Don't compact with zero input files

Summary:
We have an issue with internal service trying to run compaction with zero input files:
2014/02/07-02:26:58.386531 7f79117ec700 Compaction start summary: Base version 1420 Base level 3, seek compaction:0, inputs:[ϛ~^Qy^?],[]
2014/02/07-02:26:58.386539 7f79117ec700 Compacted 0@3 + 0@4 files => 0 bytes

There are two issues:
* inputsummary is printing out junk
* it's constantly retrying (since I guess madeProgress is true), so it prints out a lot of data in the LOG file (40GB in one day).

I read through the Level compaction picker and added some failure condition if input[0] is empty. I think PickCompaction() should not return compaction with zero input files with this change. I'm not confident enough to add an assertion though :)

Test Plan: make check

Reviewers: dhruba, haobo, sdong, kailiu

Reviewed By: haobo

CC: leveldb

Differential Revision: https://reviews.facebook.net/D16005

db/compaction.cc

@@ -197,6 +197,7 @@ static void FileSizeSummary(unsigned long long sz, char* output, int len) {
 
 static int InputSummary(std::vector<FileMetaData*>& files, char* output,
                          int len) {
+  *output = '\0';
   int write = 0;
   for (unsigned int i = 0; i < files.size(); i++) {
     int sz = len - write;
@@ -233,9 +234,7 @@ void Compaction::Summary(char* output, int len) {
     return;
   }
 
-  if (inputs_[1].size()) {
-    write += InputSummary(inputs_[1], output+write, len-write);
-  }
+  write += InputSummary(inputs_[1], output+write, len-write);
   if (write < 0 || write >= len) {
     return;
   }

db/compaction_picker.cc

@@ -178,7 +178,11 @@ bool CompactionPicker::ExpandWhileOverlapping(Compaction* c) {
   // If, after the expansion, there are files that are already under
   // compaction, then we must drop/cancel this compaction.
   int parent_index = -1;
-  if (FilesInCompaction(c->inputs_[0]) ||
+  if (c->inputs_[0].empty()) {
+    Log(options_->info_log,
+        "ExpandWhileOverlapping() failure because zero input files");
+  }
+  if (c->inputs_[0].empty() || FilesInCompaction(c->inputs_[0]) ||
       (c->level() != c->output_level() &&
        ParentRangeInCompaction(c->input_version_, &smallest, &largest, level,
                                &parent_index))) {