add command exportcp to export checkpoints

Also exported additional 17 headers on last checkpoint
to provide data for get_target blockchain method

lib/blockchain.py

@@ -437,5 +437,22 @@ class Blockchain(util.PrintError):
             height = (index + 1) * CHUNK_LEN - 1
             h = self.get_hash(height)
             target = self.get_target(height)
-            cp.append((h, target))
+            if len(h.strip('0')) == 0:
+                raise Exception('%s file has not enough data.' % self.path())
+            if index < n - 1:
+                cp.append((h, target))
+            else:
+                dgw3_headers = []
+                if os.path.exists(self.path()):
+                    with open(self.path(), 'rb') as f:
+                        lower_header = height - POW_AVERAGING_WINDOW
+                        for height in range(height, lower_header-1, -1):
+                            f.seek(height*80)
+                            hd = f.read(80)
+                            if len(hd) < 80:
+                                raise Exception(
+                                    'Expected to read a full header.'
+                                    ' This was only {} bytes'.format(len(hd)))
+                            dgw3_headers.append((height, bh2u(hd)))
+                cp.append((h, target, dgw3_headers))
         return cp

lib/commands.py

@@ -658,6 +658,15 @@ class Commands:
         to config settings (static/dynamic)"""
         return self.config.fee_per_kb()
 
+    @command('n')
+    def exportcp(self, cpfile):
+        """Export checkpoints to file"""
+        try:
+            self.network.export_checkpoints(cpfile)
+            return 'Exporting checkpoints done'
+        except Exception as e:
+            return 'Error exporting checkpoints: ' + str(e)
+
     @command('')
     def help(self):
         # for the python console
@@ -680,6 +689,7 @@ param_descriptions = {
     'requested_amount': 'Requested amount (in ZEC).',
     'outputs': 'list of ["address", amount]',
     'redeem_script': 'redeem script (hexadecimal)',
+    'cpfile': 'Checkpoints file',
 }
 
 command_options = {