[CHEF-4918] Don't destructively merge subhashes in hash_only_merge!ssd/CHEF-4918

hash_only_merge dups its inputs and then passes it to
hash_only_merge!.  Unfortunately, dup does not make a deep copy,
leading hash_only_merge to mutate deeply nested structures.

2 files changed, 12 insertions, 3 deletions

diff --git a/lib/chef/mixin/deep_merge.rb b/lib/chef/mixin/deep_merge.rb
index ad3e5803fd..5002f5dcc5 100644
--- a/lib/chef/mixin/deep_merge.rb
+++ b/lib/chef/mixin/deep_merge.rb
@@ -122,7 +122,11 @@ class Chef
         # If there are two Hashes, recursively merge.
         if merge_onto.kind_of?(Hash) && merge_with.kind_of?(Hash)
           merge_with.each do |key, merge_with_value|
-            merge_onto[key] = hash_only_merge!(merge_onto[key], merge_with_value)
+            merge_onto[key] = if merge_onto.has_key?(key)
+                                hash_only_merge(merge_onto[key], merge_with_value)
+                              else
+                                merge_with_value
+                              end
           end
           merge_onto
 
@@ -164,5 +168,3 @@ class Chef
     end
   end
 end
-
-
diff --git a/spec/unit/mixin/deep_merge_spec.rb b/spec/unit/mixin/deep_merge_spec.rb
index 0a7bbffa41..b7828a3076 100644
--- a/spec/unit/mixin/deep_merge_spec.rb
+++ b/spec/unit/mixin/deep_merge_spec.rb
@@ -347,5 +347,12 @@ describe Chef::Mixin::DeepMerge do
       merged_result["top_level_a"]["1_deep_b"].should == %w[B B B]
     end
 
+    it "does not mutate deeply-nested original hashes by default" do
+      merge_ee_hash =   {"top_level_a" => {"1_deep_a" => { "2_deep_a" => { "3_deep_a" => "foo" }}}}
+      merge_with_hash = {"top_level_a" => {"1_deep_a" => { "2_deep_a" => { "3_deep_b" => "bar" }}}}
+      @dm.hash_only_merge(merge_ee_hash, merge_with_hash)
+      merge_ee_hash.should == {"top_level_a" => {"1_deep_a" => { "2_deep_a" => { "3_deep_a" => "foo" }}}}
+      merge_with_hash.should == {"top_level_a" => {"1_deep_a" => { "2_deep_a" => { "3_deep_b" => "bar" }}}}
+    end
   end
 end