Removed __unused__ attribute

Summary:
The current build is failing on some platforms due to an __unused__ attribute.
This patch prevents the problem by using a pattern similar to MergeHelper
(assert not on the variable but inside a condition that uses the variable). We
should have better error handling in both cases in the future.

Test Plan: make clean all check

Reviewers: rven, anthony, yhchiang, sdong, igor, aekmekji

Reviewed By: aekmekji

Subscribers: dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D46623

db/compaction_iterator.cc

@@ -98,8 +98,11 @@ void CompactionIterator::NextFromInput() {
     if (!ParseInternalKey(key_, &ikey_)) {
       // If `expect_valid_internal_key_` is false, return the corrupted key
       // and let the caller decide what to do with it.
-      // TODO(noetzli): Maybe we should have a more elegant solution for this.
+      // TODO(noetzli): We should have a more elegant solution for this.
-      assert(!expect_valid_internal_key_);
+      if (expect_valid_internal_key_) {
+        assert(!"corrupted internal key is not expected");
+        break;
+      }
       current_user_key_.Clear();
       has_current_user_key_ = false;
       current_user_key_sequence_ = kMaxSequenceNumber;

db/compaction_iterator.h

@@ -90,7 +90,7 @@ class CompactionIterator {
   MergeHelper* merge_helper_;
   const std::vector<SequenceNumber>* snapshots_;
   Env* env_;
-  bool expect_valid_internal_key_ __attribute__((__unused__));
+  bool expect_valid_internal_key_;
   Statistics* stats_;
   Compaction* compaction_;
   const CompactionFilter* compaction_filter_;