Don't assume all responses have headers

CHANGELOG.md

@@ -8,7 +8,9 @@
 
 ##### Bug Fixes
 
-* None.  
+* Fix a crash when an HTTP(S) response has no headers  
+  [Eric Amorde](https://github.com/amorde)
+  [#607](https://github.com/CocoaPods/Core/pull/607)
 
 
 ## 1.9.0.beta.2 (2019-12-17)

lib/cocoapods-core/cdn_source.rb

@@ -378,7 +378,7 @@ def download_and_save_with_retries_async(partial_url, file_remote_url, etag, ret
         when 200
           File.open(path, 'w') { |f| f.write(response.response_body) }
 
-          etag_new = response.headers['etag']
+          etag_new = response.headers['etag'] unless response.headers.nil?
           debug "CDN: #{name} Relative path downloaded: #{partial_url}, save ETag: #{etag_new}"
           File.open(etag_path, 'w') { |f| f.write(etag_new) } unless etag_new.nil?
           partial_url

spec/cdn_source_spec.rb

@@ -190,6 +190,18 @@ def typhoeus_non_http_response_future(code)
         @source.versions('BeaconKit').map(&:to_s).should == %w(1.0.0)
       end
 
+      it 'handles responses with no headers' do
+        @source.expects(:download_typhoeus_impl_async).
+          with_url('http://localhost:4321/all_pods_versions_2_0_9.txt').
+          returns(typhoeus_http_response_future(200, nil, 'BeaconKit/1.0.0'))
+        @source.expects(:download_typhoeus_impl_async).
+          with_url('http://localhost:4321/Specs/2/0/9/BeaconKit/1.0.0/BeaconKit.podspec.json').
+          returns(typhoeus_http_response_future(200, nil, '{}'))
+        should.not.raise do
+          @source.versions('BeaconKit')
+        end
+      end
+
       it 'raises if unexpected HTTP error' do
         @source.expects(:download_typhoeus_impl_async).
           returns(typhoeus_http_response_future(500))