[column families] Implement refcounting ColumnFamilyData

Summary: We don't want to delete ColumnFamilyData object if somebody has references to it.

Test Plan: `make check` for now, but will need to implement bigger column family test case

Reviewers: dhruba, haobo

CC: leveldb

Differential Revision: https://reviews.facebook.net/D15111

db/version_set.cc

@@ -1163,7 +1163,7 @@ VersionSet::~VersionSet() {
     cfd.second->current->Unref();
     // List must be empty
     assert(cfd.second->dummy_versions.next_ == &cfd.second->dummy_versions);
-    delete cfd.second;
+    cfd.second->Unref();
   }
   for (auto file : obsolete_files_) {
     delete file;
@@ -3059,7 +3059,8 @@ void VersionSet::DropColumnFamily(VersionEdit* edit) {
   cfd->second->current->Unref();
   // List must be empty
   assert(cfd->second->dummy_versions.next_ == &cfd->second->dummy_versions);
-  delete cfd->second;
+  // might delete itself
+  cfd->second->Unref();
   column_family_data_.erase(cfd);
 }
 

db/version_set.h

@@ -223,11 +223,29 @@ struct ColumnFamilyData {
   Version dummy_versions;  // Head of circular doubly-linked list of versions.
   Version* current;        // == dummy_versions.prev_
   ColumnFamilyOptions options;
+  int refs;
+
+  void Ref() {
+    ++refs;
+  }
+
+  void Unref() {
+    assert(refs > 0);
+    if (refs == 1) {
+      delete this;
+    } else {
+      --refs;
+    }
+  }
 
   ColumnFamilyData(const std::string& name,
                    VersionSet* vset,
                    const ColumnFamilyOptions& options)
-      : name(name), dummy_versions(vset), current(nullptr), options(options) {}
+      : name(name),
+        dummy_versions(vset),
+        current(nullptr),
+        options(options),
+        refs(1) {}
   ~ColumnFamilyData() {}
 };