add back diffing against no file for why-run mode

2 files changed, 25 insertions, 10 deletions

diff --git a/lib/chef/util/diff.rb b/lib/chef/util/diff.rb
index 72459642de..1151722af8 100644
--- a/lib/chef/util/diff.rb
+++ b/lib/chef/util/diff.rb
@@ -33,13 +33,27 @@ class Chef
         @diff.join("\\n")
       end
 
+      def use_tempfile_if_missing(file)
+        tempfile = nil
+        unless File.exists?(file)
+          Chef::Log.debug("file #{file} does not exist to diff against, using empty tempfile")
+          tempfile = Tempfile.new("chef-diff")
+          file = tempfile.path
+        end
+        yield file
+        unless tempfile.nil?
+          tempfile.close
+          tempfile.unlink
+        end
+      end
+
       def diff(old_file, new_file)
-        # indicates calling code bug: caller is reponsible for making certain both
-        # files exist
-        raise "old file #{old_file} does not exist" unless File.exists?(old_file)
-        raise "new file #{new_file} does not exist" unless File.exists?(new_file)
-        @error = catch (:nodiff) do
-          do_diff(old_file, new_file)
+        use_tempfile_if_missing(old_file) do |old_file|
+          use_tempfile_if_missing(new_file) do |new_file|
+            @error = catch (:nodiff) do
+              do_diff(old_file, new_file)
+            end
+          end
         end
       end
 
@@ -63,6 +77,7 @@ class Chef
 
         begin
           # -u: Unified diff format
+          Chef::Log.debug("running: diff -u #{old_file} #{new_file}")
           result = shell_out("diff -u #{old_file} #{new_file}")
         rescue Exception => e
           # Should *not* receive this, but in some circumstances it seems that
diff --git a/spec/unit/util/diff_spec.rb b/spec/unit/util/diff_spec.rb
index a856902c9b..4cc72c0006 100644
--- a/spec/unit/util/diff_spec.rb
+++ b/spec/unit/util/diff_spec.rb
@@ -44,16 +44,16 @@ describe Chef::Util::Diff, :uses_diff => true do
     expect(differ).to be_a_kind_of(Chef::Util::Diff)
   end
 
-  it "should raise an exception if the old_file does not exist" do
+  it "produces a diff even if the old_file does not exist" do
     old_tempfile.close
     old_tempfile.unlink
-    expect { differ.diff(old_file, new_file) }.to raise_error
+    expect(differ.for_output).to eql(["(no diff)"])
   end
 
-  it "should raise an exception if the new_file does not exist" do
+  it "produces a diff even if the new_file does not exist" do
     new_tempfile.close
     new_tempfile.unlink
-    expect { differ.diff(old_file, new_file) }.to raise_error
+    expect(differ.for_output).to eql(["(no diff)"])
   end
 
   describe "when the two files exist with no content" do