FIX: Correct the forwarded by user small post for group inbox (#14252)

When 2ac9fd9dff5a2a9210e2b1b765e1a324bbb4f421 was done, this
affected the small post that is created when forwarding an email
into the group inbox. Instead of using the name and the email of
the user who forwarded the email, it used the original from email
and name to create the small post. So instead of something like
"Discourse Team forwarded the above email" we ended up with
"John Smith forwarded the above email" which is incorrect.

This fixes the issue by creating a staged user for the forwarding
email address (if such a user does not yet exist) and uses that
for the "forwarded" small post instead.

lib/email/receiver.rb

@@ -589,12 +589,8 @@ module Email
       # which we can extract from embedded_email_raw.
       if has_been_forwarded?
         if mail[:from].to_s =~ group_incoming_emails_regex && embedded_email[:from].errors.blank?
-          embedded_email[:from].each do |address_field|
-            from_address = address_field.address
-            from_display_name = address_field.display_name&.to_s
-            next if !from_address&.include?("@")
-            return [from_address&.downcase, from_display_name&.strip]
-          end
+          from_address, from_display_name = extract_from_fields_from_header(embedded_email, :from)
+          return [from_address, from_display_name] if from_address
         end
       end
 
@@ -604,23 +600,16 @@ module Email
       # header in more cases.
       if mail['X-Original-From'].present?
         if mail[:reply_to] && mail[:reply_to].errors.blank?
-          mail[:reply_to].each do |address_field|
-            from_address = address_field.address
-            from_display_name = address_field.display_name&.to_s
-            next if address_field.to_s != mail['X-Original-From'].to_s
-            next if !from_address&.include?("@")
-            return [from_address&.downcase, from_display_name&.strip]
-          end
+          from_address, from_display_name = extract_from_fields_from_header(
+            mail, :reply_to, comparison_headers: ['X-Original-From']
+          )
+          return [from_address, from_display_name] if from_address
         end
       end
 
       if mail[:from].errors.blank?
-        mail[:from].each do |address_field|
-          from_address = address_field.address
-          from_display_name = address_field.display_name&.to_s
-          next if !from_address&.include?("@")
-          return [from_address&.downcase, from_display_name&.strip]
-        end
+        from_address, from_display_name = extract_from_fields_from_header(mail, :from)
+        return [from_address, from_display_name] if from_address
       end
 
       return extract_from_address_and_name(mail.from) if mail.from.is_a? String
@@ -637,6 +626,24 @@ module Email
       nil
     end
 
+    def extract_from_fields_from_header(mail_object, header, comparison_headers: [])
+      mail_object[header].each do |address_field|
+        from_address = address_field.address
+        from_display_name = address_field.display_name&.to_s
+
+        comparison_failed = false
+        comparison_headers.each do |comparison_header|
+          comparison_failed = true if address_field.to_s != mail_object[comparison_header].to_s
+        end
+
+        next if comparison_failed
+        next if !from_address&.include?("@")
+        return [from_address&.downcase, from_display_name&.strip]
+      end
+
+      [nil, nil]
+    end
+
     def extract_from_address_and_name(value)
       if value[";"]
         from_display_name, from_address = value.split(";")
@@ -948,6 +955,10 @@ module Email
       embedded = Mail.new(embedded_email_raw)
       email, display_name = parse_from_field(embedded)
 
+      if forwarded_by_address && forwarded_by_name
+        forwarded_by_user = stage_sender_user(forwarded_by_address, forwarded_by_name)
+      end
+
       return false if email.blank? || !email["@"]
 
       post = forwarded_email_create_topic(destination: destination,
@@ -974,13 +985,28 @@ module Email
                        post_type: post_type,
                        skip_validations: user.staged?)
         else
-          post.topic.add_small_action(user, "forwarded")
+          if forwarded_by_user
+            post.topic.topic_allowed_users.find_or_create_by!(user_id: forwarded_by_user.id)
+          end
+          post.topic.add_small_action(forwarded_by_user || user, "forwarded")
         end
       end
 
       true
     end
 
+    def forwarded_by_sender
+      @forwarded_by_sender ||= extract_from_fields_from_header(@mail, :from)
+    end
+
+    def forwarded_by_address
+      @forwarded_by_address ||= forwarded_by_sender&.first
+    end
+
+    def forwarded_by_name
+      @forwarded_by_name ||= forwarded_by_sender&.first
+    end
+
     def forwarded_email_quote_forwarded(destination, user)
       embedded = embedded_email_raw
       raw = <<~EOF
@@ -1367,7 +1393,11 @@ module Email
     end
 
     def stage_from_user
-      @from_user ||= find_or_create_user!(@from_email, @from_display_name).tap do |u|
+      @from_user ||= stage_sender_user(@from_email, @from_display_name)
+    end
+
+    def stage_sender_user(email, display_name)
+      find_or_create_user!(email, display_name).tap do |u|
         log_and_validate_user(u)
       end
     end