Use an extra layer of ephemeral deps

Author: charliermarsh

crates/uv-python/src/environment.rs

@@ -174,7 +174,7 @@ impl PythonEnvironment {
     /// N.B. This function also works for system Python environments and users depend on this.
     pub fn from_root(root: impl AsRef<Path>, cache: &Cache) -> Result<Self, Error> {
         debug!(
-            "Checking for Python environment at `{}`",
+            "Checking for Python environment at: `{}`",
             root.as_ref().user_display()
         );
         match root.as_ref().try_exists() {

crates/uv/src/commands/project/environment.rs

@@ -17,6 +17,69 @@ use crate::commands::project::{
 use crate::printer::Printer;
 use crate::settings::{NetworkSettings, ResolverInstallerSettings};
 
+/// An ephemeral [`PythonEnvironment`] for running an individual command.
+#[derive(Debug)]
+pub(crate) struct EphemeralEnvironment(PythonEnvironment);
+
+impl From<PythonEnvironment> for EphemeralEnvironment {
+    fn from(environment: PythonEnvironment) -> Self {
+        Self(environment)
+    }
+}
+
+impl From<EphemeralEnvironment> for PythonEnvironment {
+    fn from(environment: EphemeralEnvironment) -> Self {
+        environment.0
+    }
+}
+
+impl EphemeralEnvironment {
+    /// Set the ephemeral overlay for a Python environment.
+    #[allow(clippy::result_large_err)]
+    pub(crate) fn set_overlay(&self, contents: impl AsRef<[u8]>) -> Result<(), ProjectError> {
+        let site_packages = self
+            .0
+            .site_packages()
+            .next()
+            .ok_or(ProjectError::NoSitePackages)?;
+        let overlay_path = site_packages.join("_uv_ephemeral_overlay.pth");
+        fs_err::write(overlay_path, contents)?;
+        Ok(())
+    }
+
+    /// Enable system site packages for a Python environment.
+    #[allow(clippy::result_large_err)]
+    pub(crate) fn set_system_site_packages(&self) -> Result<(), ProjectError> {
+        self.0
+            .set_pyvenv_cfg("include-system-site-packages", "true")?;
+        Ok(())
+    }
+
+    /// Set the `extends-environment` key in the `pyvenv.cfg` file to the given path.
+    ///
+    /// Ephemeral environments created by `uv run --with` extend a parent (virtual or system)
+    /// environment by adding a `.pth` file to the ephemeral environment's `site-packages`
+    /// directory. The `pth` file contains Python code to dynamically add the parent
+    /// environment's `site-packages` directory to Python's import search paths in addition to
+    /// the ephemeral environment's `site-packages` directory. This works well at runtime, but
+    /// is too dynamic for static analysis tools like ty to understand. As such, we
+    /// additionally write the `sys.prefix` of the parent environment to to the
+    /// `extends-environment` key of the ephemeral environment's `pyvenv.cfg` file, making it
+    /// easier for these tools to statically and reliably understand the relationship between
+    /// the two environments.
+    #[allow(clippy::result_large_err)]
+    pub(crate) fn set_parent_environment(
+        &self,
+        parent_environment_sys_prefix: &Path,
+    ) -> Result<(), ProjectError> {
+        self.0.set_pyvenv_cfg(
+            "extends-environment",
+            &parent_environment_sys_prefix.escape_for_python(),
+        )?;
+        Ok(())
+    }
+}
+
 /// A [`PythonEnvironment`] stored in the cache.
 #[derive(Debug)]
 pub(crate) struct CachedEnvironment(PythonEnvironment);
@@ -44,15 +107,13 @@ impl CachedEnvironment {
         printer: Printer,
         preview: PreviewMode,
     ) -> Result<Self, ProjectError> {
-        // Resolve the "base" interpreter, which resolves to an underlying parent interpreter if the
-        // given interpreter is a virtual environment.
-        let base_interpreter = Self::base_interpreter(interpreter, cache)?;
+        let interpreter = Self::base_interpreter(interpreter, cache)?;
 
         // Resolve the requirements with the interpreter.
         let resolution = Resolution::from(
             resolve_environment(
                 spec,
-                &base_interpreter,
+                &interpreter,
                 build_constraints.clone(),
                 &settings.resolver,
                 network_settings,
@@ -80,29 +141,20 @@ impl CachedEnvironment {
         // Use the canonicalized base interpreter path since that's the interpreter we performed the
         // resolution with and the interpreter the environment will be created with.
         //
-        // We also include the canonicalized `sys.prefix` of the non-base interpreter, that is, the
-        // virtual environment's path. Originally, we shared cached environments independent of the
-        // environment they'd be layered on top of. However, this causes collisions as the overlay
-        // `.pth` file can be overridden by another instance of uv. Including this element in the key
-        // avoids this problem at the cost of creating separate cached environments for identical
-        // `--with` invocations across projects. We use `sys.prefix` rather than `sys.executable` so
-        // we can canonicalize it without invalidating the purpose of the element — it'd probably be
-        // safe to just use the absolute `sys.executable` as well.
-        //
-        // TODO(zanieb): Since we're not sharing these environmments across projects, we should move
-        // [`CachedEvnvironment::set_overlay`] etc. here since the values there should be constant
-        // now.
+        // We cache environments independent of the environment they'd be layered on top of. The
+        // assumption is such that the environment will _not_ be modified by the user or uv;
+        // otherwise, we risk cache poisoning. For example, if we were to write a `.pth` file to
+        // the cached environment, it would be shared across all projects that use the same
+        // interpreter and the same cached dependencies.
         //
         // TODO(zanieb): We should include the version of the base interpreter in the hash, so if
         // the interpreter at the canonicalized path changes versions we construct a new
         // environment.
-        let environment_hash = cache_digest(&(
-            &canonicalize_executable(base_interpreter.sys_executable())?,
-            &interpreter.sys_prefix().canonicalize()?,
-        ));
+        let interpreter_hash =
+            cache_digest(&canonicalize_executable(interpreter.sys_executable())?);
 
         // Search in the content-addressed cache.
-        let cache_entry = cache.entry(CacheBucket::Environments, environment_hash, resolution_hash);
+        let cache_entry = cache.entry(CacheBucket::Environments, interpreter_hash, resolution_hash);
 
         if cache.refresh().is_none() {
             if let Ok(root) = cache.resolve_link(cache_entry.path()) {
@@ -116,7 +168,7 @@ impl CachedEnvironment {
         let temp_dir = cache.venv_dir()?;
         let venv = uv_virtualenv::create_venv(
             temp_dir.path(),
-            base_interpreter,
+            interpreter,
             uv_virtualenv::Prompt::None,
             false,
             false,
@@ -150,76 +202,6 @@ impl CachedEnvironment {
         Ok(Self(PythonEnvironment::from_root(root, cache)?))
     }
 
-    /// Set the ephemeral overlay for a Python environment.
-    #[allow(clippy::result_large_err)]
-    pub(crate) fn set_overlay(&self, contents: impl AsRef<[u8]>) -> Result<(), ProjectError> {
-        let site_packages = self
-            .0
-            .site_packages()
-            .next()
-            .ok_or(ProjectError::NoSitePackages)?;
-        let overlay_path = site_packages.join("_uv_ephemeral_overlay.pth");
-        fs_err::write(overlay_path, contents)?;
-        Ok(())
-    }
-
-    /// Clear the ephemeral overlay for a Python environment, if it exists.
-    #[allow(clippy::result_large_err)]
-    pub(crate) fn clear_overlay(&self) -> Result<(), ProjectError> {
-        let site_packages = self
-            .0
-            .site_packages()
-            .next()
-            .ok_or(ProjectError::NoSitePackages)?;
-        let overlay_path = site_packages.join("_uv_ephemeral_overlay.pth");
-        match fs_err::remove_file(overlay_path) {
-            Ok(()) => (),
-            Err(err) if err.kind() == std::io::ErrorKind::NotFound => (),
-            Err(err) => return Err(ProjectError::OverlayRemoval(err)),
-        }
-        Ok(())
-    }
-
-    /// Enable system site packages for a Python environment.
-    #[allow(clippy::result_large_err)]
-    pub(crate) fn set_system_site_packages(&self) -> Result<(), ProjectError> {
-        self.0
-            .set_pyvenv_cfg("include-system-site-packages", "true")?;
-        Ok(())
-    }
-
-    /// Disable system site packages for a Python environment.
-    #[allow(clippy::result_large_err)]
-    pub(crate) fn clear_system_site_packages(&self) -> Result<(), ProjectError> {
-        self.0
-            .set_pyvenv_cfg("include-system-site-packages", "false")?;
-        Ok(())
-    }
-
-    /// Set the `extends-environment` key in the `pyvenv.cfg` file to the given path.
-    ///
-    /// Ephemeral environments created by `uv run --with` extend a parent (virtual or system)
-    /// environment by adding a `.pth` file to the ephemeral environment's `site-packages`
-    /// directory. The `pth` file contains Python code to dynamically add the parent
-    /// environment's `site-packages` directory to Python's import search paths in addition to
-    /// the ephemeral environment's `site-packages` directory. This works well at runtime, but
-    /// is too dynamic for static analysis tools like ty to understand. As such, we
-    /// additionally write the `sys.prefix` of the parent environment to the
-    /// `extends-environment` key of the ephemeral environment's `pyvenv.cfg` file, making it
-    /// easier for these tools to statically and reliably understand the relationship between
-    /// the two environments.
-    #[allow(clippy::result_large_err)]
-    pub(crate) fn set_parent_environment(
-        &self,
-        parent_environment_sys_prefix: &Path,
-    ) -> Result<(), ProjectError> {
-        self.0.set_pyvenv_cfg(
-            "extends-environment",
-            &parent_environment_sys_prefix.escape_for_python(),
-        )?;
-        Ok(())
-    }
-
     /// Return the [`Interpreter`] to use for the cached environment, based on a given
     /// [`Interpreter`].
     ///

crates/uv/src/commands/project/mod.rs

@@ -200,9 +200,6 @@ pub(crate) enum ProjectError {
     #[error("Failed to parse PEP 723 script metadata")]
     Pep723ScriptTomlParse(#[source] toml::de::Error),
 
-    #[error("Failed to remove ephemeral overlay")]
-    OverlayRemoval(#[source] std::io::Error),
-
     #[error("Failed to find `site-packages` directory for environment")]
     NoSitePackages,
 

crates/uv/src/commands/project/run.rs

@@ -45,7 +45,7 @@ use crate::commands::pip::loggers::{
     DefaultInstallLogger, DefaultResolveLogger, SummaryInstallLogger, SummaryResolveLogger,
 };
 use crate::commands::pip::operations::Modifications;
-use crate::commands::project::environment::CachedEnvironment;
+use crate::commands::project::environment::{CachedEnvironment, EphemeralEnvironment};
 use crate::commands::project::install_target::InstallTarget;
 use crate::commands::project::lock::LockMode;
 use crate::commands::project::lock_target::LockTarget;
@@ -944,15 +944,15 @@ hint: If you are running a script with `{}` in the shebang, you may need to incl
 
     // If necessary, create an environment for the ephemeral requirements or command.
     let base_site_packages = SitePackages::from_interpreter(&base_interpreter)?;
-    let ephemeral_env = match spec {
+    let requirements_env = match spec {
         None => None,
         Some(spec)
             if can_skip_ephemeral(&spec, &base_interpreter, &base_site_packages, &settings) =>
         {
             None
         }
         Some(spec) => {
-            debug!("Syncing ephemeral requirements");
+            debug!("Syncing `--with` requirements to cached environment");
 
             // Read the build constraints from the lock file.
             let build_constraints = base_lock
@@ -1013,54 +1013,92 @@ hint: If you are running a script with `{}` in the shebang, you may need to incl
                 Err(err) => return Err(err.into()),
             };
 
-            Some(environment)
+            Some(PythonEnvironment::from(environment))
         }
     };
 
-    // If we're running in an ephemeral environment, add a path file to enable loading of
-    // the base environment's site packages. Setting `PYTHONPATH` is insufficient, as it doesn't
-    // resolve `.pth` files in the base environment.
+    // If we're layering requirements atop the project environment, run the command in an ephemeral,
+    // isolated environment. Otherwise, modifications to the "active virtual environment" would
+    // poison the cache.
+    let ephemeral_dir = requirements_env
+        .as_ref()
+        .map(|_| cache.venv_dir())
+        .transpose()?;
+
+    let ephemeral_env = ephemeral_dir
+        .as_ref()
+        .map(|dir| {
+            debug!(
+                "Creating ephemeral environment at: `{}`",
+                dir.path().simplified_display()
+            );
+
+            uv_virtualenv::create_venv(
+                dir.path(),
+                base_interpreter.clone(),
+                uv_virtualenv::Prompt::None,
+                false,
+                false,
+                false,
+                false,
+                false,
+                preview,
+            )
+        })
+        .transpose()?
+        .map(EphemeralEnvironment::from);
+
+    // If we're running in an ephemeral environment, add a path file to enable loading from the
+    // `--with` requirements environment and the project environment site packages.
     //
-    // `sitecustomize.py` would be an alternative, but it can be shadowed by an existing such
-    // module in the python installation.
+    // Setting `PYTHONPATH` is insufficient, as it doesn't resolve `.pth` files in the base
+    // environment. Adding `sitecustomize.py` would be an alternative, but it can be shadowed by an
+    // existing such module in the python installation.
     if let Some(ephemeral_env) = ephemeral_env.as_ref() {
-        let site_packages = base_interpreter
-            .site_packages()
-            .next()
-            .ok_or_else(|| ProjectError::NoSitePackages)?;
-        ephemeral_env.set_overlay(format!(
-            "import site; site.addsitedir(\"{}\")",
-            site_packages.escape_for_python()
-        ))?;
-
-        // Write the `sys.prefix` of the parent environment to the `extends-environment` key of the `pyvenv.cfg`
-        // file. This helps out static-analysis tools such as ty (see docs on
-        // `CachedEnvironment::set_parent_environment`).
-        //
-        // Note that we do this even if the parent environment is not a virtual environment.
-        // For ephemeral environments created by `uv run --with`, the parent environment's
-        // `site-packages` directory is added to `sys.path` even if the parent environment is not
-        // a virtual environment and even if `--system-site-packages` was not explicitly selected.
-        ephemeral_env.set_parent_environment(base_interpreter.sys_prefix())?;
-
-        // If `--system-site-packages` is enabled, add the system site packages to the ephemeral
-        // environment.
-        if base_interpreter.is_virtualenv()
-            && PyVenvConfiguration::parse(base_interpreter.sys_prefix().join("pyvenv.cfg"))
-                .is_ok_and(|cfg| cfg.include_system_site_packages())
-        {
-            ephemeral_env.set_system_site_packages()?;
-        } else {
-            ephemeral_env.clear_system_site_packages()?;
+        if let Some(requirements_env) = requirements_env.as_ref() {
+            let requirements_site_packages =
+                requirements_env.site_packages().next().ok_or_else(|| {
+                    anyhow!("Requirements environment has no site packages directory")
+                })?;
+            let base_site_packages = base_interpreter
+                .site_packages()
+                .next()
+                .ok_or_else(|| anyhow!("Base environment has no site packages directory"))?;
+
+            ephemeral_env.set_overlay(format!(
+                "import site; site.addsitedir(\"{}\"); site.addsitedir(\"{}\");",
+                base_site_packages.escape_for_python(),
+                requirements_site_packages.escape_for_python(),
+            ))?;
+
+            // Write the `sys.prefix` of the parent environment to the `extends-environment` key of the `pyvenv.cfg`
+            // file. This helps out static-analysis tools such as ty (see docs on
+            // `CachedEnvironment::set_parent_environment`).
+            //
+            // Note that we do this even if the parent environment is not a virtual environment.
+            // For ephemeral environments created by `uv run --with`, the parent environment's
+            // `site-packages` directory is added to `sys.path` even if the parent environment is not
+            // a virtual environment and even if `--system-site-packages` was not explicitly selected.
+            ephemeral_env.set_parent_environment(base_interpreter.sys_prefix())?;
+
+            // If `--system-site-packages` is enabled, add the system site packages to the ephemeral
+            // environment.
+            if base_interpreter.is_virtualenv()
+                && PyVenvConfiguration::parse(base_interpreter.sys_prefix().join("pyvenv.cfg"))
+                    .is_ok_and(|cfg| cfg.include_system_site_packages())
+            {
+                ephemeral_env.set_system_site_packages()?;
+            }
         }
     }
 
-    // Cast from `CachedEnvironment` to `PythonEnvironment`.
+    // Cast to `PythonEnvironment`.
     let ephemeral_env = ephemeral_env.map(PythonEnvironment::from);
 
     // Determine the Python interpreter to use for the command, if necessary.
     let interpreter = ephemeral_env
         .as_ref()
+        .or(requirements_env.as_ref())
         .map_or_else(|| &base_interpreter, |env| env.interpreter());
 
     // Check if any run command is given.
@@ -1143,6 +1181,12 @@ hint: If you are running a script with `{}` in the shebang, you may need to incl
             .as_ref()
             .map(PythonEnvironment::scripts)
             .into_iter()
+            .chain(
+                requirements_env
+                    .as_ref()
+                    .map(PythonEnvironment::scripts)
+                    .into_iter(),
+            )
             .chain(std::iter::once(base_interpreter.scripts()))
             .chain(
                 // On Windows, non-virtual Python distributions put `python.exe` in the top-level