Immutable CachedFiles

Previously, the CachedFile always behaved as if it was mutable, even
in situations where it was working against an immutable cache.  This
resulted in situations where a combination of intermittent network
connections and read-only file systems would cause an attempted
direction creation that would fail.  This change enhances the
CacheFile such that it can behave in both a mutable and immutable
fashion.

[#71928434][#54]

Author: nebhale

lib/java_buildpack/util/cache/cached_file.rb

@@ -31,13 +31,15 @@ class CachedFile
         #
         # @param [Pathname] cache_root the filesystem root for the file created and expected by this class
         # @param [String] uri a uri which uniquely identifies the file in the cache
-        def initialize(cache_root, uri)
-          FileUtils.mkdir_p cache_root
-
+        # @param [Boolean] mutable whether the cached file should be mutable
+        def initialize(cache_root, uri, mutable)
           key            = URI.escape(uri, ':/')
           @cached        = cache_root + "#{key}.cached"
           @etag          = cache_root + "#{key}.etag"
           @last_modified = cache_root + "#{key}.last_modified"
+          @mutable = mutable
+
+          FileUtils.mkdir_p cache_root if mutable
         end
 
         # Opens the cached file
@@ -60,7 +62,7 @@ def cached?
 
         # Destroys the cached file
         def destroy
-          [@cached, @etag, @last_modified].each { |f| f.delete if f.exist? }
+          [@cached, @etag, @last_modified].each { |f| f.delete if f.exist? } if @mutable
         end
 
         # Opens the etag file

lib/java_buildpack/util/cache/download_cache.rb

@@ -71,7 +71,7 @@ def get(uri, &block)
         # @param [String] uri the URI of the item
         # @return [Void]
         def evict(uri)
-          CachedFile.new(@mutable_cache_root, uri).destroy
+          CachedFile.new(@mutable_cache_root, uri, true).destroy
         end
 
         private
@@ -175,7 +175,7 @@ def cache_last_modified(response, cached_file)
         end
 
         def from_mutable_cache(uri)
-          cached_file = CachedFile.new(@mutable_cache_root, uri)
+          cached_file = CachedFile.new @mutable_cache_root, uri, true
           cached      = update URI(uri), cached_file
           [cached_file, cached]
         rescue => e
@@ -185,7 +185,7 @@ def from_mutable_cache(uri)
 
         def from_immutable_caches(uri)
           @immutable_cache_roots.each do |cache_root|
-            candidate = CachedFile.new cache_root, uri
+            candidate = CachedFile.new cache_root, uri, false
 
             next unless candidate.cached?
 

spec/java_buildpack/util/cache/cached_file_spec.rb

@@ -22,12 +22,30 @@
 describe JavaBuildpack::Util::Cache::CachedFile do
   include_context 'application_helper'
 
-  let(:file_cache) { described_class.new(app_dir, 'http://foo-uri/') }
+  let(:cache_root) { app_dir + 'cache/root' }
+
+  let(:file_cache) { described_class.new(app_dir, 'http://foo-uri/', true) }
 
   it 'should not create any files on initialization' do
     %w(cached etag last_modified).each { |extension| expect(cache_file(extension)).not_to exist }
   end
 
+  it 'should create cache_root if mutable' do
+    expect(cache_root).not_to exist
+
+    described_class.new(cache_root, 'http://foo-uri/', true)
+
+    expect(cache_root).to exist
+  end
+
+  it 'should not create cache_root if immutable' do
+    expect(cache_root).not_to exist
+
+    described_class.new(cache_root, 'http://foo-uri/', false)
+
+    expect(cache_root).not_to exist
+  end
+
   it 'should not detect cached file' do
     expect(file_cache.cached?).not_to be
   end
@@ -62,6 +80,12 @@
       %w(cached etag last_modified).each { |extension| expect(cache_file(extension)).not_to exist }
     end
 
+    it 'should not destroy all files if immutable' do
+      described_class.new(app_dir, 'http://foo-uri/', false).destroy
+
+      %w(cached etag last_modified).each { |extension| expect(cache_file(extension)).to exist }
+    end
+
     it 'should call the block with the content of the etag file' do
       expect { |b| file_cache.etag(File::RDONLY, 'test-arg', &b) }.to yield_file_with_content(/foo-etag/)
     end