[sane-devel] What's local_only and what's not?

[Louis Lagendijk]

On Wed, 2019-11-13 at 09:48 +0100, Johannes Meixner wrote:
> Hello,
> 
> On Nov 12 18:06 Olaf Meeuwissen wrote (excerpt):
> >
> > #130: https://gitlab.com/sane-project/backends/issues/130
> > #141: https://gitlab.com/sane-project/backends/issues/141
> 
> I agree with the initial reporters that the usually expected
> meaning of 'local' is in particular 'not via network'.
> 
> Therefore the special case that a scanner with a network interface
> that is directly connected by a network cable to a dedicated
> network interface on the computer where SANE runs
> is not a local scanner regardless that the scanner is directly
> attached with a cable to the computer where SANE runs.
> 
> 
> > Discussing what `sane_get_devices()` should be considered
> `local_only`,
> > we are no longer sure what is and is not local :-o
> ...
> >> FWIW, the SANE Standard, in section 4.3.3 sane_get_devices, has
> the
> >> following to say:
> ...
> >>  ... If argument
> >>  `local_only` is true, only local devices are returned (devices
> >>  directly attached to the machine that SANE is running on).
> 
> I think the word 'attached' has the implicit meaning that
> a connection method is used where usually a cable is used
> and not only electromagnetic waves like for Bluetooth
> or any other connection method that is usually wireless.
> 
> Therefore the special case that a USB cable is replaced
> with a pair of USB-thingies that replace the USB cable with
> a wireless connection is still 'directly attached'.
> 
> 
> I think the terms 'local' and 'directly attached' are
> technically vague and should be replaced with technically
> more precise terms.
> 
> In this case I think the connection type is what should make
> the difference so that the sane_get_devices() documentation
> should be more explanatory and more precise for example like:
> ------------------------------------------------------------------
> If argument 'local_only' is true, only local devices are returned.
> 
> Local devices are those that are directly attached to the machine
> that SANE is running on.
> 
> Directly attached means that a connection method is used
> where usually a direct cable connection is used and
> that is in general not a network connection.
> 
> The following connection methods are 'directly attached':
> 
> USB
> parallel port
> SCSI
> 
> All other connection methods are not 'directly attached'.
> 
> Accordingly if argument 'local_only' is true, only devices are
> returned that are connected via USB or parallel port or SCSI.
> ------------------------------------------------------------------
> Of course that could be simplified to:
> ------------------------------------------------------------------
> If argument 'local_only' is true, only devices are returned
> that are connected to the machine that SANE is running on
> via USB or parallel port or SCSI.
> ------------------------------------------------------------------
> But then is does no longer explain the reasoning behind
> so things may again break apart when a new connection method
> gets supported by SANE (e.g. Tap-proof Wired Bluetooth ;-)
> Aren't there scanners/AIOs for Mac that use Thunderbolt?
> 
> 
> FWIW,
> cf. https://en.opensuse.org/YaST_Printer
> that reads (excerpt):
> ------------------------------------------------------------------
> There is a difference between "local/remote queue"
> and "local/remote printer":
> 
> "Local queue" means a print queue which is configured on the
> local computer (strictly speaking the computer where the YaST
> printer module runs) which is usually the computer where the
> user sits in front of.
> 
> "Remote queue" means a print queue which exists on whatever
> computer in the network or on whatever network printer.
> 
> 
> "Local printer" means a printer device which is directly connected
> to the local computer (usually a USB printer).
> 
> "Remote printer" means a printer device which is not directly
> connected to the local computer (usually a printer with a
> built-in network interface). 
> ------------------------------------------------------------------
> 
> Accordingly a printer with a built-in network interface that is
> directly connected by a network cable to a dedicated network
> interface on the computer is not a local printer and
> a USB printer where the usual USB cable is replaced by
> whatever wireless technique (that may even reach out
> into the next building) is still a local printer.
> 
> 
I like Johannes's explanation. I now implemented the local_only support
in the pixma backend.
@Olaf: CI/CD seems to be broken, can you please have a look? I don't
think that my changes are to be blamed, but I may have done something
silly. The pipeline fails with:

175 $ ./tools/create-changelog.sh
176 $ ./tools/update-upstreams.sh
180 ERROR: Job failed: exit code 1

Thanks,
Louis