Treat non-existent site-packages as empty (#2413)

## Summary

It turns out this doesn't need to exist until something has been
installed into it. See, e.g., #2402.

Closes #2404.

.github/workflows/ci.yml

@@ -480,3 +480,32 @@ jobs:
       - name: "Validate global Python install"
         shell: bash -el {0}
         run: python ./scripts/check_system_python.py --uv ./uv
+
+  system-test-amazonlinux:
+    needs: build-binary-linux
+    name: "check system | amazonlinux"
+    runs-on: ubuntu-latest
+    container: amazonlinux:2023
+    steps:
+      - name: "Install base requirements"
+        run: |
+          # Needed for `actions/checkout`
+          yum install tar gzip which -y
+      - uses: actions/checkout@v4
+
+      - name: "Install Python"
+        run: yum install python3 python3-pip -y
+
+      - name: "Download binary"
+        uses: actions/download-artifact@v4
+        with:
+          name: uv-linux-${{ github.sha }}
+
+      - name: "Prepare binary"
+        run: chmod +x ./uv
+
+      - name: "Print Python path"
+        run: echo $(which python3)
+
+      - name: "Validate global Python install"
+        run: python3 scripts/check_system_python.py --uv ./uv

crates/uv-dispatch/src/lib.rs

@@ -182,8 +182,7 @@ impl<'a> BuildContext for BuildDispatch<'a> {
             let tags = self.interpreter.tags()?;
 
             // Determine the set of installed packages.
-            let site_packages =
-                SitePackages::from_executable(venv).context("Failed to list installed packages")?;
+            let site_packages = SitePackages::from_executable(venv)?;
 
             let Plan {
                 local,

crates/uv-installer/src/site_packages.rs

@@ -42,8 +42,22 @@ impl<'a> SitePackages<'a> {
         let mut by_name = FxHashMap::default();
         let mut by_url = FxHashMap::default();
 
+        // Read the site-packages directory.
+        let site_packages = match fs::read_dir(venv.site_packages()) {
+            Ok(site_packages) => site_packages,
+            Err(err) if err.kind() == std::io::ErrorKind::NotFound => {
+                return Ok(Self {
+                    venv,
+                    distributions,
+                    by_name,
+                    by_url,
+                });
+            }
+            Err(err) => return Err(err).context("Failed to read site-packages directory"),
+        };
+
         // Index all installed packages by name.
-        for entry in fs::read_dir(venv.site_packages())? {
+        for entry in site_packages {
             let entry = entry?;
             if entry.file_type()?.is_dir() {
                 let path = entry.path();

crates/uv/src/commands/pip_install.rs

@@ -143,8 +143,7 @@ pub(crate) async fn pip_install(
     let _lock = venv.lock()?;
 
     // Determine the set of installed packages.
-    let site_packages =
-        SitePackages::from_executable(&venv).context("Failed to list installed packages")?;
+    let site_packages = SitePackages::from_executable(&venv)?;
 
     // If the requirements are already satisfied, we're done. Ideally, the resolver would be fast
     // enough to let us remove this check. But right now, for large environments, it's an order of

crates/uv/src/commands/pip_sync.rs

@@ -157,8 +157,7 @@ pub(crate) async fn pip_sync(
     );
 
     // Determine the set of installed packages.
-    let site_packages =
-        SitePackages::from_executable(&venv).context("Failed to list installed packages")?;
+    let site_packages = SitePackages::from_executable(&venv)?;
 
     // Resolve any editables.
     let resolved_editables = resolve_editables(