Use a global build concurrency semaphore

crates/uv-build-frontend/src/lib.rs

@@ -12,9 +12,8 @@ use std::fmt::Write;
 use std::io;
 use std::path::{Path, PathBuf};
 use std::process::ExitStatus;
-use std::rc::Rc;
 use std::str::FromStr;
-use std::sync::LazyLock;
+use std::sync::{Arc, LazyLock};
 use std::{env, iter};
 
 use fs_err as fs;
@@ -214,11 +213,23 @@ impl Pep517Backend {
     }
 }
 
-/// Uses an [`Rc`] internally, clone freely.
-#[derive(Debug, Default, Clone)]
+/// Uses an [`Arc`] internally, clone freely.
+#[derive(Debug, Clone)]
 pub struct SourceBuildContext {
     /// An in-memory resolution of the default backend's requirements for PEP 517 builds.
-    default_resolution: Rc<Mutex<Option<Resolution>>>,
+    default_resolution: Arc<Mutex<Option<Resolution>>>,
+    /// A shared semaphore to limit the number of concurrent builds.
+    concurrent_build_slots: Arc<Semaphore>,
+}
+
+impl SourceBuildContext {
+    /// Create a [`SourceBuildContext`] with the given concurrent build limit.
+    pub fn new(concurrent_builds: usize) -> Self {
+        Self {
+            default_resolution: Arc::default(),
+            concurrent_build_slots: Arc::new(Semaphore::new(concurrent_builds)),
+        }
+    }
 }
 
 /// Holds the state through a series of PEP 517 frontend to backend calls or a single `setup.py`
@@ -290,7 +301,6 @@ impl SourceBuild {
         build_kind: BuildKind,
         mut environment_variables: FxHashMap<OsString, OsString>,
         level: BuildOutput,
-        concurrent_builds: usize,
         credentials_cache: &CredentialsCache,
     ) -> Result<Self, Error> {
         let temp_dir = build_context.cache().venv_dir()?;
@@ -379,7 +389,7 @@ impl SourceBuild {
 
             let resolved_requirements = Self::get_resolved_requirements(
                 build_context,
-                source_build_context,
+                source_build_context.clone(),
                 &pep517_backend,
                 extra_build_dependencies,
                 build_stack,
@@ -427,7 +437,7 @@ impl SourceBuild {
 
         // Create the PEP 517 build environment. If build isolation is disabled, we assume the build
         // environment is already setup.
-        let runner = PythonRunner::new(concurrent_builds, level);
+        let runner = PythonRunner::new(source_build_context.concurrent_build_slots.clone(), level);
         if build_isolation.is_isolated(package_name.as_ref()) {
             debug!("Creating PEP 517 build environment");
 
@@ -1095,7 +1105,7 @@ async fn create_pep517_build_environment(
 /// concurrency limit.
 #[derive(Debug)]
 struct PythonRunner {
-    control: Semaphore,
+    concurrent_build_slots: Arc<Semaphore>,
     level: BuildOutput,
 }
 
@@ -1107,10 +1117,10 @@ struct PythonRunnerOutput {
 }
 
 impl PythonRunner {
-    /// Create a `PythonRunner` with the provided concurrency limit and output level.
-    fn new(concurrency: usize, level: BuildOutput) -> Self {
+    /// Create a `PythonRunner` with the provided shared concurrency semaphore and output level.
+    fn new(concurrent_build_slots: Arc<Semaphore>, level: BuildOutput) -> Self {
         Self {
-            control: Semaphore::new(concurrency),
+            concurrent_build_slots,
             level,
         }
     }
@@ -1148,7 +1158,7 @@ impl PythonRunner {
             }
         }
 
-        let _permit = self.control.acquire().await.unwrap();
+        let _permit = self.concurrent_build_slots.acquire().await.unwrap();
 
         let mut child = Command::new(venv.python_executable())
             .args(["-c", script])

crates/uv-configuration/Cargo.toml

@@ -27,6 +27,7 @@ uv-pep508 = { workspace = true, features = ["schemars"] }
 uv-platform-tags = { workspace = true }
 uv-redacted = { workspace = true }
 uv-static = { workspace = true }
+
 clap = { workspace = true, features = ["derive"], optional = true }
 either = { workspace = true }
 fs-err = { workspace = true }

crates/uv-dispatch/src/lib.rs

@@ -147,7 +147,7 @@ impl<'a> BuildDispatch<'a> {
             build_options,
             hasher,
             exclude_newer,
-            source_build_context: SourceBuildContext::default(),
+            source_build_context: SourceBuildContext::new(concurrency.builds),
             build_extra_env_vars: FxHashMap::default(),
             sources,
             workspace_cache,
@@ -489,7 +489,6 @@ impl BuildContext for BuildDispatch<'_> {
             build_kind,
             environment_variables,
             build_output,
-            self.concurrency.builds,
             self.client.credentials_cache(),
         )
         .boxed_local()