blockchain: fix wrong hf version when popping multiple blocks

Since we keep track of the hf version in the db, we pick it up
from there instead of doing the full reorg call, which is quite
expensive

src/cryptonote_basic/hardfork.cpp

@@ -305,6 +305,29 @@ bool HardFork::rescan_from_chain_height(uint64_t height)
   return rescan_from_block_height(height - 1);
 }
 
+void HardFork::on_block_popped(uint64_t nblocks)
+{
+  CHECK_AND_ASSERT_THROW_MES(nblocks > 0, "nblocks must be greater than 0");
+
+  CRITICAL_REGION_LOCAL(lock);
+
+  const uint64_t new_chain_height = db.height();
+  const uint64_t old_chain_height = new_chain_height + nblocks;
+  uint8_t version;
+  uint64_t height;
+  for (height = old_chain_height - 1; height >= new_chain_height; --height)
+  {
+    versions.pop_back();
+    version = db.get_hard_fork_version(height);
+    versions.push_front(version);
+  }
+
+  // does not take voting into account
+  for (current_fork_index = heights.size() - 1; current_fork_index > 0; --current_fork_index)
+    if (height >= heights[current_fork_index].height)
+      break;
+}
+
 int HardFork::get_voted_fork_index(uint64_t height) const
 {
   CRITICAL_REGION_LOCAL(lock);