Lock the toolchains directory during toolchain operations (#4733)

crates/uv-toolchain/src/managed.rs

@@ -19,7 +19,7 @@ use crate::platform::{Arch, Libc, Os};
 use crate::python_version::PythonVersion;
 use crate::toolchain::{self, ToolchainKey};
 use crate::ToolchainRequest;
-use uv_fs::Simplified;
+use uv_fs::{LockedFile, Simplified};
 
 #[derive(Error, Debug)]
 pub enum Error {
@@ -65,6 +65,14 @@ impl InstalledToolchains {
         Self { root: root.into() }
     }
 
+    /// Lock the toolchains directory.
+    pub fn acquire_lock(&self) -> Result<LockedFile, Error> {
+        Ok(LockedFile::acquire(
+            self.root.join(".lock"),
+            self.root.user_display(),
+        )?)
+    }
+
     /// Prefer, in order:
     /// 1. The specific toolchain directory specified by the user, i.e., `UV_TOOLCHAIN_DIR`
     /// 2. A directory in the system-appropriate user-level data directory, e.g., `~/.local/uv/toolchains`
@@ -224,6 +232,7 @@ impl InstalledToolchain {
         Ok(Self { path, key })
     }
 
+    /// The path to this toolchain's Python executable.
     pub fn executable(&self) -> PathBuf {
         if cfg!(windows) {
             self.path.join("install").join("python.exe")
@@ -234,6 +243,7 @@ impl InstalledToolchain {
         }
     }
 
+    /// The [`PythonVersion`] of the toolchain.
     pub fn version(&self) -> PythonVersion {
         self.key.version()
     }

crates/uv-toolchain/src/toolchain.rs

@@ -122,6 +122,7 @@ impl Toolchain {
     ) -> Result<Self, Error> {
         let toolchains = InstalledToolchains::from_settings()?.init()?;
         let toolchain_dir = toolchains.root();
+        let _lock = toolchains.acquire_lock()?;
 
         let download = PythonDownload::from_request(&request)?;
         let client = client_builder.build();

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

@@ -31,6 +31,7 @@ pub(crate) async fn install(
 
     let toolchains = InstalledToolchains::from_settings()?.init()?;
     let toolchain_dir = toolchains.root();
+    let _lock = toolchains.acquire_lock()?;
 
     let requests: Vec<_> = if targets.is_empty() {
         if let Some(requests) = requests_from_version_file().await? {

crates/uv/src/commands/toolchain/uninstall.rs

@@ -23,6 +23,7 @@ pub(crate) async fn uninstall(
     }
 
     let toolchains = InstalledToolchains::from_settings()?.init()?;
+    let _lock = toolchains.acquire_lock()?;
 
     let requests = targets
         .iter()