Change scheduler to first wait then execute

Reviewers: dgleich

Reviewed By: dgleich

Subscribers: pullbot

Differential Revision: https://phabricator.memgraph.io/D884

src/utils/scheduler.hpp

@@ -32,9 +32,13 @@ class Scheduler {
     thread_ = std::thread([this, pause, f]() {
       auto start_time = std::chrono::system_clock::now();
 
-      while (is_working_) {
-        f();
-
+      while (true) {
+        // First wait then execute the function. We do that in that order
+        // because most of the schedulers are started at the beginning of the
+        // program and there is probably no work to do in scheduled function at
+        // the start of the program. Since Server will log some messages on
+        // the program start we let him log first and we make sure by first
+        // waiting that funcion f will not log before it.
         std::unique_lock<std::mutex> lk(mutex_);
         auto now = std::chrono::system_clock::now();
         start_time += pause;
@@ -45,6 +49,9 @@ class Scheduler {
         } else {
           start_time = now;
         }
+
+        if (!is_working_) break;
+        f();
       }
     });
   }

tests/unit/scheduler.cpp

@@ -10,20 +10,22 @@
  * value.
  */
 TEST(Scheduler, TestFunctionExecuting) {
-  std::atomic<int> x{0}, y{0};
-  std::function<void()> func{[&x, &y]() {
-    EXPECT_EQ(y.load(), x.load());
-    x++;
-  }};
+  std::atomic<int> x{0};
+  std::function<void()> func{[&x]() { ++x; }};
   Scheduler scheduler;
   scheduler.Run(std::chrono::seconds(1), func);
 
-  std::this_thread::sleep_for(std::chrono::milliseconds(980));
-  y++;
-  EXPECT_EQ(x.load(), y.load());
+  EXPECT_EQ(x, 0);
+  std::this_thread::sleep_for(std::chrono::milliseconds(900));
+  EXPECT_EQ(x, 0);
+
+  std::this_thread::sleep_for(std::chrono::milliseconds(200));
+  EXPECT_EQ(x, 1);
+
+  std::this_thread::sleep_for(std::chrono::milliseconds(2000));
+  EXPECT_EQ(x, 3);
 
   std::this_thread::sleep_for(std::chrono::milliseconds(100));
   scheduler.Stop();
-  y++;
-  EXPECT_EQ(x.load(), y.load());
+  EXPECT_EQ(x, 3);
 }