QSaveFile saves any write errors in its reimplementation of QSaveFile::writeData() to d->writeError so that QSaveFile::commit() can fail before the atomic file replacement.

However, there is no equivalent handling for QSaveFile::flush() ( = QFileDevice::flush()). Any errors from flush() are not reflected in d->writeError and commit() will proceed with the file replacement as usual.

This is a problem even if flush() is not called by the user, as QSaveFile::commit() calls QFileDevice::close() which calls QFileDevice::flush().

The above most severely breaks QSaveFile on filesystems that allow creation of files when the file system is full, e.g. common Linux filesystems ext4 and tmpfs, as a very simple testcase (write short file while FS full) causes the target file to be replaced with a zero-sized file:

QSaveFile a("file");
a.open(QSaveFile::WriteOnly);
a.write("data"); // succeeds - "data" is buffered
a.commit(); // truncates file if ext4/tmpfs filesystem was full

Other filesystems are affected as well if there is enough space to create the file but not enough space to write all the data out. I did not check which group Win/NTFS falls on. The issue is in QSaveFile platform-agnostic code.

Any other write error encountered by flush() is also ignored by commit().

Attached is a full testcase application, the issue can be reproduced with it on Linux as follows:

$ mkdir /tmp/smallfs
$ sudo mount -ttmpfs -o size=1M none /tmp/smallfs
$ ./savefiletest /tmp/smallfs/testfile data1
open: true
write: 5
commit: true
$ cat /tmp/smallfs/testfile
data1$
$ dd if=/dev/zero of=/tmp/smallfs/foo
dd: writing to '/tmp/smallfs/foo': No space left on device
2041+0 records in
2040+0 records out
$ ./savefiletest /tmp/smallfs/testfile data2
open: true
write: 5
commit: true
$ cat /tmp/smallfs/testfile
$ ls -l /tmp/smallfs/testfile
-rw-rw-r--. 1 anssiha anssiha 0 10. 4. 13:00 /tmp/smallfs/testfile

File was truncated and QSaveFile reported no errors.

The issue is present in dev branch as well.

Handling full partition is explicitly mentioned in QSaveFile documentation as supported case.