[phosphor-logging] About the "Stop emitting Entry propChanged before ifacesAdded" change reason

[Matt Spinler]

On 10/20/2021 3:39 AM, CS20 CHMa0 wrote:
> Hi Matt, I meet an issue in bmcweb while update service handle 
> firmware update error 
> https://github.com/openbmc/bmcweb/blob/master/redfish-core/lib/update_service.hpp#L321 
> After I revert the change or try to change the match rule from 
> 'PropertiesChanged' ZjQcmQRYFpfptBannerStart
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> Hi Matt,
>
> I meet an issue in bmcweb while update service handle firmware update 
> error
>
> https://github.com/openbmc/bmcweb/blob/master/redfish-core/lib/update_service.hpp#L321 
> <https://github.com/openbmc/bmcweb/blob/master/redfish-core/lib/update_service.hpp#L321> 
>
>
> After I  revert the change or try to change the match rule from 
> 'PropertiesChanged' to 'InterfacesAdded',
>
> the error handler work well in update service.
>
>      fwUpdateErrorMatcher = 
> std::make_unique<sdbusplus::bus::match::match>(
>
>          *crow::connections::systemBus,
>
> - "type='signal',member='PropertiesChanged',path_namespace='/xyz/"
>
> -        "openbmc_project/logging/entry',"
>
> - "arg0='xyz.openbmc_project.Logging.Entry'",
>
> + "interface='org.freedesktop.DBus.ObjectManager',type='signal',"
>
> +        "member='InterfacesAdded',"
>
> + "path='/xyz/openbmc_project/logging'",
>
> May I ask the reason why you update the change? 
> https://gerrit.openbmc-project.xyz/c/openbmc/phosphor-logging/+/46055 
> <https://gerrit.openbmc-project.xyz/c/openbmc/phosphor-logging/+/46055>
>

Hi,
I guess I saw this as a bug fix.  The code creates the interface on 
D-Bus with the defer-interfaces-added parameter set to true so it 
wouldn't emit interfacesAdded at the point, then sets all the property 
values, and then explicitly emits the IA signal.   Others can chime in, 
but I didn't see it as proper D-Bus behavior to emit propertiesChanged 
before InterfacesAdded, since in fact no property is changing after the 
interface was added.

It seems like every application does their own thing here, so maybe we 
can come up with some official best practices for how to emit signals 
for new interfaces (unless it's there and I missed it).


> Thanks
>
> Brian
>
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