[OSProcess] #safeToForkSqueak when running headless

> Hi folks.
>
> I?m running into an issue when forking images with OSProcess in headless images. In older versions of OSProcess #forkSqueak would try to fork in any case, independent of the presence of or the connection state to XDisplayControlPlugin.
> In dtl.57 #forkSqueak was changed to explicitly check if XDisplayControlPlugin is present and can be controlled. This is obviously a good thing for when the image runs headful. In my case however I can no longer fork my headless image, even though I don?t need an X session.
>
> Important: apparently #safeToForkSqueak answers true if the XDisplayControlPlugin is available, even in a headless image. But since we don?t need windows, we don?t install X libraries on our server and so #safeToForkSqueak answers false. So the whole thing boils down to a situation where Squeak / Pharo is run in an environment where the XDisplayPlugin can?t be loaded but where forking is still possible.
>
> I don?t know how to do this in a cross platform way but for Pharo we could extend #safeToForkSqueak with a headless check like this:
>
> safeToForkSqueak
> "The forkSqueak method is currently supported only for X11 displays with
> XDisplayControlPlugin available. If the plugin is not available, the child Squeak
> VM will continue interacting with the X display, which leads to errors in the
> parent Squeak, including VM crashes."
>
> ^self canControlXDisplay
> or: [ Smalltalk isHeadless ]
>
>
> I?m going to dirty patch this for my images because I really need it but I think some version of this patch would be a good thing to integrate into OSProcess.
>

> On Wed, Aug 13, 2014 at 5:23 AM, Max Leske <[hidden email]> wrote:
>
> >
> > On 13.08.2014, at 14:00, [hidden email]
> > wrote:
> >
> > Date: Wed, 13 Aug 2014 07:45:08 -0400
> > From: "David T. Lewis" <[hidden email]>
> > Subject: Re: [squeak-dev] [OSProcess] #safeToForkSqueak when running
> > headless
> > To: The general-purpose Squeak developers list
> > <[hidden email]>
> > Message-ID: <[hidden email]>
> > Content-Type: text/plain; charset=us-ascii
> >
> > On Wed, Aug 13, 2014 at 09:42:50AM +0200, Max Leske wrote:
> >
> > Hi folks.
> >
> > I?m running into an issue when forking images with OSProcess in headless
> > images. In older versions of OSProcess #forkSqueak would try to fork in any
> > case, independent of the presence of or the connection state to
> > XDisplayControlPlugin.
> > In dtl.57 #forkSqueak was changed to explicitly check if
> > XDisplayControlPlugin is present and can be controlled. This is obviously a
> > good thing for when the image runs headful. In my case however I can no
> > longer fork my headless image, even though I don?t need an X session.
> >
> > Important: apparently #safeToForkSqueak answers true if the
> > XDisplayControlPlugin is available, even in a headless image. But since we
> > don?t need windows, we don?t install X libraries on our server and so
> > #safeToForkSqueak answers false. So the whole thing boils down to a
> > situation where Squeak / Pharo is run in an environment where the
> > XDisplayPlugin can?t be loaded but where forking is still possible.
> >
> > I don?t know how to do this in a cross platform way but for Pharo we could
> > extend #safeToForkSqueak with a headless check like this:
> >
> > safeToForkSqueak
> > "The forkSqueak method is currently supported only for X11 displays with
> > XDisplayControlPlugin available. If the plugin is not available, the child
> > Squeak
> > VM will continue interacting with the X display, which leads to errors in
> > the
> > parent Squeak, including VM crashes."
> >
> > ^self canControlXDisplay
> >  or: [ Smalltalk isHeadless ]
> >
> >
> > I?m going to dirty patch this for my images because I really need it but I
> > think some version of this patch would be a good thing to integrate into
> > OSProcess.
> >
> >
> > Hi Max,
> >
> > Thanks for this fix. I'll add it to the OSP repository as soon as I have a
> > chance (but maybe not until Sunday).
> >
> >
> > As a side note, I have also found that an active RFB (VNC) connection
> > presents
> > a similar issue. If I have a headless image (such as a SqueakSource
> > server), and
> > if I have a VNC session connected to that image, then a forkSqueak does
> > bad things
> > because it results in two instances of the VM/image connected to the same
> > socket.
> > This is the same kind of issue as with the X11 server socket connection,
> > so I should
> > probably find a way to handle this in the safeToForkSqueak checks.
> >
> >
> > We explicitly check for that and forbid forking if there???s an RFB session.
> > We also do a couple of other things before forking to make sure we don???t
> > run into problems. One of the most important things for us is to stop WAKom
> > before the fork and restart it afterwards. Otherwise the child would
> > sometimes steal the socket or would crash etc.
> > Forking is realy a bit of a can of worms and I???m really happy that it
> > still works so reliably. So thanks for that :)
> >
>
> Should it be doing something to the effect of Smalltalk
> processShutDownList:/processStartUpList: around the fork?  I can see this
> is problematic cuz one won't want to shut down the kernel (Delay etc) or
> OSProcess itself.  But an arbitrary set of things may need to shutDown and
> restart and the start-up and shut-down lists are the pluggable mechanism.

> Hi folks.
>
> I?m running into an issue when forking images with OSProcess in headless images. In older versions of OSProcess #forkSqueak would try to fork in any case, independent of the presence of or the connection state to XDisplayControlPlugin.
> In dtl.57 #forkSqueak was changed to explicitly check if XDisplayControlPlugin is present and can be controlled. This is obviously a good thing for when the image runs headful. In my case however I can no longer fork my headless image, even though I don?t need an X session.
>
> Important: apparently #safeToForkSqueak answers true if the XDisplayControlPlugin is available, even in a headless image. But since we don?t need windows, we don?t install X libraries on our server and so #safeToForkSqueak answers false. So the whole thing boils down to a situation where Squeak / Pharo is run in an environment where the XDisplayPlugin can?t be loaded but where forking is still possible.
>
> I don?t know how to do this in a cross platform way but for Pharo we could extend #safeToForkSqueak with a headless check like this:
>
> safeToForkSqueak
> "The forkSqueak method is currently supported only for X11 displays with
> XDisplayControlPlugin available. If the plugin is not available, the child Squeak
> VM will continue interacting with the X display, which leads to errors in the
> parent Squeak, including VM crashes."
>
> ^self canControlXDisplay
> or: [ Smalltalk isHeadless ]
>
>
> I?m going to dirty patch this for my images because I really need it but I think some version of this patch would be a good thing to integrate into OSProcess.
>