SERVER-42931 KeyString::Value should report its memory usage as the total buffer capacity

Author: louiswilliams

src/mongo/db/storage/key_string.h

@@ -376,7 +376,8 @@ class Value {
     }
 
     int memUsageForSorter() const {
-        return sizeof(Value) + _bufSize;
+        // Use buffer capacity as a more accurate measure of memory usage.
+        return sizeof(Value) + _buffer.capacity();
     }
     /// Members for Sorter
 

src/mongo/util/shared_buffer.h

@@ -212,6 +212,10 @@ class ConstSharedBuffer {
         return _buffer.isShared();
     }
 
+    size_t capacity() const {
+        return _buffer.capacity();
+    }
+
     /**
      * Converts to a mutable SharedBuffer.
      * This is only legal to call if you have exclusive access to the underlying buffer.