New unit test for iterator with snapshot

Summary:
I played with the reported bug about iterator with snapshot:
https://code.google.com/p/leveldb/issues/detail?id=200.

I turned the original test program
(https://code.google.com/p/leveldb/issues/attachmentText?id=200&aid=2000000000&name=test.cc&token=7uOUQW-HFlbAFMUm7EqtaAEy7Tw%3A1378320724136)
into a new unit test, but I cannot reproduce the problem. Notice lines
31-34 in above link. I have ran the new test with and without such Put()
operations. Both succeed.

So this diff simply adds the test, without changing any source codes.

Test Plan: run new test.

Reviewers: dhruba, haobo, emayanke

Reviewed By: dhruba

CC: leveldb

Differential Revision: https://reviews.facebook.net/D12735

db/db_impl.cc

@@ -3143,7 +3143,6 @@ Status DB::Open(const Options& options, const std::string& dbname, DB** dbptr) {
   impl->mutex_.Unlock();
 
   if (options.compaction_style == kCompactionStyleUniversal) {
-    std::string property;
     int num_files;
     for (int i = 1; i < impl->NumberLevels(); i++) {
       num_files = impl->versions_->NumLevelFiles(i);

db/db_test.cc

@@ -1283,6 +1283,44 @@ TEST(DBTest, IterMultiWithDelete) {
   } while (ChangeOptions());
 }
 
+TEST(DBTest, IterWithSnapshot) {
+  do {
+    ASSERT_OK(Put("key1", "val1"));
+    ASSERT_OK(Put("key2", "val2"));
+    ASSERT_OK(Put("key3", "val3"));
+    ASSERT_OK(Put("key4", "val4"));
+    ASSERT_OK(Put("key5", "val5"));
+
+    const Snapshot *snapshot = db_->GetSnapshot();
+    ReadOptions options;
+    options.snapshot = snapshot;
+    Iterator* iter = db_->NewIterator(options);
+
+    // Put more values after the snapshot
+    ASSERT_OK(Put("key100", "val100"));
+    ASSERT_OK(Put("key101", "val101"));
+
+    iter->Seek("key5");
+    ASSERT_EQ(IterStatus(iter), "key5->val5");
+    if (!CurrentOptions().merge_operator) {
+      // TODO: merge operator does not support backward iteration yet
+      iter->Prev();
+      ASSERT_EQ(IterStatus(iter), "key4->val4");
+      iter->Prev();
+      ASSERT_EQ(IterStatus(iter), "key3->val3");
+
+      iter->Next();
+      ASSERT_EQ(IterStatus(iter), "key4->val4");
+      iter->Next();
+      ASSERT_EQ(IterStatus(iter), "key5->val5");
+      iter->Next();
+      ASSERT_TRUE(!iter->Valid());
+    }
+    db_->ReleaseSnapshot(snapshot);
+    delete iter;
+  } while (ChangeOptions());
+}
+
 TEST(DBTest, Recover) {
   do {
     ASSERT_OK(Put("foo", "v1"));