At the moment, the camera manager shared pointer is released when the last
listener is removed, and recreated when the first listener is added. This
is the same behaviour that the alsa and v4l2 monitors implement with their
respective udev, inotify monitors.
However, for `libcamera::CameraManager`, this is likely not the best way
for multiple reasons:
(a) it is a complex object with significant construction and starting cost,
which includes starting threads and usually loading shared libraries;
(b) usually one listener is added right after creating, and it is removed
right before destruction, in which there are real no advantages;
(c) the camera manager, being a shared resource, might very well be kept
alive by some other component, in which case there is again not much
real benefit.
So simplify the code by getting a camera manager reference at the beginning
and keeping it until the libcamera monitor is destroyed.
This also fixes a race condition where a hot-plugged camera might not have
been detected if the libcamera event was emitted between these two:
collect_existing_devices(impl);
start_monitor(impl);
The `impl::{add,remove}Camera` functions do the same thing except
for one value, the type of the hotplug event. Add a private method
to `impl` that implements the common parts.
There is an issue in the id allocation mechanism which can result
in the different devices having the same id. Specifically, consider
the scenario where there are only two cameras, which have just been
added. In this case `impl::devices` looks like this:
(0, camA) | (1, camB) | (?, nullptr) | ...
Now assume that `camA` is removed, after which the array appears
as follows:
(1, camB) | (1, nullptr) | (?, nullptr) | ...
Then assume that a new camera appears. When `get_free_id()` runs,
when `i == 1`, it will observe that `devices[i].camera == nullptr`,
so it selects `1` as the id. Leading to the following:
(1, camB) | (1, camC) | (?, nullptr) | ...
This is of course incorrect. The set of ids must be unique. When
wireplumber is faced with this situation it destroys the device
object for `camB` when `camC` is emitted.
Fix this by simply not moving elements in the `devices` array,
leaving everything where it is. In which case the array looks
like this:
(nullptr) | (camB) | (nullptr) | ... // after `camA` removal
(camC) | (camB) | (nullptr) | ... // after `camC` appearance
Note that `device::id` is removed, and the id is now derived from
the position in `impl::devices`.
C++20 introduced designated initializers similar to the ones found
in C99, however, in C++ designated initializers cannot be mixed
with non-designated initializers. GCC rejects mixed initializers
with an error.
Find a free id from the available ids instead of just taking the current
number of devices, which would give assign the same id to multiple
cameras when: added 0, added 1, removed 0, added 1.
Previously, if "add_listener" was called on the monitor device, then it
did not necessarily emit events about all devices because it called
`enum_devices()` which does not emit events about already existing
devices. So the very first listener would get all existing devices,
but subsequent ones would not get events about the existence of devices
that have already been seen by the monitor. Fix that by simply emitting
events about all existing devices if the current listener is not the first.
libcamera's CameraManager runs the event handlers on its own thread,
thus synchronization is needed to ensure correct functionality.
Implement that by collecting hotplug events into a mutex protected queue
and signalling the main loop where they can be processed and the
"object_info" event can be safely emitted.
Using a shared_ptr removes the need for manually calling
`libcamera_manager_release()` to drop the reference as it is done
automatically whenever the shared_ptr is destroyed or reset.
Previously, the libcamera manager `impl` object was neither properly
constructed neither properly destructed. As a consequence, for example,
the shared pointers in the `devices` array weren't properly destructed,
although this has been somewhat mitigated by a previous change
that modifed `clear_devices()`.
Previously, in `remove_device()`, the last device would be copied into
the slot of the to-be-remove device. The problem with this is that it
left the shared_ptr untouched in the previously last slot, and hence
creating an extra reference. Fix this by moving instead of copying.
A similar problem is present in `clear_devices()` which also
did not properly dispose of the shared_ptrs. Fix that by
calling `reset()` on each device's camera pointer.
