Fix incorrect Hailo reference counting and virtual device reuse

Previously, TARGET_REF_COUNT was only incremented during the first virtual device allocation,
but not for additional model instances. This caused the VDevice to be deallocated prematurely
when multiple models were loaded, as each model called close() and decremented the count.

Additionally, TARGET was not reset to None after releasing the virtual device, which caused
reuse of a released virtual device and raised exceptions on subsequent model creation.

This commit moves the TARGET_REF_COUNT increment outside the allocation block,
and resets TARGET to None after the virtual device is released when the count reaches zero.

Addresses #1300

Signed-off-by: Florent Gastoud (main) <[email protected]>

Author: gh9869827

picamera2/devices/hailo/hailo.py

@@ -25,7 +25,8 @@ def __init__(self, hef_path, batch_size=None, output_type='FLOAT32'):
         self.hef = HEF(hef_path)
         if Hailo.TARGET is None:
             Hailo.TARGET = VDevice(params)
-            Hailo.TARGET_REF_COUNT += 1
+        # increment `TARGET_REF_COUNT` every time a model is loaded
+        Hailo.TARGET_REF_COUNT += 1
         self.target = Hailo.TARGET
         self.infer_model = self.target.create_infer_model(hef_path)
         self.infer_model.set_batch_size(1 if batch_size is None else batch_size)
@@ -184,3 +185,6 @@ def close(self):
         Hailo.TARGET_REF_COUNT -= 1
         if Hailo.TARGET_REF_COUNT == 0:
             self.target.release()
+            # Reset `TARGET` to None so that subsequent __init__ calls can create
+            # a new VDevice, since this one has been released
+            Hailo.TARGET = None