Add basic detection/recovery of corrupt partial downloads

scripts/download_satellite.py

@@ -1,10 +1,11 @@
 #!/usr/bin/env nix-shell
-#!nix-shell --quiet -p python3 -i python
+#!nix-shell --quiet -p python312 -p python312Packages.pillow -i python
 
 # https://svc.pictometry.com/Image/D2B06344-7A2D-5BD0-FC89-DFDDC9888C41/wmts?SERVICE=WMTS&REQUEST=GetCapabilities
 
 import os
 import urllib.request
+from PIL import Image
 
 MAX_ZOOM=20
 
@@ -22,7 +23,12 @@ for z in range(12, MAX_ZOOM+1):
             file = os.path.normpath(os.path.join(os.path.dirname(__file__), f"../satellite/{z}/{x}/{y}.png"))
             os.makedirs(os.path.dirname(file), exist_ok=True)
             if os.path.isfile(file):
-                print(f"{progress} {file} already exists; skipping")
-                continue
-            print(f"{progress} downloading {file}")
+                try: # does it already exist and look good?
+                    Image.open(file).verify() # CRC check on PNGs; nothing on other file types? https://pillow.readthedocs.io/en/latest/_modules/PIL/PngImagePlugin.html#ChunkStream.verify
+                    print(f"{progress} {file} already exists; skipping")
+                    continue
+                except OSError as e:
+                    print(f"{progress} replacing corrupt {file}")
+            else:
+                print(f"{progress} downloading {file}")
             urllib.request.urlretrieve(url, file)