Make sure sparse mode is used on secrets where it is explicit

Sparse mode is marked in xxx_keys item with:

> mode: "sparse"

but when decrypting secrets, each node is trying to read the sparse format first
(xxx_key_[node]) and then fallback to normal xxx_keys.
This adds a performance penalty both on reading secrets and during refresh.

With this patch, sparse format is checked only when secret is marked as sparse.

This makes refresh a fast no-op (it was already a no-op with
chef#269 now it is faster) since the only
cost is now searching nodes matching search_query.

Change-Id: I38f511b9f590240775085a386b387c476d3a1f5c
Signed-off-by: Grégoire Seux <[email protected]>

lib/chef-vault/item_keys.rb

@@ -39,7 +39,7 @@ def [](key)
       ckey = @cache[key]
       return ckey unless ckey.nil?
       # check if the key is saved in sparse mode
-      skey = sparse_key(sparse_id(key))
+      skey = sparse_key(sparse_id(key)) if sparse?
       if skey
         skey[key]
       else
@@ -213,6 +213,10 @@ def self.load(vault, name)
 
     # @private
 
+    def sparse?
+      @raw_data["mode"] == "sparse"
+    end
+
     def sparse_id(key, item_id = @raw_data["id"])
       "#{item_id.chomp("_keys")}_key_#{key}"
     end