Use unique temp folder during lint for parallel execution

CHANGELOG.md

@@ -30,6 +30,10 @@ To install release candidates run `[sudo] gem install cocoapods --pre`
 
 ##### Bug Fixes
 
+* Use unique temp folder during lint for parallel execuition    
+  [Dimitris Koutsogiorgas](https://github.com/dnkoutso)
+  [#5117](https://github.com/CocoaPods/CocoaPods/issues/5117)
+
 * Only check for valid Swift version for pod targets that use Swift  
   [Dimitris Koutsogiorgas](https://github.com/dnkoutso)
   [#6733](https://github.com/CocoaPods/CocoaPods/pull/6733) 

lib/cocoapods/validator.rb

@@ -242,7 +242,7 @@ module Pod
     # @return [Pathname] the temporary directory used by the linter.
     #
     def validation_dir
-      Pathname(Dir.tmpdir) + 'CocoaPods/Lint'
+      @validation_dir ||= Pathname(Dir.mktmpdir(['CocoaPods-Lint-', "-#{spec.name}"]))
     end
 
     # @return [String] the SWIFT_VERSION to use for validation.

spec/unit/validator_spec.rb

@@ -88,7 +88,7 @@ module Pod
         validator.quick = true
         validator.stubs(:validate_url)
         validator.validate
-        validator.validation_dir.should.be == Pathname.new(Dir.tmpdir) + 'CocoaPods/Lint'
+        validator.validated?.should.be.true
       end
 
       describe '#only_subspec' do