"Show Source" frequently fails

message_example.txt

Steps to reproduce

1. Open a message
2. Select "Show source" action button

Expected behavior

The source of the message should be shown.

Actual behavior

Throws a 500:

Apache log:

"GET /apps/mail/api/messages/141034/source HTTP/2.0" 500

Nextcloud log:

{"reqId":"fZXTBIIsQKW1h5usNz1V","level":3,"time":"2023-09-20T20:33:07+00:00","remoteAddr":"<IP>","user":"<user>","app":"index","method":"GET","url":"/apps/mail/api/messages/141034/source","message":"Malformed UTF-8 characters, possibly incorrectly encoded","userAgent":"Mozilla/5.0 (Android 13; Mobile; rv:109.0) Gecko/117.0 Firefox/117.0","version":"27.1.0.7","exception":{"Exception":"JsonException","Message":"Malformed UTF-8 characters, possibly incorrectly encoded","Code":5,"Trace":[{"file":"<documentroot>/lib/public/AppFramework/Http/JSONResponse.php","line":66,"function":"json_encode","args":[[null],4194305]},{"file":"<documentroot>/lib/private/AppFramework/Http/Dispatcher.php","line":182,"function":"render","class":"OCP\AppFramework\Http\JSONResponse","type":"->","args":[]},{"file":"<documentroot>/lib/private/AppFramework/App.php","line":183,"function":"dispatch","class":"OC\AppFramework\Http\Dispatcher","type":"->","args":[["OCA\Mail\Controller\MessagesController"],"getSource"]},{"file":"<documentroot>/lib/private/Route/Router.php","line":315,"function":"main","class":"OC\AppFramework\App","type":"::","args":["OCA\Mail\Controller\MessagesController","getSource",["OC\AppFramework\DependencyInjection\DIContainer"],["141034","mail.messages.getSource"]]},{"file":"<documentroot>/lib/base.php","line":1068,"function":"match","class":"OC\Route\Router","type":"->","args":["/apps/mail/api/messages/141034/source"]},{"file":"<documentroot>/index.php","line":36,"function":"handleRequest","class":"OC","type":"::","args":[]}],"File":"<documentroot>/lib/public/AppFramework/Http/JSONResponse.php","Line":66,"CustomMessage":"--"}}

Mail app version

3.4.0

Mailserver or service

dovecot imapd

Operating system

openSUSE Tumbleweed

PHP engine version

PHP 8.2

Web server

Apache (supported)

Database

PostgreSQL

Additional info

From the mail source:

Content-Type: text/plain; charset=Cp1252 Content-Transfer-Encoding: 7bit

The offending byte might be "FC" for "ü" which is correct but neither UTF-8 nor 7bit.

Start at the MessagesController getSource endpoint shown in the trace and follow how the message source reaches JSONResponse.php. Reproduce the request with the attached message_example.txt, then inspect handling of the Cp1252 source and malformed UTF-8. Done means Show source returns the message source instead of a 500 for this input, with a regression test covering the encoding case.