tests: Fix order of used memory counter reset (#6012)

used memory counter was previously reset after starting service. This
opened up the possibility that a few cycles of engine shard memory
account ran, which incremented used memory, and then this value was
reset. This resulted in lost bytes and tests which rely on this value,
for example to measure eviction, would fail.

Here the order is moved so that used memory is reset first and then
service is started.

Signed-off-by: Abhijat Malviya <[email protected]>

Author: abhijat

src/server/dragonfly_test.cc

@@ -564,7 +564,6 @@ TEST_F(DflyEngineTest, StickyEviction) {
 #endif
 
 TEST_F(DflyEngineTest, ZeroAllocationEviction) {
-  GTEST_SKIP() << "Currently being fixed, unblock CI";
   max_memory_limit = 500000;  // 0.5mb
   shard_set->TEST_EnableCacheMode();
 

src/server/test_utils.cc

@@ -249,8 +249,11 @@ void BaseFamilyTest::ResetService() {
   pp_->Run();
   service_ = std::make_unique<Service>(pp_.get());
 
-  service_->Init(nullptr, {});
+  // Must be reset before starting the service. Engine shard heartbeat task updates this
+  // value, and if reset after some invocations of heartbeat have run, the accumulated data is
+  // lost and can cause test failure.
   used_mem_current = 0;
+  service_->Init(nullptr, {});
 
   TEST_current_time_ms = absl::GetCurrentTimeNanos() / 1000000;
   auto default_ns = &namespaces->GetDefaultNamespace();