Ctrl+S said "Saved." The file was 0 bytes.

This is a submission for DEV's Summer Bug Smash: Smash Stories powered by Sentry . Written with the help of AI (Claude). The bug, the fix, the validation setup, and every claim below are mine, and were verified against the real codebase and a real full disk. The report Someone lost a Magic: The Gathering decklist. They were playing on Cockatrice — the open-source MTG client — with their decks on a drive that had quietly filled up while Oracle pushed an update in the background. They added a card, hit Ctrl+S, and Cockatrice said it saved. The debug log agreed: [2026-05-28 22:31:42.031 I] Saved deck to "G:/cockatrice300/data/decks/edh-b2-gitrog-reanimate.cod" with format 1 - true - true . Success. The file was 0 bytes. The deck was gone. That was issue #6952 , filed by Mekkiss. The steps to reproduce are four lines long and completely damning: Have a full disk. Open a deck on the full disk Add one card to it Save the deck (ctrl+s) Observe that the deck is now a 0 byte file. Three ways to be wrong at once The save path lived in DeckLoader::saveToFile() . Stripped down, it looked like this: QFile file ( fileName ); if ( ! file . open ( QIODevice :: WriteOnly | QIODevice :: Text )) { qCWarning ( DeckLoaderLog ) << "Could not create or open file:" << fileName ; return std :: nullopt ; } bool success = false ; switch ( fmt ) { /* ... saveToFile_Native / saveToFile_Plain ... */ } file . flush (); file . close (); qCInfo ( DeckLoaderLog ) << "Saved deck to " << fileName << "with format" << fmt << "-" << success ; There are three independent failures stacked on top of each other here, and you need all three to lose data: 1. WriteOnly truncates on open. The instant open() succeeds, the existing deck is 0 bytes. Not after a successful write — at open time . The old deck is already destroyed before a single byte of the new one is written. On a full disk, open() still succeeds: truncating a file doesn't need free space. It frees space. 2. The serializers always returned true . sa