Fix source/storage flip in generate tiles

tilekiln/generator.py

@@ -2,7 +2,7 @@
 The code here pulls creates multiple kilns to generate the tiles in parallel
 '''
 import multiprocessing as mp
-from typing import Iterable
+from collections.abc import Collection
 
 import psycopg
 
@@ -37,7 +37,12 @@ def worker(tile: Tile) -> None:
 
 
 def generate(config: Config, source_kwargs, storage_kwargs,  # type: ignore[no-untyped-def]
-             tiles: Iterable[Tile], num_processes: int) -> None:
+             tiles: Collection[Tile], num_processes: int) -> None:
+
+    # If there are no processes and no tiles then there's nothing to do.
+    if num_processes == 0 and len(tiles) == 0:
+        return
+
     with mp.Pool(num_processes, setup, (config, source_kwargs, storage_kwargs)) as pool:
         pool.imap_unordered(worker, tiles)
         pool.close()

tilekiln/scripts/generate.py

@@ -49,14 +49,14 @@ def tiles(config: int, num_threads: int,
 
     click.echo(f"Rendering {len(tiles)} tiles over {threads} threads")
 
-    source_kwargs = {"dbname": storage_dbname,
-                     "host": storage_host,
-                     "port": storage_port,
-                     "user": storage_username}
-    storage_kwargs = {"dbname": source_dbname,
-                      "host": source_host,
-                      "port": source_port,
-                      "user": source_username}
+    source_kwargs = {"dbname": source_dbname,
+                     "host": source_host,
+                     "port": source_port,
+                     "user": source_username}
+    storage_kwargs = {"dbname": storage_dbname,
+                      "host": storage_host,
+                      "port": storage_port,
+                      "user": storage_username}
     tilekiln.generator.generate(c, source_kwargs, storage_kwargs, tqdm(tiles), threads)