mark 'blockchain_headers' file as sparse on windows

based on fyookball/electrum@647a6cc26dfe2d0d942159207c6e72e1b0e8d94d

electrum/network.py

@@ -675,9 +675,10 @@ class Network(PrintError):
         length = 80 * len(constants.net.CHECKPOINTS) * 2016
         if not os.path.exists(filename) or os.path.getsize(filename) < length:
             with open(filename, 'wb') as f:
-                if length>0:
+                if length > 0:
                     f.seek(length-1)
                     f.write(b'\x00')
+            util.ensure_sparse_file(filename)
         with b.lock:
             b.update_size()
 

electrum/util.py

@@ -383,6 +383,16 @@ def android_check_data_dir():
     return data_dir
 
 
+def ensure_sparse_file(filename):
+    # On modern Linux, no need to do anything.
+    # On Windows, need to explicitly mark file.
+    if os.name == "nt":
+        try:
+            os.system('fsutil sparse setflag "{}" 1'.format(filename))
+        except Exception as e:
+            print_error('error marking file {} as sparse: {}'.format(filename, e))
+
+
 def get_headers_dir(config):
     return android_headers_dir() if 'ANDROID_DATA' in os.environ else config.path