mempool: Make GetMinFee() with custom size protected

The version of GetMinFee() with a custom size specification is and
should only be used by tests. Mark it as protected and use a derived
class exposing GetMinFee() as public in tests.

src/test/mempool_tests.cpp

@@ -16,6 +16,12 @@ BOOST_FIXTURE_TEST_SUITE(mempool_tests, TestingSetup)
 
 static constexpr auto REMOVAL_REASON_DUMMY = MemPoolRemovalReason::REPLACED;
 
+class MemPoolTest final : public CTxMemPool
+{
+public:
+    using CTxMemPool::GetMinFee;
+};
+
 BOOST_AUTO_TEST_CASE(MempoolRemoveTest)
 {
     // Test CTxMemPool::remove functionality
@@ -423,7 +429,7 @@ BOOST_AUTO_TEST_CASE(MempoolAncestorIndexingTest)
 
 BOOST_AUTO_TEST_CASE(MempoolSizeLimitTest)
 {
-    CTxMemPool& pool = *Assert(m_node.mempool);
+    auto& pool = static_cast<MemPoolTest&>(*Assert(m_node.mempool));
     LOCK2(cs_main, pool.cs);
     TestMemPoolEntryHelper entry;
 

src/txmempool.h

@@ -453,6 +453,8 @@ protected:
 
     bool m_is_loaded GUARDED_BY(cs){false};
 
+    CFeeRate GetMinFee(size_t sizelimit) const;
+
 public:
 
     static const int ROLLING_FEE_HALFLIFE = 60 * 60 * 12; // public only for testing
@@ -707,7 +709,6 @@ public:
     CFeeRate GetMinFee() const {
         return GetMinFee(m_max_size_bytes);
     }
-    CFeeRate GetMinFee(size_t sizelimit) const;
 
     /** Remove transactions from the mempool until its dynamic size is <= sizelimit.
       *  pvNoSpendsRemaining, if set, will be populated with the list of outpoints