Avoid deadlocks when multiple uv processes lock resources (#6790)

This is achieved by updating the `LockedFile::acquire` API to be async —
as in some cases we were attempting to acquire the lock synchronously,
i.e., without yielding, which blocked the runtime.

Closes https://github.com/astral-sh/uv/issues/6691 — I tested with the
reproduction there and a local release build and no longer reproduce the
deadlock with these changes.

Some additional context in the [internal Discord
thread](1278478941)

crates/uv-fs/Cargo.toml

@@ -24,6 +24,7 @@ fs-err = { workspace = true }
 fs2 = { workspace = true }
 path-slash = { workspace = true }
 serde = { workspace = true, optional = true }
+tokio = { workspace = true, optional = true}
 tempfile = { workspace = true }
 tracing = { workspace = true }
 urlencoding = { workspace = true }
@@ -33,4 +34,4 @@ junction = { workspace = true }
 
 [features]
 default = []
-tokio = ["fs-err/tokio", "backoff/tokio"]
+tokio = ["dep:tokio", "fs-err/tokio", "backoff/tokio"]

crates/uv-fs/src/lib.rs

@@ -314,8 +314,8 @@ pub fn is_temporary(path: impl AsRef<Path>) -> bool {
 pub struct LockedFile(fs_err::File);
 
 impl LockedFile {
-    pub fn acquire(path: impl AsRef<Path>, resource: impl Display) -> Result<Self, std::io::Error> {
-        let file = fs_err::File::create(path.as_ref())?;
+    /// Inner implementation for [`LockedFile::acquire_blocking`] and [`LockedFile::acquire`].
+    fn lock_file_blocking(file: fs_err::File, resource: &str) -> Result<Self, std::io::Error> {
         trace!("Checking lock for `{resource}`");
         match file.file().try_lock_exclusive() {
             Ok(()) => {
@@ -328,19 +328,42 @@ impl LockedFile {
                 warn_user!(
                     "Waiting to acquire lock for {} (lockfile: {})",
                     resource,
-                    path.user_display(),
+                    file.path().user_display(),
                 );
                 file.file().lock_exclusive().map_err(|err| {
                     // Not an fs_err method, we need to build our own path context
                     std::io::Error::new(
                         std::io::ErrorKind::Other,
-                        format!("Could not lock {}: {}", path.as_ref().user_display(), err),
+                        format!("Could not lock {}: {}", file.path().user_display(), err),
                     )
                 })?;
                 Ok(Self(file))
             }
         }
     }
+
+    /// The same as [`LockedFile::acquire`], but for synchronous contexts. Do not use from an async
+    /// context, as this can block the runtime while waiting for another process to release the
+    /// lock.
+    pub fn acquire_blocking(
+        path: impl AsRef<Path>,
+        resource: impl Display,
+    ) -> Result<Self, std::io::Error> {
+        let file = fs_err::File::create(path.as_ref())?;
+        let resource = resource.to_string();
+        Self::lock_file_blocking(file, &resource)
+    }
+
+    /// Acquire a cross-process lock for a resource using a file at the provided path.
+    #[cfg(feature = "tokio")]
+    pub async fn acquire(
+        path: impl AsRef<Path>,
+        resource: impl Display,
+    ) -> Result<Self, std::io::Error> {
+        let file = fs_err::File::create(path.as_ref())?;
+        let resource = resource.to_string();
+        tokio::task::spawn_blocking(move || Self::lock_file_blocking(file, &resource)).await?
+    }
 }
 
 impl Drop for LockedFile {