Add query_cache_unit-t to build and init Prometheus registry

- tests/unit/Makefile: Register query_cache_unit-t in the build system
- test_init.cpp: Create a real Prometheus registry in test_init_query_
  cache() since the Query_Cache constructor registers metrics via
  GloVars.prometheus_registry (nullptr would crash)

test/tap/test_helpers/test_init.cpp

@@ -81,6 +81,13 @@ int test_init_query_cache() {
 		return 0;
 	}
 
+	// The Query_Cache constructor registers Prometheus metrics via
+	// GloVars.prometheus_registry. Provide a real registry so the
+	// constructor doesn't crash on nullptr dereference.
+	if (GloVars.prometheus_registry == nullptr) {
+		GloVars.prometheus_registry = std::make_shared<prometheus::Registry>();
+	}
+
 	GloMyQC = new MySQL_Query_Cache();
 	GloPgQC = new PgSQL_Query_Cache();
 

test/tap/tests/unit/Makefile

@@ -231,7 +231,7 @@ $(ODIR)/test_init.o: $(TEST_HELPERS_DIR)/test_init.cpp | $(ODIR)
 # Unit test targets
 # ===========================================================================
 
-UNIT_TESTS := smoke_test-t
+UNIT_TESTS := smoke_test-t query_cache_unit-t
 
 .PHONY: all
 all: $(UNIT_TESTS)
@@ -250,6 +250,11 @@ smoke_test-t: smoke_test-t.cpp $(TEST_HELPERS_OBJ) $(LIBPROXYSQLAR)
 		$(LIBPROXYSQLAR_FULL) $(STATIC_LIBS) $(MYLIBS) \
 		$(ALLOW_MULTI_DEF) -o $@
 
+query_cache_unit-t: query_cache_unit-t.cpp $(TEST_HELPERS_OBJ) $(LIBPROXYSQLAR)
+	$(CXX) $< $(TEST_HELPERS_OBJ) $(IDIRS) $(LDIRS) $(OPT) \
+		$(LIBPROXYSQLAR_FULL) $(STATIC_LIBS) $(MYLIBS) \
+		$(ALLOW_MULTI_DEF) -o $@
+
 
 # ===========================================================================
 # Clean