Do not submit multiple simultaneous seek-compaction requests.

Summary:
The code was such that if multi-threaded-compactions as well
as seek compaction are enabled then it submits multiple
compaction request for the same range of keys. This causes
extraneous sst-files to accumulate at various levels.

Test Plan:
I am not able to write a very good unit test for this one
but can easily reproduce this bug with 'dbstress' with the
following options.

batch=1;maxk=100000000;ops=100000000;ro=0;fm=2;bpl=10485760;of=500000; wbn=3; mbc=20; mb=2097152; wbs=4194304; dds=1; sync=0;  t=32; bs=16384; cs=1048576; of=500000; ./db_stress --disable_seek_compaction=0 --mmap_read=0 --threads=$t --block_size=$bs --cache_size=$cs --open_files=$of --verify_checksum=1 --db=/data/mysql/leveldb/dbstress.dir --sync=$sync --disable_wal=1 --disable_data_sync=$dds --write_buffer_size=$wbs --target_file_size_base=$mb --target_file_size_multiplier=$fm --max_write_buffer_number=$wbn --max_background_compactions=$mbc --max_bytes_for_level_base=$bpl --reopen=$ro --ops_per_thread=$ops --max_key=$maxk --test_batches_snapshots=$batch

Reviewers: leveldb, emayanke

Reviewed By: emayanke

Differential Revision: https://reviews.facebook.net/D11055

db/version_set.cc

@@ -2061,14 +2061,23 @@ Compaction* VersionSet::PickCompaction() {
   }
 
   // Find compactions needed by seeks
-  if (c == nullptr && (current_->file_to_compact_ != nullptr)) {
+  FileMetaData* f = current_->file_to_compact_;
+  if (c == nullptr && f != nullptr && !f->being_compacted) {
+
     level = current_->file_to_compact_level_;
+    int parent_index = -1;
 
     // Only allow one level 0 compaction at a time.
+    // Do not pick this file if its parents at level+1 are being compacted.
     if (level != 0 || compactions_in_progress_[0].empty()) {
+      if(!ParentRangeInCompaction(&f->smallest, &f->largest, level,
+                                  &parent_index)) {
         c = new Compaction(level, MaxFileSizeForLevel(level),
                 MaxGrandParentOverlapBytes(level), NumberLevels(), true);
-      c->inputs_[0].push_back(current_->file_to_compact_);
+        c->inputs_[0].push_back(f);
+        c->parent_index_ = parent_index;
+        current_->file_to_compact_ = nullptr;
+      }
     }
   }