Add test for UnregisterAllValidationInterfaces bug

Bug in MainSignalsInstance::Clear could cause validation interface callbacks to
be deleted during execution if UnregisterAllValidationInterfaces was called
more than once.

Bug was introduced in https://github.com/bitcoin/bitcoin/pull/18524 and is
fixed by https://github.com/bitcoin/bitcoin/pull/18551

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()