{android,win32}/build.py: move common code to python/build/

python/build/download.py

@@ -0,0 +1,40 @@
+import os
+import hashlib
+import urllib.request
+
+def file_md5(path):
+    """Calculate the MD5 checksum of a file and return it in hexadecimal notation."""
+
+    with open(path, 'rb') as f:
+        m = hashlib.md5()
+        while True:
+            data = f.read(65536)
+            if len(data) == 0:
+                # end of file
+                return m.hexdigest()
+            m.update(data)
+
+def download_and_verify(url, md5, parent_path):
+    """Download a file, verify its MD5 checksum and return the local path."""
+
+    os.makedirs(parent_path, exist_ok=True)
+    path = os.path.join(parent_path, os.path.basename(url))
+
+    try:
+        calculated_md5 = file_md5(path)
+        if md5 == calculated_md5: return path
+        os.unlink(path)
+    except FileNotFoundError:
+        pass
+
+    tmp_path = path + '.tmp'
+
+    print("download", url)
+    urllib.request.urlretrieve(url, tmp_path)
+    calculated_md5 = file_md5(tmp_path)
+    if calculated_md5 != md5:
+        os.unlink(tmp_path)
+        raise "MD5 mismatch"
+
+    os.rename(tmp_path, path)
+    return path