core: check block version for alt chains too

This is incompatible with block version voting

src/cryptonote_core/blockchain.cpp

@@ -1259,6 +1259,14 @@ bool Blockchain::handle_alternative_block(const block& b, const crypto::hash& id
     return false;
   }
 
+  // this is a cheap test
+  if (!m_hardfork->check_for_height(b, block_height))
+  {
+    LOG_PRINT_L1("Block with id: " << id << std::endl << "has old version for height " << block_height);
+    bvc.m_verifivation_failed = true;
+    return false;
+  }
+
   //block is not related with head of main chain
   //first of all - look in alternative chains container
   auto it_prev = m_alternative_chains.find(b.prev_id);

src/cryptonote_core/hardfork.cpp

@@ -114,6 +114,19 @@ bool HardFork::check(const cryptonote::block &block) const
   return do_check(::get_block_version(block), ::get_block_vote(block));
 }
 
+bool HardFork::do_check_for_height(uint8_t block_version, uint8_t voting_version, uint64_t height) const
+{
+  int fork_index = get_voted_fork_index(height);
+  return block_version == heights[fork_index].version
+      && voting_version >= heights[fork_index].version;
+}
+
+bool HardFork::check_for_height(const cryptonote::block &block, uint64_t height) const
+{
+  CRITICAL_REGION_LOCAL(lock);
+  return do_check_for_height(::get_block_version(block), ::get_block_vote(block), height);
+}
+
 bool HardFork::add(uint8_t block_version, uint8_t voting_version, uint64_t height)
 {
   CRITICAL_REGION_LOCAL(lock);

src/cryptonote_core/hardfork.h

@@ -113,6 +113,20 @@ namespace cryptonote
      */
     bool check(const cryptonote::block &block) const;
 
+    /**
+     * @brief same as check, but for a particular height, rather than the top
+     *
+     * NOTE: this does not play well with voting, and relies on voting to be
+     * disabled (that is, forks happen on the scheduled date, whether or not
+     * enough blocks have voted for the fork).
+     *
+     * returns true if no error, false otherwise
+     *
+     * @param block the new block
+     * @param height which height to check for
+     */
+    bool check_for_height(const cryptonote::block &block, uint64_t height) const;
+
     /**
      * @brief add a new block
      *
@@ -211,6 +225,7 @@ namespace cryptonote
 
     uint8_t get_block_version(uint64_t height) const;
     bool do_check(uint8_t block_version, uint8_t voting_version) const;
+    bool do_check_for_height(uint8_t block_version, uint8_t voting_version, uint64_t height) const;
     int get_voted_fork_index(uint64_t height) const;
     uint8_t get_effective_version(uint8_t voting_version) const;
     bool add(uint8_t block_version, uint8_t voting_version, uint64_t height);