test(duckdb): fix extension loading tests to support parallel execution

ibis/backends/duckdb/tests/test_client.py

@@ -8,13 +8,27 @@
 from ibis.conftest import LINUX, SANDBOXED
 
 
+@pytest.fixture(scope="session")
+def ext_directory(tmpdir_factory):
+    # A session-scoped temp directory to cache extension downloads per session.
+    # Coupled with the xdist_group below, this ensures that the extension
+    # loading tests always run in the same process and a common temporary
+    # directory isolated from other duckdb tests, avoiding issues with
+    # downloading extensions in parallel.
+    return str(tmpdir_factory.mktemp("exts"))
+
+
 @pytest.mark.xfail(
     LINUX and SANDBOXED,
     reason="nix on linux cannot download duckdb extensions or data due to sandboxing",
     raises=sa.exc.OperationalError,
 )
-def test_connect_extensions():
-    con = ibis.duckdb.connect(extensions=["s3", "sqlite"])
+@pytest.mark.xdist_group(name="duckdb-extensions")
+def test_connect_extensions(ext_directory):
+    con = ibis.duckdb.connect(
+        extensions=["s3", "sqlite"],
+        extension_directory=ext_directory,
+    )
     results = con.raw_sql(
         """
         SELECT loaded FROM duckdb_extensions()
@@ -29,8 +43,9 @@ def test_connect_extensions():
     reason="nix on linux cannot download duckdb extensions or data due to sandboxing",
     raises=duckdb.IOException,
 )
-def test_load_extension():
-    con = ibis.duckdb.connect()
+@pytest.mark.xdist_group(name="duckdb-extensions")
+def test_load_extension(ext_directory):
+    con = ibis.duckdb.connect(extension_directory=ext_directory)
     con.load_extension("s3")
     con.load_extension("sqlite")
     results = con.raw_sql(