Fix engine.

Summary:
Test engine update.

https://docs.google.com/document/d/1rnFsCFock-K3ZbuvMiCYWhGS_m7-DZV37JtxWRClD_8/edit

Reviewers: mislav.bradac, dtomicevic, buda

Reviewed By: buda

Subscribers: pullbot

Differential Revision: https://phabricator.memgraph.io/D167

src/transactions/engine.hpp

@@ -21,7 +21,7 @@ class Engine : Lockable<SpinLock> {
  public:
   using sptr = std::shared_ptr<Engine>;
 
-  Engine() : counter(1) {}
+  Engine() : counter(0) {}
 
   // Begins transaction and runs given functions in same atomic step.
   // Functions will be given Transaction&
@@ -77,9 +77,15 @@ class Engine : Lockable<SpinLock> {
     finalize(t);
   }
 
-  Id last_known_active() {
+  /*
+   *@brief Return oldest active transaction in the active transaction pool. In
+   *case none exist return None.
+   *@return Id of transaction
+   */
+  Option<Id> oldest_active() {
     auto guard = this->acquire_unique();
-    return active.front();
+    if (active.size() == 0) return Option<Id>();
+    return Option<Id>(active.front());
   }
 
   // total number of transactions started from the beginning of time

tests/concurrent/transaction_engine.cpp

@@ -38,8 +38,7 @@ int main() {
   for (int i = 0; i < THREADS; ++i) sum_computed += sums[i];
 
   uint64_t sum_actual = 0;
-  for (uint64_t i = 2; i <= THREADS * TRANSACTIONS + 1; ++i) sum_actual += i;
+  for (uint64_t i = 1; i <= THREADS * TRANSACTIONS; ++i) sum_actual += i;
-  // the range is strange because the first transaction gets transaction id 2
 
   std::cout << sum_computed << " " << sum_actual << std::endl;
   permanent_assert(sum_computed == sum_actual, "sums have to be the same");

tests/unit/transaction_engine.cpp

@@ -0,0 +1,58 @@
+#include "gtest/gtest.h"
+
+#include <vector>
+
+#include "transactions/engine.hpp"
+#include "transactions/transaction.hpp"
+
+TEST(Engine, Count) {
+  tx::Engine eng;
+  EXPECT_EQ(eng.count(), 0);
+}
+
+TEST(Engine, CountFive) {
+  tx::Engine eng;
+  EXPECT_EQ(eng.count(), (uint64_t)0);
+  std::vector<tx::Transaction *> V;
+  for (int i = 0; i < 5; ++i) {
+    V.push_back(eng.begin());
+    EXPECT_EQ(eng.count(), (uint64_t)(i + 1));
+  }
+  EXPECT_EQ(eng.size(), (uint64_t)5);
+  for (int i = 0; i < 5; ++i) V[i]->commit();
+  EXPECT_EQ(eng.count(), (uint64_t)5);
+}
+
+TEST(Engine, LastKnownActiveEmpty) {
+  tx::Engine eng;
+  EXPECT_EQ(eng.oldest_active().is_present(), false);
+}
+
+TEST(Engine, LastKnownActive) {
+  tx::Engine eng;
+  std::vector<tx::Transaction *> V;
+  for (int i = 0; i < 5; ++i) {
+    V.push_back(eng.begin());
+    EXPECT_EQ(eng.size(), (size_t)i + 1);
+  }
+  for (int i = 0; i < 5; ++i) {
+    EXPECT_EQ(eng.oldest_active().get(), Id(i + 1));
+    V[i]->commit();
+  }
+  EXPECT_EQ(eng.oldest_active().is_present(), false);
+}
+
+TEST(Engine, Size) {
+  tx::Engine eng;
+  std::vector<tx::Transaction *> V;
+  for (int i = 0; i < 5; ++i) {
+    V.push_back(eng.begin());
+    EXPECT_EQ(eng.size(), (size_t)i + 1);
+  }
+  for (int i = 0; i < 5; ++i) V[i]->commit();
+}
+
+int main(int argc, char **argv) {
+  ::testing::InitGoogleTest(&argc, argv);
+  return RUN_ALL_TESTS();
+}