mempool: add AssumeCalculateMemPoolAncestors helper function

There are quite a few places that assume CalculateMemPoolAncestors
will return a value without raising an error. This helper function
adds logging (and Assume for debug builds) that ensures robustness
but increases visibility in case of unexpected failures

src/txmempool.cpp

@@ -254,6 +254,20 @@ util::Result<CTxMemPool::setEntries> CTxMemPool::CalculateMemPoolAncestors(
                                             limits);
 }
 
+CTxMemPool::setEntries CTxMemPool::AssumeCalculateMemPoolAncestors(
+    std::string_view calling_fn_name,
+    const CTxMemPoolEntry &entry,
+    const Limits& limits,
+    bool fSearchForParents /* = true */) const
+{
+    auto result{Assume(CalculateMemPoolAncestors(entry, limits, fSearchForParents))};
+    if (!result) {
+        LogPrintLevel(BCLog::MEMPOOL, BCLog::Level::Error, "%s: CalculateMemPoolAncestors failed unexpectedly, continuing with empty ancestor set (%s)\n",
+                      calling_fn_name, util::ErrorString(result).original);
+    }
+    return std::move(result).value_or(CTxMemPool::setEntries{});
+}
+
 void CTxMemPool::UpdateAncestorsOf(bool add, txiter it, setEntries &setAncestors)
 {
     const CTxMemPoolEntry::Parents& parents = it->GetMemPoolParentsConst();