ProcessManager Singleton

[Barnabás Pőcze]

Hi


2025. 04. 22. 23:31 keltezéssel, Christian Rauch írta:
> Hi,
> 
> The "libcamera::ProcessManager" and hence the "libcamera::CameraManager" (via "CameraManager::Private") can only be instantiated once in a process. I have two questions regarding the reason for this and about the way this is currently implemented. Since this is a design decision and "not a bug", I am posting this on the "devel" mailing list. Let me know if I should move this somewhere else.
> 
> The "CameraManager" as well as the "ProcessManager" maintain a static "self_" and check this when they are instantiated. However, when instantiating a second "CameraManager" the "CameraManager::self_"-check is actually never reached as the process will quit already at the "ProcessManager::self_" check via a "Fatal" log and its "std::abort()". At least the "CameraManager::self_"-check appears redundant to me.
> 
> Q1: Is the constraint that either the "CameraManager" or the "ProcessManager" must not be instantiated multiple times? Why is this the case? If the constraint is on the "ProcessManager", what is the purpose of the "CameraManager::self_"-check, which is never reached?
> 
> Having the Fatal log / std::abort() triggered for additional instances in the same process has some unexpected implications. While a single "CameraManager" instance can easily be managed within the same code base, this is not really possible when dealing with dynamically loaded objects. Let's assume a plugin system, where plugins are loaded from shared object files into the same process / address space. Loading a "Raspberry Pi" plugin to handle specifically Raspberry Pi camera modules would instantiate the first "CameraManager". When a second "UVC" plugin is loaded for generic USB cameras, this would also instantiate a "CameraManager", trigger the "abort" and crash the whole process.
> 
> Q2: Is the "abort" really required? If there can really be only a single "CameraManager" (and/or "ProcessManager") instance, why can't different sections of the code, or different plugins in the example above, share a common static "ProcessManager" singleton?

See https://bugs.libcamera.org/show_bug.cgi?id=246 ideally no
singletons would exist. I believe this is the plan.

There are at least three singletons in libcamera at the moment:
   * the virtual pipeline handler
   * the camera manager
   * the process manager

There are patches to replace the global ProcessManager with pidfd:
   https://lists.libcamera.org/pipermail/libcamera-devel/2025-March/049339.html
Then the virtual pipeline handler needs to be amended, and then the
CameraManager's singleton requirement can finally be removed (hopefully).


Regards,
Barnabás Pőcze