node: Track last block that received a blockTip notification

Also signal m_tip_block_cv when StopRPC is called, for
consistency with g_best_block_cv. This is handled in
StopRPC instead of OnRPCStopped() because the latter
is deleted in a later commit.

Co-authored-by: TheCharlatan <seb.kung@gmail.com>
Co-authored-by: Ryan Ofsky <ryan@ofsky.org>

src/init.cpp

@@ -284,7 +284,7 @@ void Shutdown(NodeContext& node)
 
     StopHTTPRPC();
     StopREST();
-    StopRPC();
+    StopRPC(&node);
     StopHTTPServer();
     for (const auto& client : node.chain_clients) {
         client->flush();

src/node/interfaces.cpp

@@ -139,7 +139,7 @@ public:
         // Stop RPC for clean shutdown if any of waitfor* commands is executed.
         if (args().GetBoolArg("-server", false)) {
             InterruptRPC();
-            StopRPC();
+            StopRPC(m_context);
         }
     }
     bool shutdownRequested() override { return ShutdownRequested(*Assert(m_context)); };

src/node/kernel_notifications.cpp

@@ -50,6 +50,12 @@ namespace node {
 
 kernel::InterruptResult KernelNotifications::blockTip(SynchronizationState state, CBlockIndex& index)
 {
+    {
+        LOCK(m_tip_block_mutex);
+        m_tip_block = index.GetBlockHash();
+        m_tip_block_cv.notify_all();
+    }
+
     uiInterface.NotifyBlockTip(state, &index);
     if (m_stop_at_height && index.nHeight >= m_stop_at_height) {
         if (!m_shutdown()) {

src/node/kernel_notifications.h

@@ -7,6 +7,10 @@
 
 #include <kernel/notifications_interface.h>
 
+#include <sync.h>
+#include <threadsafety.h>
+#include <uint256.h>
+
 #include <atomic>
 #include <cstdint>
 
@@ -34,7 +38,7 @@ public:
     KernelNotifications(util::SignalInterrupt& shutdown, std::atomic<int>& exit_status, node::Warnings& warnings)
         : m_shutdown(shutdown), m_exit_status{exit_status}, m_warnings{warnings} {}
 
-    [[nodiscard]] kernel::InterruptResult blockTip(SynchronizationState state, CBlockIndex& index) override;
+    [[nodiscard]] kernel::InterruptResult blockTip(SynchronizationState state, CBlockIndex& index) override EXCLUSIVE_LOCKS_REQUIRED(!m_tip_block_mutex);
 
     void headerTip(SynchronizationState state, int64_t height, int64_t timestamp, bool presync) override;
 
@@ -52,6 +56,12 @@ public:
     int m_stop_at_height{DEFAULT_STOPATHEIGHT};
     //! Useful for tests, can be set to false to avoid shutdown on fatal error.
     bool m_shutdown_on_fatal_error{true};
+
+    Mutex m_tip_block_mutex;
+    std::condition_variable m_tip_block_cv;
+    //! The block for which the last blockTip notification was received for.
+    uint256 m_tip_block;
+
 private:
     util::SignalInterrupt& m_shutdown;
     std::atomic<int>& m_exit_status;

src/rpc/server.cpp

@@ -11,6 +11,7 @@
 #include <common/system.h>
 #include <logging.h>
 #include <node/context.h>
+#include <node/kernel_notifications.h>
 #include <rpc/server_util.h>
 #include <rpc/util.h>
 #include <sync.h>
@@ -311,16 +312,19 @@ void InterruptRPC()
     });
 }
 
-void StopRPC()
+void StopRPC(const std::any& context)
 {
     static std::once_flag g_rpc_stop_flag;
     // This function could be called twice if the GUI has been started with -server=1.
     assert(!g_rpc_running);
-    std::call_once(g_rpc_stop_flag, []() {
+    std::call_once(g_rpc_stop_flag, [&]() {
         LogDebug(BCLog::RPC, "Stopping RPC\n");
         WITH_LOCK(g_deadline_timers_mutex, deadlineTimers.clear());
         DeleteAuthCookie();
         g_rpcSignals.Stopped();
+        node::NodeContext& node = EnsureAnyNodeContext(context);
+        // The notifications interface doesn't exist between initialization step 4a and 7.
+        if (node.notifications) node.notifications->m_tip_block_cv.notify_all();
     });
 }
 

src/rpc/server.h

@@ -9,6 +9,7 @@
 #include <rpc/request.h>
 #include <rpc/util.h>
 
+#include <any>
 #include <functional>
 #include <map>
 #include <stdint.h>
@@ -178,7 +179,7 @@ extern CRPCTable tableRPC;
 
 void StartRPC();
 void InterruptRPC();
-void StopRPC();
+void StopRPC(const std::any& context);
 UniValue JSONRPCExec(const JSONRPCRequest& jreq, bool catch_errors);
 
 #endif // BITCOIN_RPC_SERVER_H