Make m_tip_block an std::optional

This change avoids ambiguity when no tip is connected and it is
compared to uint256::ZERO.

src/init.cpp

@@ -1807,7 +1807,7 @@ bool AppInitMain(NodeContext& node, interfaces::BlockAndHeaderTipInfo* tip_info)
     {
         WAIT_LOCK(kernel_notifications.m_tip_block_mutex, lock);
         kernel_notifications.m_tip_block_cv.wait(lock, [&]() EXCLUSIVE_LOCKS_REQUIRED(kernel_notifications.m_tip_block_mutex) {
-            return !kernel_notifications.m_tip_block.IsNull() || ShutdownRequested(node);
+            return kernel_notifications.m_tip_block || ShutdownRequested(node);
         });
     }
 

src/node/interfaces.cpp

@@ -973,7 +973,9 @@ public:
         {
             WAIT_LOCK(notifications().m_tip_block_mutex, lock);
             notifications().m_tip_block_cv.wait_for(lock, timeout, [&]() EXCLUSIVE_LOCKS_REQUIRED(notifications().m_tip_block_mutex) {
-                return (notifications().m_tip_block != current_tip && notifications().m_tip_block != uint256::ZERO) || chainman().m_interrupt;
+                // We need to wait for m_tip_block to be set AND for the value
+                // to differ from the current_tip value.
+                return (notifications().m_tip_block && notifications().m_tip_block != current_tip) || chainman().m_interrupt;
             });
         }
         // Must release m_tip_block_mutex before locking cs_main, to avoid deadlocks.

src/node/kernel_notifications.h

@@ -59,7 +59,7 @@ public:
     //! The block for which the last blockTip notification was received.
     //! It's first set when the tip is connected during node initialization.
     //! Might be unset during an early shutdown.
-    uint256 m_tip_block GUARDED_BY(m_tip_block_mutex){uint256::ZERO};
+    std::optional<uint256> m_tip_block GUARDED_BY(m_tip_block_mutex);
 
 private:
     const std::function<bool()>& m_shutdown_request;

src/test/validation_chainstate_tests.cpp

@@ -72,7 +72,8 @@ BOOST_FIXTURE_TEST_CASE(chainstate_update_tip, TestChain100Setup)
     ChainstateManager& chainman = *Assert(m_node.chainman);
     const auto get_notify_tip{[&]() {
         LOCK(m_node.notifications->m_tip_block_mutex);
-        return m_node.notifications->m_tip_block;
+        BOOST_REQUIRE(m_node.notifications->m_tip_block);
+        return *m_node.notifications->m_tip_block;
     }};
     uint256 curr_tip = get_notify_tip();