When importing a protected file as an unprivileged user (should become impossible after work in T233575), I was presented with a "mark source file" checkbox and chose to make the edit, but import proceeded without an error. No failures were recorded in the logs for either the source or target wiki, no failure metrics were incremented, but the edit was not made.
Change 598686 had a related patch set uploaded (by Thiemo Kreuz (WMDE); owner: Thiemo Kreuz (WMDE)):
[mediawiki/extensions/FileImporter@master] Improve comments and PHPDoc documentation
I played around with this locally and found that I always get a proper error message when either the deletion or inserting the "Now Commons" template failed. The main issue I can see is that this error message is easy to miss. It's a yellow box below the green box that says the import was successful (which it was).
However, my test setup was not specifically for a write-protected source file. I think this is worth another test.
Change 598955 had a related patch set uploaded (by Thiemo Kreuz (WMDE); owner: Thiemo Kreuz (WMDE)):
[mediawiki/extensions/FileImporter@master] Fix incomplete error reporting on remote edit actions
I found the issue! \o/
Open question: Should we display the internal error message to the user? As of now, the messages are neutral:
It should be rare for a deletion to fail (maybe if the file was already deleted by another administrator). But edits can easily fail. However, the error message we get is most probably not localized, but English. Something like "You don't have the necessary rights …". Is this useful?
Possible TODOs:
Change 598955 merged by jenkins-bot:
[mediawiki/extensions/FileImporter@master] Fix incomplete error reporting on remote edit actions
Change 598686 merged by jenkins-bot:
[mediawiki/extensions/FileImporter@master] Improve comments and PHPDoc documentation