fix: create a more local lock when creating writable fixtures.

Previously, the lock location would block all writers from executing
a fixture even though they wouldn't step on each others feet.

Now, a script destination is used to assure locks are created close
to the destination when creating writable fixtures, typically removing
the need for multiple writers to wait on each other unnecessarily.

tests/tools/src/lib.rs

@@ -509,11 +509,18 @@ fn scripted_fixture_read_only_with_args_inner(
         |d| (true, d.to_owned()),
     );
 
-    let _marker = gix_lock::Marker::acquire_to_hold_resource(
-        script_basename,
-        gix_lock::acquire::Fail::AfterDurationWithBackoff(Duration::from_secs(6 * 60)),
-        None,
-    )?;
+    // We may that destination_dir is already unique (i.e. temp-dir) - thus there is no need for a lock,
+    // and we can execute scripts in parallel.
+    let _marker = destination_dir
+        .is_none()
+        .then(|| {
+            gix_lock::Marker::acquire_to_hold_resource(
+                script_basename,
+                gix_lock::acquire::Fail::AfterDurationWithBackoff(Duration::from_secs(6 * 60)),
+                None,
+            )
+        })
+        .transpose()?;
     let failure_marker = script_result_directory.join("_invalid_state_due_to_script_failure_");
     if force_run || !script_result_directory.is_dir() || failure_marker.is_file() {
         if failure_marker.is_file() {