Unexpected behavior in KIO::MimeTypeFinderJob

[Harald Sitter]

On Wed, Aug 7, 2024 at 12:10 PM David Faure <faure at kde.org> wrote:
>
> On mercredi 7 août 2024 11:54:20 heure d’été d’Europe centrale David Faure
> wrote:
> > On mercredi 7 août 2024 08:30:43 heure d’été d’Europe centrale Stefano
> > Crocco
> > wrote:
> > > Hello to everyone,
> > > while investigating a bug in Konqueror, I just found what in my opinion is
> > > an unexpected behavior of KIO::MimeTypeFinderJob. If I'm reading the code
> > > correctly, when it uses KIO::get() to determine the mimetype [1], it lets
> > > the TransferJob go on even after it detected the mimetype.
> > >
> > > The documentation for KIO::get() states:
> > > "Special case: if you want to determine the MIME type of the file first,
> > > and then read it with the appropriate component, you can still use a
> > > KIO::get() directly. When that job emits the mimeType signal, (which is
> > > guaranteed to happen before it emits any data), put the job on hold".
> > > Since the task of MimeTypeFinderJob is finding the mimetype of the URL, I
> > > expected it would put the job on hold as soon as it has determined the
> > > mimetype, coherently with the KIO::get() documentation.
> >
> > Right, that's what KRun::foundMimeType was doing, and I did that in
> > MimeTypeFinderJob initially.
> >
> > But see commit 621585f0e818cd9f75cdd3bcc8665da7ac708283
> > and bug 452729. Seems weird. Talk to Harald ;)
>
> Of course in KF5 it was a bit different because another process could resume
> the slave on hold, made available by klauncher. AFAIK nowadays that's gone so
> resuming the slave on hold only works if the *same* process is going to fetch
> that URL. I guess that's the issue. If another process gets started instead
> (to open that URL in e.g. an image viewer), that slave on hold stays there
> forever. We need a way to discard it then, somehow.

I don't see the problem. I mean, the docs are wrong, but beyond that?

MimeTypeFinderJob is a KCompositeJob. It subjob()s the TransferJob.
subjob calls setParent. When the MimeTypeFinderJob gets deleted the
TransferJob gets deleted and that triggers the Scheduler to terminate
the Worker process.

HS