Allow fallback to Python download on non-critical discovery errors (#10908)

Closes #10898

In #10716, I broke fallback to downloading Python versions by throwing a
different error kind.

Author: zanieb

crates/uv-python/src/installation.rs

@@ -92,36 +92,48 @@ impl PythonInstallation {
         let request = request.unwrap_or(&PythonRequest::Default);
 
         // Search for the installation
-        match Self::find(request, environments, preference, cache) {
-            Ok(venv) => Ok(venv),
-            // If missing and allowed, perform a fetch
-            Err(Error::MissingPython(err))
-                if preference.allows_managed()
-                    && python_downloads.is_automatic()
-                    && client_builder.connectivity.is_online() =>
-            {
-                if let Some(request) = PythonDownloadRequest::from_request(request) {
-                    debug!("Requested Python not found, checking for available download...");
-                    match Self::fetch(
-                        request.fill()?,
-                        client_builder,
-                        cache,
-                        reporter,
-                        python_install_mirror,
-                        pypy_install_mirror,
-                    )
-                    .await
-                    {
-                        Ok(installation) => Ok(installation),
-                        Err(Error::Download(downloads::Error::NoDownloadFound(_))) => {
-                            Err(Error::MissingPython(err))
-                        }
-                        Err(err) => Err(err),
-                    }
-                } else {
-                    Err(Error::MissingPython(err))
-                }
-            }
+        let err = match Self::find(request, environments, preference, cache) {
+            Ok(installation) => return Ok(installation),
+            Err(err) => err,
+        };
+
+        let downloads_enabled = preference.allows_managed()
+            && python_downloads.is_automatic()
+            && client_builder.connectivity.is_online();
+
+        if !downloads_enabled {
+            return Err(err);
+        }
+
+        match err {
+            // If Python is missing, we should attempt a download
+            Error::MissingPython(_) => {}
+            // If we raised a non-critical error, we should attempt a download
+            Error::Discovery(ref err) if !err.is_critical() => {}
+            // Otherwise, this is fatal
+            _ => return Err(err),
+        }
+
+        // If we can't convert the request to a download, throw the original error
+        let Some(request) = PythonDownloadRequest::from_request(request) else {
+            return Err(err);
+        };
+
+        debug!("Requested Python not found, checking for available download...");
+        match Self::fetch(
+            request.fill()?,
+            client_builder,
+            cache,
+            reporter,
+            python_install_mirror,
+            pypy_install_mirror,
+        )
+        .await
+        {
+            Ok(installation) => Ok(installation),
+            // Throw the original error if we couldn't find a download
+            Err(Error::Download(downloads::Error::NoDownloadFound(_))) => Err(err),
+            // But if the download failed, throw that error
             Err(err) => Err(err),
         }
     }

crates/uv/tests/it/python_install.rs

@@ -188,16 +188,12 @@ fn python_install_automatic() {
             .env("UV_TEST_PYTHON_PATH", context.bin_dir.as_os_str())
             .arg("-p").arg("3.11")
             .arg("python").arg("-c").arg("import sys; print(sys.version_info[:2])"), @r###"
-        success: false
-        exit_code: 2
+        success: true
+        exit_code: 0
         ----- stdout -----
+        (3, 11)
 
         ----- stderr -----
-        error: Failed to inspect Python interpreter from search path at `[BIN]/python3`
-          Caused by: Querying Python at `[BIN]/python3` failed with exit status exit status: 1
-
-        [stderr]
-        error: intentionally broken python executable
         "###);
     }
 }