Raise an exception when attachments are added after `mail` was called.

Closes #16163

Adding attachments after a call to `mail` will result in invalid emails.
This is related to the fact, that `mail` is making the required preparations
before the email is ready to be sent. These change depending on your
added attachments.

1 files changed, 39 insertions, 0 deletions

diff --git a/actionmailer/test/base_test.rb b/actionmailer/test/base_test.rb
index 229ded8e04..6116d1e29f 100644
--- a/actionmailer/test/base_test.rb
+++ b/actionmailer/test/base_test.rb
@@ -232,6 +232,45 @@ class BaseTest < ActiveSupport::TestCase
     end
   end
 
+  test "adding attachments after mail was called raises exception" do
+    class LateAttachmentMailer < ActionMailer::Base
+      def welcome
+        mail body: "yay", from: "welcome@example.com", to: "to@example.com"
+        attachments['invoice.pdf'] = 'This is test File content'
+      end
+    end
+
+    e = assert_raises(RuntimeError) { LateAttachmentMailer.welcome }
+    assert_match(/Can't add attachments after `mail` was called./, e.message)
+  end
+
+  test "adding inline attachments after mail was called raises exception" do
+    class LateInlineAttachmentMailer < ActionMailer::Base
+      def welcome
+        mail body: "yay", from: "welcome@example.com", to: "to@example.com"
+        attachments.inline['invoice.pdf'] = 'This is test File content'
+      end
+    end
+
+    e = assert_raises(RuntimeError) { LateInlineAttachmentMailer.welcome }
+    assert_match(/Can't add attachments after `mail` was called./, e.message)
+  end
+
+  test "accessing attachments works after mail was called" do
+    class LateAttachmentAccessorMailer < ActionMailer::Base
+      def welcome
+        attachments['invoice.pdf'] = 'This is test File content'
+        mail body: "yay", from: "welcome@example.com", to: "to@example.com"
+
+        unless attachments.map(&:filename) == ["invoice.pdf"]
+          raise Minitest::Assertion, "Should allow access to attachments"
+        end
+      end
+    end
+
+    assert_nothing_raised { LateAttachmentAccessorMailer.welcome }
+  end
+
   # Implicit multipart
   test "implicit multipart" do
     email = BaseMailer.implicit_multipart