[entity-manager] Issue about entity-manager getting stuck

[Ed Tanous]

On Tue, Jan 26, 2021 at 10:34 PM Scron Chang (張仲延)
<Scron.Chang at quantatw.com> wrote:
>
> Hi all,
>
> I am using openbmc/entity-manager in this version: "f094125cd3bdbc8737dc8035a6e9ac252f6e8840" and I found calling Dbus makes entity-manager get stuck.
>
> Reproduce this by following steps:
> 1. systemctl stop xyz.openbmc_project.EntityManager
> 2. open another terminal and do this while-loop: "while true; do busctl ; sleep 1; done"
> 3. systemctl start xyz.openbmc_project.EntityManager
> I think the root cause is this function: "nameOwnerChangedMatch." (Please refer to this line: https://github.com/openbmc/entity-manager/blob/f094125cd3bdbc8737dc8035a6e9ac252f6e8840/src/EntityManager.cpp#L1859.)

My first thought is: Don't run an empty busctl in a loop then, but I'm
guessing that's not what you're really trying to do.  If we had more
ideas about what you were really hoping to accomplish, we might have
some better advice for how to proceed.

The intent of that code is to reconfigure entity-manager when
interfaces are changed, so if you're constantly attaching and
detaching to dbus, entity-manager (and object manager) never sees the
system as "up" and keeps waiting for the system to finish stabilizing
before it runs the config logic.

In your specific case above, the code could be a little smarter, and
ignore unique names in that check, only caring about newly-defined
well known names, but without knowing your real use case, it's hard to
know if that would help.

>
> Manually calling Dbus or calling Dbus in a script makes NameOwnerChanged signal and thus triggers the function: "propertiesChangedCallback" repeatedly. Meanwhile, the async_wait in propertiesChangedCallback gets returned because of the operation_aborted.

Personal opinion: Don't call busctl continuously in a script.  It's
inefficient, and causes problems like this.

> So here is the conclusion:
> Manually calling Dbus in a period that is less than 5 seconds leads entity-manager keeping to trigger new async_wait and abort the old one. However, the async_wait never gets done.
>
> Is this a bug of entity-manager, or I get something wrong. Please help me with this.

IMO, entity-manager is working as intended, but lets try to figure out
what you're really trying to do, and see if we can find you a
solution.

>
> Scron Chang
> E-Mail  Scron.Chang at quantatw.com
>