[Rocksdb] Fix Crash on finding a db with no log files. Error out instead

Summary:
If the vector returned by GetUpdatesSince is empty, it is still returned to the
user. This causes it throw an std::range error.
The probable file list is checked and it returns an IOError status instead of OK now.

Test Plan: added a unit test.

Reviewers: dhruba, heyongqiang

Reviewed By: heyongqiang

CC: leveldb

Differential Revision: https://reviews.facebook.net/D9771

db/db_impl.cc

@@ -956,6 +956,10 @@ Status DBImpl::FindProbableWALFiles(std::vector<LogFile>* const allLogs,
   for( size_t i = startIndex; i < allLogs->size(); ++i) {
     result->push_back(allLogs->at(i));
   }
+  if (result->empty()) {
+    return Status::IOError(
+        "No probable files. Check if the db contains log files");
+  }
   return Status::OK();
 }
 

db/db_test.cc

@@ -2681,6 +2681,13 @@ TEST(DBTest, TransactionLogIteratorStallAtLastRecord) {
   ASSERT_TRUE(iter->Valid());
 }
 
+TEST(DBTest, TransactionLogIteratorJustEmptyFile) {
+  Options options = OptionsForLogIterTest();
+  DestroyAndReopen(&options);
+  unique_ptr<TransactionLogIterator> iter;
+  Status status = dbfull()->GetUpdatesSince(0, &iter);
+  ASSERT_TRUE(!status.ok());
+}
 TEST(DBTest, ReadCompaction) {
   std::string value(4096, '4'); // a string of size 4K
   {