#231](https://github.com/heroku/buildpacks-python/pull/231))
+- The `bin` directory in the Pip dependencies layer is now always added to `PATH` instead of only when an installed dependency has an entry point script. ([#232](https://github.com/heroku/buildpacks-python/pull/232))
 
 ## [0.12.1] - 2024-07-15
 

src/layers/pip_dependencies.rs

@@ -64,9 +64,6 @@ impl Layer for PipDependenciesLayer<'_> {
                     "--cache-dir",
                     &self.pip_cache_dir.to_string_lossy(),
                     "--no-input",
-                    // Prevent warning about the `bin/` directory not being on `PATH`, since it
-                    // will be added automatically by libcnb/lifecycle later.
-                    "--no-warn-script-location",
                     "--progress",
                     "off",
                     // Install dependencies into the user `site-packages` directory (set by `PYTHONUSERBASE`),
@@ -110,6 +107,16 @@ impl Layer for PipDependenciesLayer<'_> {
 /// Environment variables that will be set by this layer.
 fn generate_layer_env(layer_path: &Path) -> LayerEnv {
     LayerEnv::new()
+        // We set `PATH` explicitly, since lifecycle will only add the bin directory to `PATH` if it
+        // exists - and we want to support the scenario of installing a debugging package with CLI at
+        // run-time, when none of the dependencies installed at build-time had an entrypoint script.
+        .chainable_insert(
+            Scope::All,
+            ModificationBehavior::Prepend,
+            "PATH",
+            layer_path.join("bin"),
+        )
+        .chainable_insert(Scope::All, ModificationBehavior::Delimiter, "PATH", ":")
         // `PYTHONUSERBASE` overrides the default user base directory, which is used by Python to
         // compute the path of the user `site-packages` directory:
         // https://docs.python.org/3/using/cmdline.html#envvar-PYTHONUSERBASE
@@ -149,17 +156,24 @@ mod tests {
     #[test]
     fn pip_dependencies_layer_env() {
         let mut base_env = Env::new();
+        base_env.insert("PATH", "/base");
         base_env.insert("PYTHONUSERBASE", "this-should-be-overridden");
 
         let layer_env = generate_layer_env(Path::new("/layer-dir"));
 
         assert_eq!(
             utils::environment_as_sorted_vector(&layer_env.apply(Scope::Build, &base_env)),
-            [("PYTHONUSERBASE", "/layer-dir")]
+            [
+                ("PATH", "/layer-dir/bin:/base"),
+                ("PYTHONUSERBASE", "/layer-dir"),
+            ]
         );
         assert_eq!(
             utils::environment_as_sorted_vector(&layer_env.apply(Scope::Launch, &base_env)),
-            [("PYTHONUSERBASE", "/layer-dir")]
+            [
+                ("PATH", "/layer-dir/bin:/base"),
+                ("PYTHONUSERBASE", "/layer-dir"),
+            ]
         );
     }
 }

tests/pip_test.rs

@@ -61,7 +61,7 @@ fn pip_basic_install_and_cache_reuse() {
             &formatdoc! {"
                 LANG=C.UTF-8
                 LD_LIBRARY_PATH=/layers/heroku_python/python/lib:/layers/heroku_python/dependencies/lib
-                PATH=/layers/heroku_python/python/bin:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin
+                PATH=/layers/heroku_python/dependencies/bin:/layers/heroku_python/python/bin:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin
                 PIP_DISABLE_PIP_VERSION_CHECK=1
                 PYTHONHOME=/layers/heroku_python/python
                 PYTHONUNBUFFERED=1