Ensure mtime of site packages is updated during wheel installation (#2545)

Closes https://github.com/astral-sh/uv/issues/2530

crates/install-wheel-rs/src/linker.rs

@@ -3,6 +3,7 @@
 
 use std::path::Path;
 use std::str::FromStr;
+use std::time::SystemTime;
 
 use fs_err as fs;
 use fs_err::{DirEntry, File};
@@ -263,6 +264,31 @@ fn clone_wheel_files(
         count += 1;
     }
 
+    // The directory mtime is not updated when cloning and the mtime is used by CPython's
+    // import mechanisms to determine if it should look for new packages in a directory.
+    // Here, we force the mtime to be updated to ensure that packages are importable without
+    // manual cache invalidation.
+    //
+    // <https://github.com/python/cpython/blob/8336cb2b6f428246803b02a4e97fce49d0bb1e09/Lib/importlib/_bootstrap_external.py#L1601>
+    let now = SystemTime::now();
+
+    // `File.set_modified` is not available in `fs_err` yet
+    #[allow(clippy::disallowed_types)]
+    match std::fs::File::open(site_packages.as_ref()) {
+        Ok(dir) => {
+            if let Err(err) = dir.set_modified(now) {
+                debug!(
+                    "Failed to update mtime for {}: {err}",
+                    site_packages.as_ref().display()
+                );
+            }
+        }
+        Err(err) => debug!(
+            "Failed to open {} to update mtime: {err}",
+            site_packages.as_ref().display()
+        ),
+    }
+
     Ok(count)
 }
 

crates/uv/tests/pip_install.rs

@@ -1,13 +1,12 @@
 #![cfg(all(feature = "python", feature = "pypi"))]
 
-use std::process::Command;
-
 use anyhow::Result;
 use assert_cmd::prelude::*;
 use assert_fs::prelude::*;
 use base64::{prelude::BASE64_STANDARD as base64, Engine};
 use indoc::indoc;
 use itertools::Itertools;
+use std::process::Command;
 use url::Url;
 
 use common::{uv_snapshot, TestContext, EXCLUDE_NEWER, INSTA_FILTERS};
@@ -2843,3 +2842,48 @@ fn install_index_with_relative_links_authenticated() {
 
     context.assert_command("import anyio").success();
 }
+
+/// The modified time of `site-packages` should change on package installation.
+#[cfg(unix)]
+#[test]
+fn install_site_packages_mtime_updated() -> Result<()> {
+    use std::os::unix::fs::MetadataExt;
+
+    let context = TestContext::new("3.12");
+
+    let site_packages = context.site_packages();
+
+    // `mtime` is only second-resolution so we include the nanoseconds as well
+    let metadata = site_packages.metadata()?;
+    let pre_mtime = metadata.mtime();
+    let pre_mtime_ns = metadata.mtime_nsec();
+
+    // Install a package.
+    uv_snapshot!(command(&context)
+        .arg("anyio")
+        .arg("--strict"), @r###"
+    success: true
+    exit_code: 0
+    ----- stdout -----
+
+    ----- stderr -----
+    Resolved 3 packages in [TIME]
+    Downloaded 3 packages in [TIME]
+    Installed 3 packages in [TIME]
+     + anyio==4.0.0
+     + idna==3.4
+     + sniffio==1.3.0
+    "###
+    );
+
+    let metadata = site_packages.metadata()?;
+    let post_mtime = metadata.mtime();
+    let post_mtime_ns = metadata.mtime_nsec();
+
+    assert!(
+        (post_mtime, post_mtime_ns) > (pre_mtime, pre_mtime_ns),
+        "Expected newer mtime than {pre_mtime}.{pre_mtime_ns} but got {post_mtime}.{post_mtime_ns}"
+    );
+
+    Ok(())
+}