[MiqFtpStorage] Add #magic_number_for support

Since we need a connection for any download/upload to work (sets @ftp on
the instance), we need a slight override of `#magic_number_for` to wrap
it with a connection.

lib/gems/pending/util/object_storage/miq_ftp_storage.rb

@@ -32,6 +32,11 @@ def download(*download_args)
     with_connection { super }
   end
 
+  # Override for connection handling
+  def magic_number_for(*magic_number_for_args)
+    with_connection { super }
+  end
+
   # Specific version of Net::FTP#storbinary that doesn't use an existing local
   # file, and only uploads a specific size (byte_count) from the input_file
   def upload_single(dest_uri)

spec/util/object_storage/miq_ftp_storage_spec.rb

@@ -115,21 +115,26 @@
       # Nothing written, just printed the streamed file in the above command
       expect(source_data.size).to eq(10.megabytes)
     end
+  end
 
-    # Note, we are using `#download` in the spec, but really, this is a feature
-    # for `#download_single`.  `#download` really doesn't make use of this
-    # directly.
-    it "respects the `byte_count` attribute" do
-      downloaded_data = StringIO.new
-      subject.instance_variable_set(:@byte_count, 42)
-      subject.download(downloaded_data, source_path)
+  describe "#magic_number_for" do
+    let(:source_file) { existing_ftp_file(10.megabytes) }
+    let(:source_path) { File.basename(source_file.path) }
 
-      # Sanity check that what we are downloading is the size we expect
-      expect(source_path).to exist_on_ftp_server
-      expect(source_path).to have_size_on_ftp_server_of(10.megabytes)
+    it "returns 256 bytes by default" do
+      result = subject.magic_number_for(source_path)
+
+      expect(result.size).to eq(256)
+      expect(result).to      eq("0" * 256)
+    end
 
-      expect(downloaded_data.string.size).to eq(42)
-      expect(downloaded_data.string).to      eq("0" * 42)
+    describe "with a hash of accepted magics" do
+      it "returns key for the passed in magic number value" do
+        magics = { :zero => "000", :one => "1", :foo => "bar" }
+        result = subject.magic_number_for(source_path, :accepted => magics)
+
+        expect(result).to eq(:zero)
+      end
     end
   end
 end