[Cache] Remove cache when CocoaPods version changes

CHANGELOG.md

@@ -6,13 +6,21 @@ To install release candidates run `[sudo] gem install cocoapods --pre`
 
 ## Master
 
-### Bug Fixes
+##### Enhancements
+
+* The download cache will automatically be reset when changing CocoaPods
+  versions.  
+  [Samuel Giddins](https://github.com/segiddins)
+  [#3542](https://github.com/CocoaPods/CocoaPods/issues/3542)
+
+##### Bug Fixes
 
 * Public headers of vendored frameworks are now automatically linked in
-  the sandbox. That allows transitive inclusion of headers from other pods.
+  the sandbox. That allows transitive inclusion of headers from other pods.  
   [Vincent Isambart](https://github.com/vincentisambart)
   [#3161](https://github.com/CocoaPods/CocoaPods/issues/3161)
 
+
 ## 0.37.2
 
 ##### Enhancements

Gemfile.lock

@@ -200,4 +200,4 @@ DEPENDENCIES
   xcodeproj!
 
 BUNDLED WITH
-   1.10.0
+   1.10.2

lib/cocoapods/downloader/cache.rb

@@ -19,7 +19,7 @@ module Pod
       #
       def initialize(root)
         @root = Pathname(root)
-        @root.mkpath
+        ensure_matching_version
       end
 
       # Downloads the Pod from the given `request`
@@ -40,6 +40,21 @@ module Pod
 
       private
 
+      # Ensures the cache on disk was created with the same CocoaPods version as
+      # is currently running.
+      #
+      # @return [Void]
+      #
+      def ensure_matching_version
+        version_file = root + 'VERSION'
+        version = version_file.read.strip if version_file.file?
+
+        root.rmtree if version != Pod::VERSION && root.exist?
+        root.mkpath
+
+        version_file.open('w') { |f| f << Pod::VERSION }
+      end
+
       # @param  [Request] request
       #         the request to be downloaded.
       #

spec/unit/downloader/cache_spec.rb

@@ -30,6 +30,15 @@ module Pod
       @cache.root.should.be.directory?
     end
 
+    it 'implodes when the cache is from a different CocoaPods version' do
+      root = Pathname(Dir.mktmpdir)
+      root.+('VERSION').open('w') { |f| f << '0.0.0' }
+      root.+('FILE').open('w') { |f| f << '0.0.0' }
+      @cache = Downloader::Cache.new(root)
+      root.+('VERSION').read.should == Pod::VERSION
+      root.+('FILE').should.not.exist?
+    end
+
     it 'groups subspecs by platform' do
       @spec = Specification.new do |s|
         s.ios.deployment_target = '6.0'