Reduce memory requirement by clearing header list (#82)

Author: mariano54

src/blockchain.py

@@ -706,7 +706,7 @@ async def pre_validate_blocks(
             if pos is not None:
                 pos = bytes32(pos)
             results[i] = val, pos
-        pool.shutdown()
+        pool.shutdown(wait=True)
         return results
 
     @staticmethod

src/full_node.py

@@ -386,6 +386,8 @@ async def _sync(self) -> OutboundMessageGenerator:
             f"Validated weight of headers. Downloaded {len(headers)} headers, tip height {tip_height}"
         )
         assert tip_height == fork_point_height + len(headers)
+        self.store.clear_potential_headers()
+        headers.clear()
 
         # Download blocks in batches, and verify them as they come in. We download a few batches ahead,
         # in case there are delays.

src/store.py

@@ -183,6 +183,9 @@ def add_potential_header(self, block: HeaderBlock) -> None:
     def get_potential_header(self, height: uint32) -> Optional[HeaderBlock]:
         return self.potential_headers.get(height, None)
 
+    def clear_potential_headers(self) -> None:
+        self.potential_headers.clear()
+
     def set_potential_hashes(self, potential_hashes: List[bytes32]) -> None:
         self.potential_hashes = potential_hashes