HashBucketLock: Fix defaulted-function-deleted warning

As reported by clang 8, the move assigment operator of
StoredValueProxy is implicitly deleted because one of it's members (of
type HashBucketLock) does _not_ have a move assignment operator
itself:

    kv_engine/engines/ep/src/hash_table.h:682:27: warning: explicitly
    defaulted move assignment operator is implicitly deleted
    [-Wdefaulted-function-deleted]

        StoredValueProxy& operator=(StoredValueProxy&&) = default;
                          ^
    kv_engine/engines/ep/src/hash_table.h:720:24: note: move
    assignment operator of 'StoredValueProxy' is implicitly deleted
    because field 'lock' has a deleted move assignment operator

        HashBucketLock lock;
                       ^

    kv_engine/engines/ep/src/hash_table.h:397:9: note: copy assignment
    operator is implicitly deleted because 'HashBucketLock' has a
    user-declared move constructor

        HashBucketLock(HashBucketLock&& other)
        ^

There's also a further problem with `valueStats` as that is a
reference which also cannot be used.

Fix by defining a move-assignment operator for HashBucketLock, and use
a reference_wrapper for `valueStats`.

Change-Id: Ib2e2b6617d31ca3bb70666ec01338516da0ffb71
Reviewed-on: http://review.couchbase.org/110144
Tested-by: Build Bot <[email protected]>
Reviewed-by: Ben Huddleston <[email protected]>

Author: daverigby

engines/ep/src/hash_table.cc

@@ -92,11 +92,11 @@ HashTable::StoredValueProxy::StoredValueProxy(HashBucketLock&& hbl,
     : lock(std::move(hbl)),
       value(sv),
       valueStats(stats),
-      pre(valueStats.prologue(sv)) {
+      pre(valueStats.get().prologue(sv)) {
 }
 
 HashTable::StoredValueProxy::~StoredValueProxy() {
-    valueStats.epilogue(pre, value);
+    valueStats.get().epilogue(pre, value);
 }
 
 void HashTable::StoredValueProxy::setCommitted(CommittedState state) {

engines/ep/src/hash_table.h

@@ -398,7 +398,15 @@ class HashTable {
             : bucketNum(other.bucketNum), htLock(std::move(other.htLock)) {
         }
 
+        // Cannot copy HashBucketLock.
         HashBucketLock(const HashBucketLock& other) = delete;
+        HashBucketLock& operator=(const HashBucketLock& other) = delete;
+
+        HashBucketLock& operator=(HashBucketLock&& other) {
+            bucketNum = other.bucketNum;
+            htLock = std::move(other.htLock);
+            return *this;
+        }
 
         int getBucketNum() const {
             return bucketNum;
@@ -719,7 +727,8 @@ class HashTable {
     private:
         HashBucketLock lock;
         StoredValue* value;
-        Statistics& valueStats;
+        // Using ref wrapper to support move.
+        std::reference_wrapper<Statistics> valueStats;
         Statistics::StoredValueProperties pre;
     };