"Sorry, can not save your changes." page should include changes that could not be applied

[trac@…]

If you submit changes to a ticket and the changes could not be applied because someone else modified it, you get a page that says just that. It would be better to echo the changes that couldn't be saved back to the user so they wouldn't lose them.

Steps to reproduce:

1. Create a new ticket.
2. Open the ticket in two windows/tabs in your web browser.
3. Submit changes to a ticket in one window/tab.
4. Click Submit changes in the other window/tab (you don't need to actually make any changes)

Results:

Internal Error

Sorry, can not save your changes. This ticket has been modified by someone else since you started

Expected results:

This is an acceptable error, but it would be best if this error was displayed along with the changes that you made (this would be simple - just reproduce the original form fields and data that the user submitted), so that the changes could be saved and re-submitted if needed. Although sometimes browsers save the form field data making it available when you go Back, it would be best to not rely on it. On Safari, for example, clicking the Back button simply goes from ​http://example.com/ticket/2788#preview to ​http://example.com/ticket/2788, without changing the page (leaving the error message up, not showing the submitted data).

Workaround:

To prevent the changes from being lost when using Safari on OS X:

1. Refresh the page. You'll be asked "Are you sure you want to send a form again?".
2. In Terminal, run sudo tcpdump -A -s 1500
3. Submit the form.
4. Watch tcpdump for the form submission. It will be url-encoded. Save it somewhere.
5. Decode by opening Python, type cgi.parse_qsl(", paste url-encoded data, then type `")'.
6. Copy your data as necessary, salvaging what is still timely.

[Emmanuel Blot]

An enhanced collision detection feature has already been implemented in the current development version (available from source:/trunk​). The changes that cannot be saved to the ticket are shown in a "Preview" box.

[Emmanuel Blot]

See #4100 for the original ticket.

[anonymous]

Resubmitting the form should submit the changes successfully. If you are commenting on a ticket then when it informs you that it has changed, the view of the ticket is updated. However, resubmitting the comment continues to trigger the error even though you have been informed of both the error and the update.