bambam-gtfs: safe chunking behavior when N GTFS archives less than parallelism

rust/bambam-gtfs/src/schedule/bundle_ops.rs

@@ -78,7 +78,17 @@ pub fn batch_process(
         .map_err(|e| ScheduleError::GtfsApp(format!("failure reading directory: {e}")))?
         .collect::<Result<Vec<_>, _>>()
         .map_err(|e| ScheduleError::GtfsApp(format!("failure reading directory: {e}")))?;
-    let chunk_size = archive_paths.len() / std::cmp::max(1, parallelism);
+
+    let n_archives = archive_paths.len();
+    if n_archives == 0 {
+        let msg = format!(
+            "directory {} is empty",
+            bundle_directory_path.to_string_lossy()
+        );
+        return Err(ScheduleError::InvalidData(msg));
+    }
+
+    let chunk_size = calculate_chunk_size(n_archives, parallelism);
 
     // a progress bar shared across threads
     let bar: Arc<Mutex<Bar>> = Arc::new(Mutex::new(
@@ -700,3 +710,13 @@ fn archive_intersects_extent(gtfs: &Gtfs, extent: &Geometry) -> Result<bool, Sch
     }
     Ok(false)
 }
+
+/// safely calculates a chunk size argument which is non-zero
+fn calculate_chunk_size(archives: usize, parallelism: usize) -> usize {
+    let par_denom = std::cmp::max(1, parallelism);
+    if archives < par_denom {
+        1
+    } else {
+        archives / par_denom
+    }
+}

rust/bambam/src/app/gtfs_config/run.rs

@@ -334,7 +334,7 @@ where
     Ok(result)
 }
 
-/// finds what modes are already available via other edge lists in the config.
+/// finds what modes are already available via other edge lists via the Label model in the config.
 /// assumes that each edge list has a "multimodal" TraversalModel type.
 /// enforces that the mode list matches the listing in the label model.
 pub fn get_available_modes(base_conf: &CompassAppConfig) -> Result<Vec<String>, GtfsConfigError> {