blockchain: do not lock the blockchain lock for simple DB getters

It is safe in those cases, though might return slightly out of date
information if another thread is busy modifying the blockchain,
but it avoids potentially lengthy delays just to get things like
the current blockchain height.

src/cryptonote_core/blockchain.cpp

@@ -139,14 +139,20 @@ Blockchain::Blockchain(tx_memory_pool& tx_pool) :
 bool Blockchain::have_tx(const crypto::hash &id) const
 {
   LOG_PRINT_L3("Blockchain::" << __func__);
-  CRITICAL_REGION_LOCAL(m_blockchain_lock);
+  // WARNING: this function does not take m_blockchain_lock, and thus should only call read only
+  // m_db functions which do not depend on one another (ie, no getheight + gethash(height-1), as
+  // well as not accessing class members, even read only (ie, m_invalid_blocks). The caller must
+  // lock if it is otherwise needed.
   return m_db->tx_exists(id);
 }
 //------------------------------------------------------------------
 bool Blockchain::have_tx_keyimg_as_spent(const crypto::key_image &key_im) const
 {
   LOG_PRINT_L3("Blockchain::" << __func__);
-  CRITICAL_REGION_LOCAL(m_blockchain_lock);
+  // WARNING: this function does not take m_blockchain_lock, and thus should only call read only
+  // m_db functions which do not depend on one another (ie, no getheight + gethash(height-1), as
+  // well as not accessing class members, even read only (ie, m_invalid_blocks). The caller must
+  // lock if it is otherwise needed.
   return  m_db->has_key_image(key_im);
 }
 //------------------------------------------------------------------
@@ -287,7 +293,10 @@ bool Blockchain::scan_outputkeys_for_indexes(size_t tx_version, const txin_to_ke
 uint64_t Blockchain::get_current_blockchain_height() const
 {
   LOG_PRINT_L3("Blockchain::" << __func__);
-  CRITICAL_REGION_LOCAL(m_blockchain_lock);
+  // WARNING: this function does not take m_blockchain_lock, and thus should only call read only
+  // m_db functions which do not depend on one another (ie, no getheight + gethash(height-1), as
+  // well as not accessing class members, even read only (ie, m_invalid_blocks). The caller must
+  // lock if it is otherwise needed.
   return m_db->height();
 }
 //------------------------------------------------------------------
@@ -571,7 +580,10 @@ crypto::hash Blockchain::get_tail_id(uint64_t& height) const
 crypto::hash Blockchain::get_tail_id() const
 {
   LOG_PRINT_L3("Blockchain::" << __func__);
-  CRITICAL_REGION_LOCAL(m_blockchain_lock);
+  // WARNING: this function does not take m_blockchain_lock, and thus should only call read only
+  // m_db functions which do not depend on one another (ie, no getheight + gethash(height-1), as
+  // well as not accessing class members, even read only (ie, m_invalid_blocks). The caller must
+  // lock if it is otherwise needed.
   return m_db->top_block_hash();
 }
 //------------------------------------------------------------------
@@ -633,7 +645,10 @@ bool Blockchain::get_short_chain_history(std::list<crypto::hash>& ids) const
 crypto::hash Blockchain::get_block_id_by_height(uint64_t height) const
 {
   LOG_PRINT_L3("Blockchain::" << __func__);
-  CRITICAL_REGION_LOCAL(m_blockchain_lock);
+  // WARNING: this function does not take m_blockchain_lock, and thus should only call read only
+  // m_db functions which do not depend on one another (ie, no getheight + gethash(height-1), as
+  // well as not accessing class members, even read only (ie, m_invalid_blocks). The caller must
+  // lock if it is otherwise needed.
   try
   {
     return m_db->get_block_hash_from_height(height);
@@ -1928,7 +1943,10 @@ bool Blockchain::find_blockchain_supplement(const std::list<crypto::hash>& qbloc
 uint64_t Blockchain::block_difficulty(uint64_t i) const
 {
   LOG_PRINT_L3("Blockchain::" << __func__);
-  CRITICAL_REGION_LOCAL(m_blockchain_lock);
+  // WARNING: this function does not take m_blockchain_lock, and thus should only call read only
+  // m_db functions which do not depend on one another (ie, no getheight + gethash(height-1), as
+  // well as not accessing class members, even read only (ie, m_invalid_blocks). The caller must
+  // lock if it is otherwise needed.
   try
   {
     return m_db->get_block_difficulty(i);
@@ -2209,7 +2227,10 @@ bool Blockchain::handle_block_to_main_chain(const block& bl, block_verification_
 size_t Blockchain::get_total_transactions() const
 {
   LOG_PRINT_L3("Blockchain::" << __func__);
-  CRITICAL_REGION_LOCAL(m_blockchain_lock);
+  // WARNING: this function does not take m_blockchain_lock, and thus should only call read only
+  // m_db functions which do not depend on one another (ie, no getheight + gethash(height-1), as
+  // well as not accessing class members, even read only (ie, m_invalid_blocks). The caller must
+  // lock if it is otherwise needed.
   return m_db->get_tx_count();
 }
 //------------------------------------------------------------------