refactor: Use wait_for predicate to check for interrupt

Also use uint256::ZERO where appropriate for self-documenting code.

src/interfaces/mining.h

@@ -61,7 +61,8 @@ public:
     virtual std::optional<BlockRef> getTip() = 0;
 
     /**
-     * Waits for the tip to change
+     * Waits for the connected tip to change. If the tip was not connected on
+     * startup, this will wait.
      *
      * @param[in] current_tip block hash of the current chain tip. Function waits
      *                        for the chain tip to differ from this.

src/node/interfaces.cpp

@@ -940,19 +940,12 @@ public:
 
     BlockRef waitTipChanged(uint256 current_tip, MillisecondsDouble timeout) override
     {
-        // Interrupt check interval
+        if (timeout > std::chrono::years{100}) timeout = std::chrono::years{100}; // Upper bound to avoid UB in std::chrono
-        const MillisecondsDouble tick{1000};
-        auto now{std::chrono::steady_clock::now()};
-        auto deadline = now + timeout;
-        // std::chrono does not check against overflow
-        if (deadline < now) deadline = std::chrono::steady_clock::time_point::max();
         {
             WAIT_LOCK(notifications().m_tip_block_mutex, lock);
-            while ((notifications().m_tip_block == uint256() || notifications().m_tip_block == current_tip) && !chainman().m_interrupt) {
+            notifications().m_tip_block_cv.wait_for(lock, timeout, [&]() EXCLUSIVE_LOCKS_REQUIRED(notifications().m_tip_block_mutex) {
-                now = std::chrono::steady_clock::now();
+                return (notifications().m_tip_block != current_tip && notifications().m_tip_block != uint256::ZERO) || chainman().m_interrupt;
-                if (now >= deadline) break;
+            });
-                notifications().m_tip_block_cv.wait_until(lock, std::min(deadline, now + tick));
-            }
         }
         // Must release m_tip_block_mutex before locking cs_main, to avoid deadlocks.
         LOCK(::cs_main);

src/node/kernel_notifications.h

@@ -57,7 +57,9 @@ public:
     Mutex m_tip_block_mutex;
     std::condition_variable m_tip_block_cv GUARDED_BY(m_tip_block_mutex);
     //! The block for which the last blockTip notification was received for.
-    uint256 m_tip_block GUARDED_BY(m_tip_block_mutex);
+    //! The initial ZERO means that no block has been connected yet, which may
+    //! be true even long after startup, until shutdown.
+    uint256 m_tip_block GUARDED_BY(m_tip_block_mutex){uint256::ZERO};
 
 private:
     const std::function<bool()>& m_shutdown_request;