Changed the CompactionJobStats::output_key_prefix type from char[] to string.

Summary:
Keys in RocksDB can be arbitrary byte strings.  However, in the current
CompactionJobStats, smallest_output_key_prefix and largest_output_key_prefix
are of type char[] without having a length, which is insufficient to handle
non-null terminated strings.

This patch change their type to std::string.

Test Plan: compaction_job_stats_test

Reviewers: igor, rven, IslamAbdelRahman, kradhakrishnan, sdong

Reviewed By: sdong

Subscribers: dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D39537

db/compaction_job.cc

@@ -1252,11 +1252,10 @@ void CompactionJob::CleanupCompaction(const Status& status) {
 #ifndef ROCKSDB_LITE
 namespace {
 void CopyPrefix(
-    char* dst, size_t dst_length, const Slice& src) {
+    const Slice& src, size_t prefix_length, std::string* dst) {
-  assert(dst_length > 0);
+  assert(prefix_length > 0);
-  size_t length = src.size() > dst_length - 1 ? dst_length - 1 : src.size();
+  size_t length = src.size() > prefix_length ? prefix_length : src.size();
-  memcpy(dst, src.data(), length);
+  dst->assign(src.data(), length);
-  dst[length] = 0;
 }
 }  // namespace
 
@@ -1285,13 +1284,13 @@ void CompactionJob::UpdateCompactionJobStats(
 
     if (compact_->outputs.size() > 0U) {
       CopyPrefix(
-          compaction_job_stats_->smallest_output_key_prefix,
+          compact_->outputs[0].smallest.user_key(),
-          sizeof(compaction_job_stats_->smallest_output_key_prefix),
+          CompactionJobStats::kMaxPrefixLength,
-          compact_->outputs[0].smallest.user_key().ToString());
+          &compaction_job_stats_->smallest_output_key_prefix);
       CopyPrefix(
-          compaction_job_stats_->largest_output_key_prefix,
+          compact_->current_output()->largest.user_key(),
-          sizeof(compaction_job_stats_->largest_output_key_prefix),
+          CompactionJobStats::kMaxPrefixLength,
-          compact_->current_output()->largest.user_key().ToString());
+          &compaction_job_stats_->largest_output_key_prefix);
     }
   }
 #endif  // !ROCKSDB_LITE

db/compaction_job_stats_test.cc

@@ -457,11 +457,10 @@ uint64_t EstimatedFileSize(
 namespace {
 
 void CopyPrefix(
-    char* dst, size_t dst_length, const Slice& src) {
+    const Slice& src, size_t prefix_length, std::string* dst) {
-  assert(dst_length > 0);
+  assert(prefix_length > 0);
-  size_t length = src.size() > dst_length - 1 ? dst_length - 1 : src.size();
+  size_t length = src.size() > prefix_length ? prefix_length : src.size();
-  memcpy(dst, src.data(), length);
+  dst->assign(src.data(), length);
-  dst[length] = 0;
 }
 
 }  // namespace
@@ -499,12 +498,12 @@ CompactionJobStats NewManualCompactionJobStats(
 
   stats.num_records_replaced = num_records_replaced;
 
-  CopyPrefix(stats.smallest_output_key_prefix,
+  CopyPrefix(smallest_key,
-             sizeof(stats.smallest_output_key_prefix),
+             CompactionJobStats::kMaxPrefixLength,
-             smallest_key);
+             &stats.smallest_output_key_prefix);
-  CopyPrefix(stats.largest_output_key_prefix,
+  CopyPrefix(largest_key,
-             sizeof(stats.largest_output_key_prefix),
+             CompactionJobStats::kMaxPrefixLength,
-             largest_key);
+             &stats.largest_output_key_prefix);
 
   return stats;
 }

include/rocksdb/compaction_job_stats.h

@@ -6,6 +6,7 @@
 #pragma once
 #include <stddef.h>
 #include <stdint.h>
+#include <string>
 
 namespace rocksdb {
 struct CompactionJobStats {
@@ -45,7 +46,9 @@ struct CompactionJobStats {
 
   // 0-terminated strings storing the first 8 bytes of the smallest and
   // largest key in the output.
-  char smallest_output_key_prefix[9];
+  static const size_t kMaxPrefixLength = 8;
-  char largest_output_key_prefix[9];
+
+  std::string smallest_output_key_prefix;
+  std::string largest_output_key_prefix;
 };
 }  // namespace rocksdb