11 files changed, 116 insertions, 6 deletions
diff --git a/actionmailer/CHANGELOG.md b/actionmailer/CHANGELOG.md
index 86ecb3ee88..acdc4c7dc9 100644
--- a/actionmailer/CHANGELOG.md
+++ b/actionmailer/CHANGELOG.md
@@ -1,3 +1,29 @@
+*   `assert_emails` in block form use the given number as expected value.
+    This makes the error message much easier to understand.
+
+    *Yuji Yaginuma*
+
+*   Add support for inline images in mailer previews by using an interceptor
+    class to convert cid: urls in image src attributes to data urls.
+
+    *Andrew White*
+
+*   Mailer preview now uses `url_for` to fix links to emails for apps running on
+    a subdirectory.
+
+    *Remo Mueller*
+
+*   Mailer previews no longer crash when the `mail` method wasn't called
+    (`NullMail`).
+
+    Fixes #19849.
+
+    *Yves Senn*
+
+*   Make sure labels and values line up in mailer previews.
+
+    *Yves Senn*
+
 *   Add `assert_enqueued_emails` and `assert_no_enqueued_emails`.
 
     Example:
diff --git a/actionmailer/README.rdoc b/actionmailer/README.rdoc
index a4e660d621..e5c2ed8c77 100644
--- a/actionmailer/README.rdoc
+++ b/actionmailer/README.rdoc
@@ -146,7 +146,7 @@ The Base class has the full list of configuration options. Here's an example:
 
 The latest version of Action Mailer can be installed with RubyGems:
 
-  % [sudo] gem install actionmailer
+  % gem install actionmailer
 
 Source code can be downloaded as part of the Rails project on GitHub
 
diff --git a/actionmailer/lib/action_mailer.rb b/actionmailer/lib/action_mailer.rb
index 17d8dcc208..291a8c1e34 100644
--- a/actionmailer/lib/action_mailer.rb
+++ b/actionmailer/lib/action_mailer.rb
@@ -40,6 +40,7 @@ module ActionMailer
 
   autoload :Base
   autoload :DeliveryMethods
+  autoload :InlinePreviewInterceptor
   autoload :MailHelper
   autoload :Preview
   autoload :Previews, 'action_mailer/preview'
diff --git a/actionmailer/lib/action_mailer/base.rb b/actionmailer/lib/action_mailer/base.rb
index 754f698e48..218b7a735a 100644
--- a/actionmailer/lib/action_mailer/base.rb
+++ b/actionmailer/lib/action_mailer/base.rb
@@ -596,6 +596,7 @@ module ActionMailer
 
     class NullMail #:nodoc:
       def body; '' end
+      def header; {} end
 
       def respond_to?(string, include_all=false)
         true
diff --git a/actionmailer/lib/action_mailer/inline_preview_interceptor.rb b/actionmailer/lib/action_mailer/inline_preview_interceptor.rb
new file mode 100644
index 0000000000..dea5e52f0c
--- /dev/null
+++ b/actionmailer/lib/action_mailer/inline_preview_interceptor.rb
@@ -0,0 +1,61 @@
+require 'base64'
+
+module ActionMailer
+  # Implements a mailer preview interceptor that converts image tag src attributes
+  # that use inline cid: style urls to data: style urls so that they are visible
+  # when previewing a HTML email in a web browser.
+  #
+  # This interceptor is enabled by default, to remove it delete it from the
+  # <tt>ActionMailer::Base.preview_interceptors</tt> array:
+  #
+  #   ActionMailer::Base.preview_interceptors.delete(ActionMailer::InlinePreviewInterceptor)
+  #
+  class InlinePreviewInterceptor
+    PATTERN  = /src=(?:"cid:[^"]+"|'cid:[^']+')/i
+
+    include Base64
+
+    def self.previewing_email(message) #:nodoc:
+      new(message).transform!
+    end
+
+    def initialize(message) #:nodoc:
+      @message = message
+    end
+
+    def transform! #:nodoc:
+      return message if html_part.blank?
+
+      html_source.gsub!(PATTERN) do |match|
+        if part = find_part(match[9..-2])
+          %[src="#{data_url(part)}"]
+        else
+          match
+        end
+      end
+
+      message
+    end
+
+    private
+      def message
+        @message
+      end
+
+      def html_part
+        @html_part ||= message.html_part
+      end
+
+      def html_source
+        html_part.body.raw_source
+      end
+
+      def data_url(part)
+        "data:#{part.mime_type};base64,#{strict_encode64(part.body.raw_source)}"
+      end
+
+      def find_part(cid)
+        message.all_parts.find{ |p| p.attachment? && p.cid == cid }
+      end
+  end
+end
diff --git a/actionmailer/lib/action_mailer/preview.rb b/actionmailer/lib/action_mailer/preview.rb
index 44cf6665ba..25ad7ee721 100644
--- a/actionmailer/lib/action_mailer/preview.rb
+++ b/actionmailer/lib/action_mailer/preview.rb
@@ -21,7 +21,7 @@ module ActionMailer
 
       # :nodoc:
       mattr_accessor :preview_interceptors, instance_writer: false
-      self.preview_interceptors = []
+      self.preview_interceptors = [ActionMailer::InlinePreviewInterceptor]
     end
 
     module ClassMethods
diff --git a/actionmailer/lib/action_mailer/test_helper.rb b/actionmailer/lib/action_mailer/test_helper.rb
index 524e6e3af1..4d03a616d2 100644
--- a/actionmailer/lib/action_mailer/test_helper.rb
+++ b/actionmailer/lib/action_mailer/test_helper.rb
@@ -34,7 +34,7 @@ module ActionMailer
         original_count = ActionMailer::Base.deliveries.size
         yield
         new_count = ActionMailer::Base.deliveries.size
-        assert_equal original_count + number, new_count, "#{number} emails expected, but #{new_count - original_count} were sent"
+        assert_equal number, new_count - original_count, "#{number} emails expected, but #{new_count - original_count} were sent"
       else
         assert_equal number, ActionMailer::Base.deliveries.size
       end
diff --git a/actionmailer/test/delivery_methods_test.rb b/actionmailer/test/delivery_methods_test.rb
index 2e4e019bf7..78507ce7dc 100644
--- a/actionmailer/test/delivery_methods_test.rb
+++ b/actionmailer/test/delivery_methods_test.rb
@@ -1,5 +1,4 @@
 require 'abstract_unit'
-require 'mail'
 
 class MyCustomDelivery
 end
diff --git a/actionmailer/test/mail_helper_test.rb b/actionmailer/test/mail_helper_test.rb
index 24ccaab8df..ff6b25b0c7 100644
--- a/actionmailer/test/mail_helper_test.rb
+++ b/actionmailer/test/mail_helper_test.rb
@@ -59,6 +59,12 @@ The second
     end
   end
 
+  def use_cache
+    mail_with_defaults do |format|
+      format.html { render(inline: "<% cache(:foo) do %>Greetings from a cache helper block<% end %>") }
+    end
+  end
+
   protected
 
   def mail_with_defaults(&block)
@@ -107,5 +113,11 @@ class MailerHelperTest < ActionMailer::TestCase
     TEXT
     assert_equal expected.gsub("\n", "\r\n"), mail.body.encoded
   end
-end
 
+  def test_use_cache
+    assert_nothing_raised do
+      mail = HelperMailer.use_cache
+      assert_equal "Greetings from a cache helper block", mail.body.encoded
+    end
+  end
+end
diff --git a/actionmailer/test/message_delivery_test.rb b/actionmailer/test/message_delivery_test.rb
index e4dd269494..aaa3f218b5 100644
--- a/actionmailer/test/message_delivery_test.rb
+++ b/actionmailer/test/message_delivery_test.rb
@@ -2,7 +2,6 @@ require 'abstract_unit'
 require 'active_job'
 require 'minitest/mock'
 require 'mailers/delayed_mailer'
-require 'active_support/core_ext/numeric/time'
 
 class MessageDeliveryTest < ActiveSupport::TestCase
   include ActiveJob::TestHelper
diff --git a/actionmailer/test/test_helper_test.rb b/actionmailer/test/test_helper_test.rb
index 089933e245..0a4bc75d3e 100644
--- a/actionmailer/test/test_helper_test.rb
+++ b/actionmailer/test/test_helper_test.rb
@@ -112,6 +112,17 @@ class TestHelperMailerTest < ActionMailer::TestCase
     assert_match(/1 .* but 2/, error.message)
   end
 
+  def test_assert_emails_message
+    TestHelperMailer.test.deliver_now
+    error = assert_raise ActiveSupport::TestCase::Assertion do
+      assert_emails 2 do
+        TestHelperMailer.test.deliver_now
+      end
+    end
+    assert_match "Expected: 2", error.message
+    assert_match "Actual: 1", error.message
+  end
+
   def test_assert_no_emails_failure
     error = assert_raise ActiveSupport::TestCase::Assertion do
       assert_no_emails do