From 8dd0a7e11abad6bc32e59a5cba8769961e085312 Mon Sep 17 00:00:00 2001
From: Andrew Kryczka <andrewkr@fb.com>
Date: Wed, 11 Oct 2017 15:11:08 -0700
Subject: [PATCH] add comment in SuperVersion referencing logic

Summary:
The referencing logic is super confusing so added a comment at the part that took me longest to figure out.
Closes https://github.com/facebook/rocksdb/pull/2996

Differential Revision: D6034969

Pulled By: ajkr

fbshipit-source-id: 9cc2e744c1f79d6d57d378f86ed59238a5f583db
---
 db/column_family.cc | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/db/column_family.cc b/db/column_family.cc
index 766508c36..367d4b608 100644
--- a/db/column_family.cc
+++ b/db/column_family.cc
@@ -854,6 +854,10 @@ SuperVersion* ColumnFamilyData::GetReferencedSuperVersion(
   sv = GetThreadLocalSuperVersion(db_mutex);
   sv->Ref();
   if (!ReturnThreadLocalSuperVersion(sv)) {
+    // This Unref() corresponds to the Ref() in GetThreadLocalSuperVersion()
+    // when the thread-local pointer was populated. So, the Ref() earlier in
+    // this function still prevents the returned SuperVersion* from being
+    // deleted out from under the caller.
     sv->Unref();
   }
   return sv;