From 3192975f1d177aa9f0bbd823c6387cfbfa943610 Mon Sep 17 00:00:00 2001
From: Matt Corallo <git@bluematt.me>
Date: Tue, 27 Jun 2017 19:07:52 -0400
Subject: [PATCH] Flush CValidationInterface callbacks prior to destruction

Note that the CScheduler thread cant be running at this point,
it has already been stopped with the rest of the init threadgroup.
Thus, just calling any remaining loose callbacks during Shutdown()
is sane.
---
 src/init.cpp                | 13 +++++++++++++
 src/scheduler.cpp           |  9 +++++++++
 src/scheduler.h             |  3 +++
 src/test/test_bitcoin.cpp   |  1 +
 src/validationinterface.cpp |  4 ++++
 src/validationinterface.h   |  2 ++
 6 files changed, 32 insertions(+)

diff --git a/src/init.cpp b/src/init.cpp
index 94bba6820e..12f29a4ca8 100644
--- a/src/init.cpp
+++ b/src/init.cpp
@@ -215,6 +215,19 @@ void Shutdown()
         fFeeEstimatesInitialized = false;
     }
 
+    // FlushStateToDisk generates a SetBestChain callback, which we should avoid missing
+    FlushStateToDisk();
+
+    // After there are no more peers/RPC left to give us new data which may generate
+    // CValidationInterface callbacks, flush them...
+    GetMainSignals().FlushBackgroundCallbacks();
+
+    // Any future callbacks will be dropped. This should absolutely be safe - if
+    // missing a callback results in an unrecoverable situation, unclean shutdown
+    // would too. The only reason to do the above flushes is to let the wallet catch
+    // up with our current chain to avoid any strange pruning edge cases and make
+    // next startup faster by avoiding rescan.
+
     {
         LOCK(cs_main);
         if (pcoinsTip != NULL) {
diff --git a/src/scheduler.cpp b/src/scheduler.cpp
index a76a87e10a..35bf0da4be 100644
--- a/src/scheduler.cpp
+++ b/src/scheduler.cpp
@@ -191,3 +191,12 @@ void SingleThreadedSchedulerClient::AddToProcessQueue(std::function<void (void)>
     }
     MaybeScheduleProcessQueue();
 }
+
+void SingleThreadedSchedulerClient::EmptyQueue() {
+    bool should_continue = true;
+    while (should_continue) {
+        ProcessQueue();
+        LOCK(m_cs_callbacks_pending);
+        should_continue = !m_callbacks_pending.empty();
+    }
+}
diff --git a/src/scheduler.h b/src/scheduler.h
index 82036afdf0..6a079f7749 100644
--- a/src/scheduler.h
+++ b/src/scheduler.h
@@ -101,6 +101,9 @@ private:
 public:
     SingleThreadedSchedulerClient(CScheduler *pschedulerIn) : m_pscheduler(pschedulerIn) {}
     void AddToProcessQueue(std::function<void (void)> func);
+
+    // Processes all remaining queue members on the calling thread, blocking until queue is empty
+    void EmptyQueue();
 };
 
 #endif
diff --git a/src/test/test_bitcoin.cpp b/src/test/test_bitcoin.cpp
index e6a8cc779d..3ba81ed17b 100644
--- a/src/test/test_bitcoin.cpp
+++ b/src/test/test_bitcoin.cpp
@@ -94,6 +94,7 @@ TestingSetup::~TestingSetup()
         UnregisterNodeSignals(GetNodeSignals());
         threadGroup.interrupt_all();
         threadGroup.join_all();
+        GetMainSignals().FlushBackgroundCallbacks();
         GetMainSignals().UnregisterBackgroundSignalScheduler();
         UnloadBlockIndex();
         delete pcoinsTip;
diff --git a/src/validationinterface.cpp b/src/validationinterface.cpp
index 8edc7c398d..bf20d606f8 100644
--- a/src/validationinterface.cpp
+++ b/src/validationinterface.cpp
@@ -44,6 +44,10 @@ void CMainSignals::UnregisterBackgroundSignalScheduler() {
     m_internals.reset(nullptr);
 }
 
+void CMainSignals::FlushBackgroundCallbacks() {
+    m_internals->m_schedulerClient.EmptyQueue();
+}
+
 CMainSignals& GetMainSignals()
 {
     return g_signals;
diff --git a/src/validationinterface.h b/src/validationinterface.h
index fbfe273b10..568da66df2 100644
--- a/src/validationinterface.h
+++ b/src/validationinterface.h
@@ -79,6 +79,8 @@ public:
     void RegisterBackgroundSignalScheduler(CScheduler& scheduler);
     /** Unregister a CScheduler to give callbacks which should run in the background - these callbacks will now be dropped! */
     void UnregisterBackgroundSignalScheduler();
+    /** Call any remaining callbacks on the calling thread */
+    void FlushBackgroundCallbacks();
 
     void UpdatedBlockTip(const CBlockIndex *, const CBlockIndex *, bool fInitialDownload);
     void TransactionAddedToMempool(const CTransactionRef &);