Do not do internal item_realloc() every time

refcount == 2 is "linked and I own it"
code was doing a replace if refcount != 1 (which would mean unlinked and I own
it, in this case. If that code ever actually misfired some weird stuff might
happen).

Author: dormando

items.c

@@ -345,6 +345,7 @@ void do_item_unlink_nolock(item *it, const uint32_t hv) {
 void do_item_remove(item *it) {
     MEMCACHED_ITEM_REMOVE(ITEM_key(it), it->nkey, it->nbytes);
     assert((it->it_flags & ITEM_SLABBED) == 0);
+    assert(it->refcount > 0);
 
     if (refcount_decr(&it->refcount) == 0) {
         item_free(it);

memcached.c

@@ -3162,7 +3162,9 @@ enum delta_result_type do_add_delta(conn *c, const char *key, const size_t nkey,
 
     snprintf(buf, INCR_MAX_STORAGE_LEN, "%llu", (unsigned long long)value);
     res = strlen(buf);
-    if (res + 2 > it->nbytes || it->refcount != 1) { /* need to realloc */
+    /* refcount == 2 means we are the only ones holding the item, and it is
+     * linked. */
+    if (res + 2 > it->nbytes || it->refcount != 2) { /* need to realloc */
         item *new_it;
         new_it = do_item_alloc(ITEM_key(it), it->nkey, atoi(ITEM_suffix(it) + 1), it->exptime, res + 2, hv);
         if (new_it == 0) {