Activation of multiple services provided by the same executable

[Darren Garvey]

Hi,

We have a situation where some of our processes expose themselves over dbus
with more than one well-known name. There are a few reasons this may be
done:

* For optimisation - moving two or more increasingly tightly coupled
components into one process;
* Legacy - providing legacy adaptor interfaces from a newer daemon.
* Prototyping - starting with a single-process implementation, knowing that
we want to provide separate interfaces over different bus-names.

In these situations, we still provide service files for each bus-name and
they all point to the same executable. My expectation was that dbus-daemon
would take care of process spawning and avoid any race conditions during
activation. IIUC dbus-daemon provides this guarantee for concurrent
requests to activate a service, but doesn't appear to support concurrent
requests to activate different services that are provided by the same
executable. We're seeing multiple instances of an executable started
occasionally which we believe is caused by this.

This question has been asked in the past:

http://old.nabble.com/Providing-several-well-known-names-vs.-activation-td31627361.html

Is this use-case still unsupported? Has there been any work done to patch
dbus to make this work? If not, we might consider patching dbus to make it
check the "Exec" in a service file in a similar way to how it checks the
service name, to fix any raciness in this situation.

Before we attempt that, can anyone give us any reasons why this design is
fundamentally broken or why such a patch wouldn't be accepted upstream?

Cheers,

Darren
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