Avoid trying to fetch the same project in parallel with a map of mutexes

crates/brioche-core/src/project.rs

@@ -985,6 +985,17 @@ async fn fetch_project_from_registry(
     brioche: &Brioche,
     project_hash: ProjectHash,
 ) -> anyhow::Result<PathBuf> {
+    // Use a mutex to ensure we don't try to fetch the same project more
+    // than once at a time
+    static FETCH_PROJECTS_MUTEX: tokio::sync::Mutex<
+        BTreeMap<ProjectHash, Arc<tokio::sync::Mutex<()>>>,
+    > = tokio::sync::Mutex::const_new(BTreeMap::new());
+    let project_mutex = {
+        let mut fetch_projects = FETCH_PROJECTS_MUTEX.lock().await;
+        fetch_projects.entry(project_hash).or_default().clone()
+    };
+    let _guard = project_mutex.lock().await;
+
     let local_path = brioche.home.join("projects").join(project_hash.to_string());
 
     if tokio::fs::try_exists(&local_path).await? {