Replace executables with broken symlinks during uv python install (#9706)

I somehow got in a state where we'd fail to install with

```
error: Failed to install cpython-3.13.0-macos-aarch64-none
  Caused by: Executable already exists at `/Users/zb/.local/bin/python3` but is not managed by uv; use `--force` to replace it
error: Failed to install cpython-3.13.0-macos-aarch64-none
  Caused by: Executable already exists at `/Users/zb/.local/bin/python` but is not managed by uv; use `--force` to replace it
```

but `python` / `python3` _were_ managed by uv, they just were linked to
an installation that was deleted.

This updates the logic to replace broken executables that are broken
symlinks. We apply this to broken links regardless of whether or not we
think the target is managed by uv.

Author: zanieb

crates/uv/src/commands/python/install.rs

@@ -358,32 +358,50 @@ pub(crate) async fn install(
                         target.simplified_display()
                     );
 
+                    // Check if the existing link is valid
+                    let valid_link = target
+                        .read_link()
+                        .and_then(|target| target.try_exists())
+                        .inspect_err(|err| debug!("Failed to inspect executable with error: {err}"))
+                        .unwrap_or(true);
+
                     // Figure out what installation it references, if any
-                    let existing = find_matching_bin_link(
-                        installations
-                            .iter()
-                            .copied()
-                            .chain(existing_installations.iter()),
-                        &target,
-                    );
+                    let existing = valid_link
+                        .then(|| {
+                            find_matching_bin_link(
+                                installations
+                                    .iter()
+                                    .copied()
+                                    .chain(existing_installations.iter()),
+                                &target,
+                            )
+                        })
+                        .flatten();
 
                     match existing {
                         None => {
                             // There's an existing executable we don't manage, require `--force`
-                            if !force {
-                                errors.push((
-                                    installation.key(),
-                                    anyhow::anyhow!(
-                                        "Executable already exists at `{}` but is not managed by uv; use `--force` to replace it",
-                                        to.simplified_display()
-                                    ),
-                                ));
-                                continue;
+                            if valid_link {
+                                if !force {
+                                    errors.push((
+                                        installation.key(),
+                                        anyhow::anyhow!(
+                                            "Executable already exists at `{}` but is not managed by uv; use `--force` to replace it",
+                                            to.simplified_display()
+                                        ),
+                                    ));
+                                    continue;
+                                }
+                                debug!(
+                                    "Replacing existing executable at `{}` due to `--force`",
+                                    target.simplified_display()
+                                );
+                            } else {
+                                debug!(
+                                    "Replacing broken symlink at `{}`",
+                                    target.simplified_display()
+                                );
                             }
-                            debug!(
-                                "Replacing existing executable at `{}` due to `--force`",
-                                target.simplified_display()
-                            );
                         }
                         Some(existing) if existing == *installation => {
                             // The existing link points to the same installation, so we're done unless

crates/uv/tests/it/python_install.rs

@@ -839,3 +839,40 @@ fn python_install_unknown() {
     error: `./foo` is not a valid Python download request; see `uv python help` for supported formats and `uv python list --only-downloads` for available versions
     "###);
 }
+
+#[cfg(unix)]
+#[test]
+fn python_install_preview_broken_link() {
+    use assert_fs::prelude::PathCreateDir;
+    use fs_err::os::unix::fs::symlink;
+
+    let context: TestContext = TestContext::new_with_versions(&[])
+        .with_filtered_python_keys()
+        .with_filtered_exe_suffix();
+
+    let bin_python = context.temp_dir.child("bin").child("python3.13");
+
+    // Create a broken symlink
+    context.temp_dir.child("bin").create_dir_all().unwrap();
+    symlink(context.temp_dir.join("does-not-exist"), &bin_python).unwrap();
+
+    // Install
+    uv_snapshot!(context.filters(), context.python_install().arg("--preview").arg("3.13"), @r###"
+    success: true
+    exit_code: 0
+    ----- stdout -----
+
+    ----- stderr -----
+    Installed Python 3.13.1 in [TIME]
+     + cpython-3.13.1-[PLATFORM] (python3.13)
+    "###);
+
+    // We should replace the broken symlink
+    insta::with_settings!({
+        filters => context.filters(),
+    }, {
+        insta::assert_snapshot!(
+            read_link_path(&bin_python), @"[TEMP_DIR]/managed/cpython-3.13.1-[PLATFORM]/bin/python3.13"
+        );
+    });
+}