Fall back to QFile when QSaveFile fails on CIFS/SMB

[rva79]

Problem

Saving to a CIFS/SMB mount fails in Omawrite with Could not save …, even when the share is mounted read-write and a normal file write succeeds.

Backend::saveTo uses QSaveFile. On Linux that creates an unnamed temp file with O_TMPFILE, then materializes it with linkat. CIFS does not support O_TMPFILE and returns ENOENT rather than EOPNOTSUPP. Qt only treats EOPNOTSUPP and EISDIR as “not supported”, so it never falls back to a named temp file and QSaveFile::open() fails.

QSaveFile::setDirectWriteFallback(true) does not help: that path only retries on EACCES.

Reproduced with Qt 6.11:

• open(dir, O_TMPFILE | O_RDWR) on CIFS → ENOENT
• mkstemp / QFile on the same directory → success
• QSaveFile::open on the same directory → "No such file or directory"

Change

Keep QSaveFile as the default (atomic replace on local filesystems). If that fails, write the destination with QFile so saves still work on CIFS/SMB.

Recovery snapshots stay on QSaveFile; those go to AppDataLocation on a local disk.

Test plan

• ./bin/test (12 passed)
• Save a new Markdown file to a CIFS/SMB mount from Omawrite (Ctrl+Shift+S)
• Save again to the same path (Ctrl+S)
• Save to a local directory still works