python/build: add support for fallback download URLs

python/build/download.py

@@ -1,13 +1,50 @@
+from typing import Sequence, Union
 import os
+import sys
 import urllib.request
 
 from .verify import verify_file_digest
 
-def download_and_verify(url, md5, parent_path):
+def __to_string_sequence(x: Union[str, Sequence[str]]) -> Sequence[str]:
+    if isinstance(x, str):
+        return (x,)
+    else:
+        return x
+
+def __get_any(x: Union[str, Sequence[str]]) -> str:
+    if isinstance(x, str):
+        return x
+    else:
+        return x[0]
+
+def __download_one(url: str, path: str) -> None:
+    print("download", url)
+    urllib.request.urlretrieve(url, path)
+
+def __download(urls: Sequence[str], path: str) -> None:
+    for url in urls[:-1]:
+        try:
+            __download_one(url, path)
+            return
+        except:
+            print("download error:", sys.exc_info()[0])
+    __download_one(urls[-1], path)
+
+def __download_and_verify_to(urls: Sequence[str], md5: str, path: str) -> None:
+    __download(urls, path)
+    if not verify_file_digest(path, md5):
+        raise RuntimeError("Digest mismatch")
+
+def download_basename(urls: Union[str, Sequence[str]]) -> str:
+    return os.path.basename(__get_any(urls))
+
+def download_and_verify(urls: Union[str, Sequence[str]], md5: str, parent_path: str) -> str:
     """Download a file, verify its MD5 checksum and return the local path."""
 
+    base = download_basename(urls)
+
     os.makedirs(parent_path, exist_ok=True)
-    path = os.path.join(parent_path, os.path.basename(url))
+    path = os.path.join(parent_path, base)
 
     try:
         if verify_file_digest(path, md5): return path
@@ -17,11 +54,6 @@ def download_and_verify(url, md5, parent_path):
 
     tmp_path = path + '.tmp'
 
-    print("download", url)
-    urllib.request.urlretrieve(url, tmp_path)
-    if not verify_file_digest(tmp_path, md5):
-        os.unlink(tmp_path)
-        raise RuntimeError("Digest mismatch")
-
+    __download_and_verify_to(__to_string_sequence(urls), md5, tmp_path)
     os.rename(tmp_path, path)
     return path