QTemporaryFile: file not being created in Qt 5.11/5.12

Christian Ehrlicher

I don't see it as bug here - a temporary file may not exist on the disk at all so exists() may return false. As soon as you call fileName() you force Qt to create a file on disk and it exists there.

koahnig

@Christian-Ehrlicher

That is disputable.

According to the documentation of bool QTemporaryFile::open(QIODevice::OpenMode flags) it says:
Creates a unique file name for the temporary file, and opens it. You can get the unique name later by calling fileName(). The file is guaranteed to have been created by this function (i.e., it has never existed before).

For the open used in the code bool QTemporaryFile::open() it states only:
A QTemporaryFile will always be opened in QIODevice::ReadWrite mode, this allows easy access to the data in the file. This function will return true upon success and will set the fileName() to the unique filename used.

Also there seems to be a change in functionality between versions according to the first post.

Christian Ehrlicher

@koahnig said in QTemporaryFile: file not being created in Qt 5.11:

That is disputable.

Then please discuss this on the LKML, O_TMPFILE, when supported, doesn't create visible directory entry. As soon as you want it to be visible (= calling fileName()) this changes. See also e.g. here: http://man7.org/linux/man-pages/man2/open.2.html

sierdzio

@Christian-Ehrlicher said in QTemporaryFile: file not being created in Qt 5.11:

@koahnig said in QTemporaryFile: file not being created in Qt 5.11:

That is disputable.

Then please discuss this on the LKML, O_TMPFILE, when supported, doesn't create visible directory entry. As soon as you want it to be visible (= calling fileName()) this changes. See also e.g. here: http://man7.org/linux/man-pages/man2/open.2.html

That's fine and I agree with that.

But, if it worked one way in Qt 5.9 and works in a different way in 5.11, then it's a regression - so definitely a bug and also a high priority one.

Aritz

@sierdzio They closed my bug report as invalid. This is affecting Qt 5.12.6 also, BTW.

SGaist

Hi,

Can you post the link to the bug report ?

Aritz

@SGaist https://bugreports.qt.io/browse/QTBUG-80157

JonB

I have read both sides of this: the bug report + Qt documentation, and the Linux open docs for O_TMPFILE. I can see both sides of the argument, and of course understand that Qt wants to use the OS facility.

I admit I have not tried it, but how does, say, Qt docs stating stuff like

The file name of the temporary file can be found by calling fileName().
A temporary file will have some static part of the name and some part that is calculated to be unique.
and will be placed into the temporary path as returned by QDir::tempPath().

marry with the O_TMPFILE docs of

Create an unnamed temporary regular file
(2) can never be reached via any pathname
(4) do not require the caller to devise unique names

I am also intrigued that apparently the implementation has changed from Qt 5.9 to 5.11, I wonder why now.

It's easy for me to say, but some hints in the Qt docs about this possible behaviour might be welcome. I fully understand that it is not Qt's job to document everything which might be going on under each platform, but there are other precedents in the docs which do give the user a clue about platform-specific behaviour, or what to expect/not expect....

Christian Ehrlicher

The support for O_TMPFILE was added in 5.10 and there was even a big changelog entry for this: https://code.qt.io/cgit/qt/qtbase.git/tree/dist/changes-5.10.0/?h=v5.10.0

sierdzio

@Christian-Ehrlicher said in QTemporaryFile: file not being created in Qt 5.11/5.12:

The support for O_TMPFILE was added in 5.10 and there was even a big changelog entry for this: https://code.qt.io/cgit/qt/qtbase.git/tree/dist/changes-5.10.0/?h=v5.10.0

Great, thanks for the link.

I'd still say there should be a note about this in Qt docs for Qt >5.10. Many other such OS "curiosities" are mentioned in the docs.

Christian Ehrlicher

@sierdzio said in QTemporaryFile: file not being created in Qt 5.11/5.12:

I'd still say there should be a note about this in Qt docs

Feel free to provide a patch ;)

sierdzio · @sierdzio said in QTemporaryFile: file not being created in Qt 5.11/5.12:

@Christian-Ehrlicher said in QTemporaryFile: file not being created in Qt 5.11/5.12:

@sierdzio said in QTemporaryFile: file not being created in Qt 5.11/5.12:

I'd still say there should be a note about this in Qt docs

Feel free to provide a patch ;)

Oh I wish, but no time right now.

I'll bookmark this, though and hopefully come back with a patch some day.

aha_1980 · @sierdzio said in QTemporaryFile: file not being created in Qt 5.11/5.12:

@sierdzio

I'll bookmark this, though and hopefully come back with a patch some day.

Too late :) But you can review the patch: https://codereview.qt-project.org/c/qt/qtbase/+/282081

sierdzio

Great, thanks! Looks good. I may be lacking some powers as I don't see any +1 button I can click.

SGaist

It's in the dialog you get when you hit the Reply button.

sierdzio

Ah OK found it under REPLY. That gerrit UI...

JonB

@aha_1980
Whoever made that doc change (you? some bot?!), it's perfect!

aha_1980

@JonB It was me. I copied the text from the 5.10 changelog :)

koahnig

Thanks to all being involved in the discussion.
When answering with 1st and 2nd post I did not expect to create so much fuzz around this issue. With the mass of comments and different references. Therefore, I miss a crucial point here.

I am happy with the functionality, since there are good reasons as I expect with the summarized expertize being involved in the discussion. After a couple of trials I was also able to review @aha_1980 's addition to the documentation, which clarifies things generally.
I guess there is still a different behaviour of exist() after the 2 versions of open to be callable. Since the second says that the file is definitely created, it has to be the first open() as used by @Aritz
Personally I would add a short reference to the detailed information section on linux for the first open() version. Something like:
See note on Linux above

If my assumption is wrong and the issue with exists() is for both open versions the same, we have a different ambiguity in the documentation respectively a bug in the description for bool QTemporaryFile::open(QIODevice::OpenMode flags) . In this case a reference or modification in documentation is required for second open version.