Always pass MergeContext as pointer, not reference

Summary: To follow the coding convention and make sure when passing reference as a parameter it is also const, pass MergeContext as a pointer to mem tables.

Test Plan: make all check

Reviewers: ljin, igor

Reviewed By: igor

Subscribers: leveldb, dhruba, yhchiang

Differential Revision: https://reviews.facebook.net/D23085

Author: siying

db/db_impl.cc

@@ -3449,10 +3449,10 @@ Status DBImpl::GetImpl(const ReadOptions& options,
   LookupKey lkey(key, snapshot);
   PERF_TIMER_STOP(get_snapshot_time);
 
-  if (sv->mem->Get(lkey, value, &s, merge_context)) {
+  if (sv->mem->Get(lkey, value, &s, &merge_context)) {
     // Done
     RecordTick(stats_, MEMTABLE_HIT);
-  } else if (sv->imm->Get(lkey, value, &s, merge_context)) {
+  } else if (sv->imm->Get(lkey, value, &s, &merge_context)) {
     // Done
     RecordTick(stats_, MEMTABLE_HIT);
   } else {
@@ -3537,9 +3537,9 @@ std::vector<Status> DBImpl::MultiGet(
     assert(mgd_iter != multiget_cf_data.end());
     auto mgd = mgd_iter->second;
     auto super_version = mgd->super_version;
-    if (super_version->mem->Get(lkey, value, &s, merge_context)) {
+    if (super_version->mem->Get(lkey, value, &s, &merge_context)) {
       // Done
-    } else if (super_version->imm->Get(lkey, value, &s, merge_context)) {
+    } else if (super_version->imm->Get(lkey, value, &s, &merge_context)) {
       // Done
     } else {
       super_version->current->Get(options, lkey, value, &s, &merge_context);

db/db_impl_readonly.cc

@@ -61,7 +61,7 @@ Status DBImplReadOnly::Get(const ReadOptions& read_options,
   SuperVersion* super_version = cfd->GetSuperVersion();
   MergeContext merge_context;
   LookupKey lkey(key, snapshot);
-  if (super_version->mem->Get(lkey, value, &s, merge_context)) {
+  if (super_version->mem->Get(lkey, value, &s, &merge_context)) {
   } else {
     super_version->current->Get(read_options, lkey, value, &s, &merge_context);
   }

db/memtable.cc

@@ -430,7 +430,7 @@ static bool SaveValue(void* arg, const char* entry) {
 }
 
 bool MemTable::Get(const LookupKey& key, std::string* value, Status* s,
-                   MergeContext& merge_context) {
+                   MergeContext* merge_context) {
   // The sequence number is updated synchronously in version_set.h
   if (IsEmpty()) {
     // Avoiding recording stats for speed.
@@ -454,7 +454,7 @@ bool MemTable::Get(const LookupKey& key, std::string* value, Status* s,
     saver.value = value;
     saver.status = s;
     saver.mem = this;
-    saver.merge_context = &merge_context;
+    saver.merge_context = merge_context;
     saver.merge_operator = ioptions_.merge_operator;
     saver.logger = ioptions_.info_log;
     saver.inplace_update_support = moptions_.inplace_update_support;

db/memtable.h

@@ -118,7 +118,7 @@ class MemTable {
   //   store MergeInProgress in s, and return false.
   // Else, return false.
   bool Get(const LookupKey& key, std::string* value, Status* s,
-           MergeContext& merge_context);
+           MergeContext* merge_context);
 
   // Attempts to update the new_value inplace, else does normal Add
   // Pseudocode

db/memtable_list.cc

@@ -62,7 +62,7 @@ int MemTableList::size() const {
 // Return the most recent value found, if any.
 // Operands stores the list of merge operations to apply, so far.
 bool MemTableListVersion::Get(const LookupKey& key, std::string* value,
-                              Status* s, MergeContext& merge_context) {
+                              Status* s, MergeContext* merge_context) {
   for (auto& memtable : memlist_) {
     if (memtable->Get(key, value, s, merge_context)) {
       return true;

db/memtable_list.h

@@ -46,7 +46,7 @@ class MemTableListVersion {
   // Search all the memtables starting from the most recent one.
   // Return the most recent value found, if any.
   bool Get(const LookupKey& key, std::string* value, Status* s,
-           MergeContext& merge_context);
+           MergeContext* merge_context);
 
   void AddIterators(const ReadOptions& options,
                     std::vector<Iterator*>* iterator_list, Arena* arena);