Merge #18551: Do not clear validationinterface entries being executed

2276339a17 Add test for UnregisterAllValidationInterfaces bug (Russell Yanofsky)
3c61abbbc8 Do not clear validationinterface entries being executed (Pieter Wuille)

Pull request description:

  The previous code for MainSignalsInstance::Clear would decrement the reference
  count of every interface, including ones that were already Unregister()ed but
  still being executed.

  This fixes the issue pointed out here: https://github.com/bitcoin/bitcoin/pull/18524/files#r404395685 . It's not currently observable.

ACKs for top commit:
  jonasschnelli:
    utACK 2276339a17 - reviewed code and test (thanks @ryanofsky for adding the test).
  MarcoFalke:
    ACK 2276339a17 🎎
  ryanofsky:
    Code review ACK 2276339a17. No change to bugfix, just rebased and new test commit added since last review

Tree-SHA512: c1d68e7c681a45c6cadc84e407c2266bcb4b12d34264e1232a61c4eadb74b551231c5a3b1d041de39f507aef4dfa7d4589b8bfe1833f069c739c6270d2a05dbe

src/Makefile.test.include

@@ -239,6 +239,7 @@ BITCOIN_TESTS =\
   test/util_tests.cpp \
   test/validation_block_tests.cpp \
   test/validation_flush_tests.cpp \
+  test/validationinterface_tests.cpp \
   test/versionbits_tests.cpp
 
 if ENABLE_WALLET

src/test/validationinterface_tests.cpp

@@ -0,0 +1,63 @@
+// Copyright (c) 2020 The Bitcoin Core developers
+// Distributed under the MIT software license, see the accompanying
+// file COPYING or http://www.opensource.org/licenses/mit-license.php.
+
+#include <boost/test/unit_test.hpp>
+
+#include <consensus/validation.h>
+#include <primitives/block.h>
+#include <scheduler.h>
+#include <util/check.h>
+#include <validationinterface.h>
+
+BOOST_AUTO_TEST_SUITE(validationinterface_tests)
+
+class TestInterface : public CValidationInterface
+{
+public:
+    TestInterface(std::function<void()> on_call = nullptr, std::function<void()> on_destroy = nullptr)
+        : m_on_call(std::move(on_call)), m_on_destroy(std::move(on_destroy))
+    {
+    }
+    virtual ~TestInterface()
+    {
+        if (m_on_destroy) m_on_destroy();
+    }
+    void BlockChecked(const CBlock& block, const BlockValidationState& state) override
+    {
+        if (m_on_call) m_on_call();
+    }
+    static void Call()
+    {
+        CBlock block;
+        BlockValidationState state;
+        GetMainSignals().BlockChecked(block, state);
+    }
+    std::function<void()> m_on_call;
+    std::function<void()> m_on_destroy;
+};
+
+// Regression test to ensure UnregisterAllValidationInterfaces calls don't
+// destroy a validation interface while it is being called. Bug:
+// https://github.com/bitcoin/bitcoin/pull/18551
+BOOST_AUTO_TEST_CASE(unregister_all_during_call)
+{
+    bool destroyed = false;
+
+    CScheduler scheduler;
+    GetMainSignals().RegisterBackgroundSignalScheduler(scheduler);
+    RegisterSharedValidationInterface(std::make_shared<TestInterface>(
+        [&] {
+            // First call should decrements reference count 2 -> 1
+            UnregisterAllValidationInterfaces();
+            BOOST_CHECK(!destroyed);
+            // Second call should not decrement reference count 1 -> 0
+            UnregisterAllValidationInterfaces();
+            BOOST_CHECK(!destroyed);
+        },
+        [&] { destroyed = true; }));
+    TestInterface::Call();
+    BOOST_CHECK(destroyed);
+}
+
+BOOST_AUTO_TEST_SUITE_END()

src/validationinterface.cpp

@@ -67,8 +67,8 @@ public:
     void Clear()
     {
         LOCK(m_mutex);
-        for (auto it = m_list.begin(); it != m_list.end();) {
+        for (const auto& entry : m_map) {
-            it = --it->count ? std::next(it) : m_list.erase(it);
+            if (!--entry.second->count) m_list.erase(entry.second);
         }
         m_map.clear();
     }