Store::File#cache! will retry after Errno::EMLINK

== Problem

    Errno::EMLINK: Too many links @ dir_s_mkdir

== Full Backtrace (carrierwave 0.9)

    /opt/rubies/ruby-2.1.5/lib/ruby/2.1.0/fileutils.rb:247 :in `mkdir`
    /opt/rubies/ruby-2.1.5/lib/ruby/2.1.0/fileutils.rb:247 :in `fu_mkdir`
    /opt/rubies/ruby-2.1.5/lib/ruby/2.1.0/fileutils.rb:224 :in `block (2 levels) in mkdir_p`
    /opt/rubies/ruby-2.1.5/lib/ruby/2.1.0/fileutils.rb:222 :in `reverse_each`
    /opt/rubies/ruby-2.1.5/lib/ruby/2.1.0/fileutils.rb:222 :in `block in mkdir_p`
    /opt/rubies/ruby-2.1.5/lib/ruby/2.1.0/fileutils.rb:208 :in `each`
    /opt/rubies/ruby-2.1.5/lib/ruby/2.1.0/fileutils.rb:208 :in `mkdir_p`
    [GEM_ROOT]/gems/carrierwave-0.9.0/lib/carrierwave/sanitized_file.rb:290 :in `mkdir!`
    [GEM_ROOT]/gems/carrierwave-0.9.0/lib/carrierwave/sanitized_file.rb:209 :in `copy_to`
    [GEM_ROOT]/gems/carrierwave-0.9.0/lib/carrierwave/uploader/cache.rb:131 :in `block in cache!`
    [GEM_ROOT]/gems/carrierwave-0.9.0/lib/carrierwave/uploader/callbacks.rb:17 :in `with_callbacks`
    [GEM_ROOT]/gems/carrierwave-0.9.0/lib/carrierwave/uploader/cache.rb:122 :in `cache!`
    [GEM_ROOT]/gems/carrierwave-0.9.0/lib/carrierwave/mount.rb:327 :in `cache`
    [GEM_ROOT]/gems/carrierwave-0.9.0/lib/carrierwave/mount.rb:179 :in `file=`
    [GEM_ROOT]/gems/carrierwave-0.9.0/lib/carrierwave/orm/activerecord.rb:38 :in `file=`

== Why

Some file systems only allow a limited number of subdirectories.
Ext3 for example only allow ~32k subdirectories.

== Reproduce

Let's imagine a rails app with a User#avatar

    require 'tempfile'
    32001.times { User.new avatar: Tempfile.new("foo") }
    # This will create 32001 dirs within /public/uploads/tmp

== Fix

Clean cache after caching failed.

== Related Pull Requests / Issues / Links

* [How to: Delete cache garbage directories](https://github.com/carrierwaveuploader/carrierwave/wiki/How-to:-Delete-cache-garbage-directories)
* [Remove the cached file after storing it #107](#107)
* [Remove the cached file after storing it #125](#125)
* [delete cache garbage dirs #338](#338)
* [delete cache garbage dirs issue #338 #346](#346)
* [Issue 338 #393](#393)
* [tmp files not being deleted #1489](#1489)

lib/carrierwave/storage/file.rb

@@ -64,6 +64,14 @@ def retrieve!(identifier)
       #
       def cache!(new_file)
         new_file.move_to(::File.expand_path(uploader.cache_path, uploader.root), uploader.permissions, uploader.directory_permissions, true)
+      rescue Errno::EMLINK => e
+        raise(e) if @cache_called
+        @cache_called = true
+
+        # NOTE: Remove cached files older than 10 minutes
+        clean_cache!(600)
+
+        cache!(new_file)
       end
 
       ##

spec/storage/file_spec.rb

@@ -1,8 +1,16 @@
 # encoding: utf-8
 
 require 'spec_helper'
+require 'support/file_utils_helper'
+require 'tempfile'
 
 describe CarrierWave::Storage::File do
+  include FileUtilsHelper
+
+  subject(:storage) { described_class.new(@uploader) }
+
+  let(:tempfile) { Tempfile.new("foo") }
+  let(:sanitized_temp_file) { CarrierWave::SanitizedFile.new(tempfile) }
 
   before do
     @uploader_class = Class.new(CarrierWave::Uploader::Base)
@@ -39,6 +47,15 @@
     end
   end
 
+  describe '#cache!' do
+    context "when FileUtils.mkdir_p raises Errno::EMLINK" do
+      before { fake_failed_mkdir_p }
+      after { storage.cache!(sanitized_temp_file) }
+
+      it { is_expected.to receive(:clean_cache!).with(600) }
+    end
+  end
+
   describe '#clean_cache!' do
     before do
       five_days_ago_int  = 1369894322

spec/support/file_utils_helper.rb

@@ -0,0 +1,15 @@
+module FileUtilsHelper
+  # NOTE: Make FileUtils.mkdir_p to raise `Errno::EMLINK` only once
+  def fake_failed_mkdir_p
+    original_mkdir_p = FileUtils.method(:mkdir_p)
+    mkdir_p_called = false
+    allow(FileUtils).to receive(:mkdir_p) do |args|
+      if mkdir_p_called
+        original_mkdir_p.call(*args)
+      else
+        mkdir_p_called = true
+        raise Errno::EMLINK
+      end
+    end
+  end
+end