Fix bug in KeyMayExist

Summary: In KeyMayExist.db_test we do a Flush which causes sst file to be written and added as open file in TableCache, but block cache for the file is not populated. So value_found should have been false where it was true and KeyMayExist.db_test should not have passed earlier. But it passed because BlockReader in table/table.cc takes 2 default arguments at the end called for_compaction and no_io. Although I passed no_io=true from InternalGet to BlockReader, but it understood for_compaction=true and defaulted no_io to false. This is a bug and although will be removed by Dhruba's new patch to incorporate no_io in readoptions, I'm submitting this patch to fix this bug independently of that patch.

Test Plan: make all check

Reviewers: dhruba, haobo

Reviewed By: dhruba

CC: leveldb

Differential Revision: https://reviews.facebook.net/D12537

db/db_test.cc

@@ -843,8 +843,7 @@ TEST(DBTest, KeyMayExist) {
     value.clear();
     value_found = false;
     ASSERT_TRUE(db_->KeyMayExist(ropts, "a", &value, &value_found));
-    ASSERT_TRUE(value_found);
-    ASSERT_EQ("b", value);
+    ASSERT_TRUE(!value_found);
 
     ASSERT_OK(db_->Delete(WriteOptions(), "a"));
     ASSERT_TRUE(!db_->KeyMayExist(ropts, "a", &value));

table/table.cc

@@ -421,7 +421,7 @@ Status Table::InternalGet(const ReadOptions& options, const Slice& k,
     } else {
       bool didIO = false;
       Iterator* block_iter = BlockReader(this, options, iiter->value(),
-                                         &didIO, no_io);
+                                         &didIO, false, no_io);
 
       if (no_io && !block_iter) { // couldn't get block from block_cache
         // Update Saver.state to Found because we are only looking for whether