Consolidate logic for checking for a virtual environment (#14214)

We were checking whether a path was an executable in a virtual
environment or the base directory of a virtual environment in multiple
places in the codebase. This PR consolidates this logic into one place.

Closes #13947.

crates/uv-fs/src/lib.rs

@@ -575,6 +575,30 @@ pub fn is_temporary(path: impl AsRef<Path>) -> bool {
         .is_some_and(|name| name.starts_with(".tmp"))
 }
 
+/// Checks if the grandparent directory of the given executable is the base
+/// of a virtual environment.
+///
+/// The procedure described in PEP 405 includes checking both the parent and
+/// grandparent directory of an executable, but in practice we've found this to
+/// be unnecessary.
+pub fn is_virtualenv_executable(executable: impl AsRef<Path>) -> bool {
+    executable
+        .as_ref()
+        .parent()
+        .and_then(Path::parent)
+        .is_some_and(is_virtualenv_base)
+}
+
+/// Returns `true` if a path is the base path of a virtual environment,
+/// indicated by the presence of a `pyvenv.cfg` file.
+///
+/// The procedure described in PEP 405 includes scanning `pyvenv.cfg`
+/// for a `home` key, but in practice we've found this to be
+/// unnecessary.
+pub fn is_virtualenv_base(path: impl AsRef<Path>) -> bool {
+    path.as_ref().join("pyvenv.cfg").is_file()
+}
+
 /// A file lock that is automatically released when dropped.
 #[derive(Debug)]
 pub struct LockedFile(fs_err::File);

crates/uv-python/src/discovery.rs

@@ -888,13 +888,8 @@ impl Error {
                 | InterpreterError::BrokenSymlink(BrokenSymlink { path, .. }) => {
                     // If the interpreter is from an active, valid virtual environment, we should
                     // fail because it's broken
-                    if let Some(Ok(true)) = matches!(source, PythonSource::ActiveEnvironment)
-                        .then(|| {
-                            path.parent()
-                                .and_then(Path::parent)
-                                .map(|path| path.join("pyvenv.cfg").try_exists())
-                        })
-                        .flatten()
+                    if matches!(source, PythonSource::ActiveEnvironment)
+                        && uv_fs::is_virtualenv_executable(path)
                     {
                         true
                     } else {

crates/uv-python/src/interpreter.rs

@@ -993,14 +993,9 @@ impl InterpreterInfo {
                         .symlink_metadata()
                         .is_ok_and(|metadata| metadata.is_symlink())
                     {
-                        let venv = executable
-                            .parent()
-                            .and_then(Path::parent)
-                            .map(|path| path.join("pyvenv.cfg").is_file())
-                            .unwrap_or(false);
                         Error::BrokenSymlink(BrokenSymlink {
                             path: executable.to_path_buf(),
-                            venv,
+                            venv: uv_fs::is_virtualenv_executable(executable),
                         })
                     } else {
                         Error::NotFound(executable.to_path_buf())

crates/uv-python/src/virtualenv.rs

@@ -130,14 +130,14 @@ pub(crate) fn virtualenv_from_working_dir() -> Result<Option<PathBuf>, Error> {
 
     for dir in current_dir.ancestors() {
         // If we're _within_ a virtualenv, return it.
-        if dir.join("pyvenv.cfg").is_file() {
+        if uv_fs::is_virtualenv_base(dir) {
             return Ok(Some(dir.to_path_buf()));
         }
 
         // Otherwise, search for a `.venv` directory.
         let dot_venv = dir.join(".venv");
         if dot_venv.is_dir() {
-            if !dot_venv.join("pyvenv.cfg").is_file() {
+            if !uv_fs::is_virtualenv_base(&dot_venv) {
                 return Err(Error::MissingPyVenvCfg(dot_venv));
             }
             return Ok(Some(dot_venv));

crates/uv-virtualenv/src/virtualenv.rs

@@ -85,7 +85,7 @@ pub(crate) fn create(
             } else if metadata.is_dir() {
                 if allow_existing {
                     debug!("Allowing existing directory");
-                } else if location.join("pyvenv.cfg").is_file() {
+                } else if uv_fs::is_virtualenv_base(location) {
                     debug!("Removing existing directory");
 
                     // On Windows, if the current executable is in the directory, guard against