Build source distributions at top-level of cache (#8905)

## Summary

See: https://github.com/astral-sh/uv/issues/8884. We build in a
directory that's deep within the cache; to help with file name length
limits, we should build at the top-level of the cache.

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

@@ -21,7 +21,7 @@ use std::rc::Rc;
 use std::str::FromStr;
 use std::sync::LazyLock;
 use std::{env, iter};
-use tempfile::{tempdir_in, TempDir};
+use tempfile::TempDir;
 use tokio::io::AsyncBufReadExt;
 use tokio::process::Command;
 use tokio::sync::{Mutex, Semaphore};
@@ -30,7 +30,7 @@ use tracing::{debug, info_span, instrument, Instrument};
 use uv_configuration::{BuildKind, BuildOutput, ConfigSettings, LowerBound, SourceStrategy};
 use uv_distribution::RequiresDist;
 use uv_distribution_types::{IndexLocations, Resolution};
-use uv_fs::{rename_with_retry, PythonExt, Simplified};
+use uv_fs::{PythonExt, Simplified};
 use uv_pep440::Version;
 use uv_pep508::PackageName;
 use uv_pypi_types::{Requirement, VerbatimParsedUrl};
@@ -656,16 +656,7 @@ impl SourceBuild {
     pub async fn build(&self, wheel_dir: &Path) -> Result<String, Error> {
         // The build scripts run with the extracted root as cwd, so they need the absolute path.
         let wheel_dir = std::path::absolute(wheel_dir)?;
-
-        // Prevent clashes from two uv processes building distributions in parallel.
-        let tmp_dir = tempdir_in(&wheel_dir)?;
-        let filename = self
-            .pep517_build(tmp_dir.path(), &self.pep517_backend)
-            .await?;
-
-        let from = tmp_dir.path().join(&filename);
-        let to = wheel_dir.join(&filename);
-        rename_with_retry(from, to).await?;
+        let filename = self.pep517_build(&wheel_dir, &self.pep517_backend).await?;
         Ok(filename)
     }
 

crates/uv-distribution/src/source/mod.rs

@@ -1751,6 +1751,13 @@ impl<'a, T: BuildContext> SourceDistributionBuilder<'a, T> {
             }
         }
 
+        // Build into a temporary directory, to prevent partial builds.
+        let build = self
+            .build_context
+            .cache()
+            .environment()
+            .map_err(Error::CacheWrite)?;
+
         // Build the wheel.
         fs::create_dir_all(&cache_shard)
             .await
@@ -1773,10 +1780,18 @@ impl<'a, T: BuildContext> SourceDistributionBuilder<'a, T> {
             )
             .await
             .map_err(Error::Build)?
-            .wheel(cache_shard)
+            .wheel(build.path())
             .await
             .map_err(Error::Build)?;
 
+        // Move the wheel to the cache.
+        rename_with_retry(
+            build.path().join(&disk_filename),
+            cache_shard.join(&disk_filename),
+        )
+        .await
+        .map_err(Error::CacheWrite)?;
+
         // Read the metadata from the wheel.
         let filename = WheelFilename::from_str(&disk_filename)?;
         let metadata = read_wheel_metadata(&filename, &cache_shard.join(&disk_filename))?;