[BUG] Threads not terminating after camera is closed

Start with the attached test program and its doCapture_mp4 function, focusing on H264Encoder, FfmpegOutput, start_encoder, stop_encoder, and camera close behavior. Reproduce the thread counts on the stated Raspberry Pi setup, then trace which encoder threads remain after each stop and close; done means no encoder-created threads survive camera shutdown.

Hello,

Bug Description

I am working on a Flask-based web server for controlling Raspberry Pi cameras with picamera2 (raspi-cam-srv)
Among others, this includes motion detection with recording of .mp4 videos.

Users had recognized that the number of threads of the server process is increasing with time.

To analyze the situation, I have written a small test program which monitors the number of threads after specific process steps for different scenarios.

For results, see Console Output, below.

Obviously, already the import of Picamera2 starts 2 threads (only observed on Pi5).

Within the loop, the encoder is started and stopped 5 times, while the camera keeps running.
Each encoder start seems to start 9 threads. However, these threads do not in all cases terminate after the encoder has been stopped.
After the first run, 9 threads are still alive after the camera had been closed and the reference to the camera has been removed.

In the example, all threads seem to have terminated after the second cycle (only the 3 threads from import survive).
However, I have also seen cases where these threads lived for hours.

With other encoders, the behavior was as expected: No additional threads survived after the camera was closed.

To Reproduce

The attached test program can be used to reproduce the described bahavior.

TestPicamera2Threads.zip

Expected Behaviour

I would expect that no additional threads survive after the encoder has been stopped and the camera has been closed.

Console Output, Screenshots

Below is the function for recording .mp4 videos:

def doCapture_mp4(ser, count, pause):
    """Capture MP4"""
    print(f'\nCapturing mp4')
    picam2 = None
    print(f'picam2: None               #Threads: {countThreads(PROCESSNAME)[2]}')
    picam2 = Picamera2(0)
    print(f'picam2: Instantiated       #Threads: {countThreads(PROCESSNAME)[2]}')
    config = picam2.create_video_configuration()
    picam2.configure(config)
    print(f'picam2: configured         #Threads: {countThreads(PROCESSNAME)[2]}')
    picam2.start(show_preview=False)
    print(f'picam2: started            #Threads: {countThreads(PROCESSNAME)[2]}')
    time.sleep(2)
    for n in range(0, count):
        fn = f'test_{ser}_{n}.mp4'
        print(fn)
        encoder = H264Encoder()
        encoder.output = FfmpegOutput(fn, audio=False)
        picam2.start_encoder(encoder)
        print(f'picam2: encoding mp4       #Threads: {countThreads(PROCESSNAME)[2]}')
        time.sleep(pause)
        picam2.stop_encoder(encoder)
        time.sleep(2)
        print(f'picam2: encoder stopped    #Threads: {countThreads(PROCESSNAME)[2]}')
    picam2.stop()
    print(f'picam2: Stopped            #Threads: {countThreads(PROCESSNAME)[2]}')
    picam2.close()
    print(f'picam2: closed             #Threads: {countThreads(PROCESSNAME)[2]}')
    del picam2
    gc.collect()
    print(f"picam2: del'd              #Threads: {countThreads(PROCESSNAME)[2]}")

For recording .mp4 videos with H264Encoder and FfmpegOutput, the test gives the following output:

Start                      #Threads: 1
Picamera2 imported         #Threads: 3
Encoders imported          #Threads: 3
Select capture mode: 1=jpg, 2=mjpg(JpegEncoder), 3=mjpg(MJPEGEncoder), 4=mp4 0:terminate
4

Capturing mp4
picam2: None               #Threads: 3
picam2: Instantiated       #Threads: 9
picam2: configured         #Threads: 9
picam2: started            #Threads: 11
test_1_0.mp4
picam2: encoding mp4       #Threads: 20
picam2: encoder stopped    #Threads: 20
test_1_1.mp4
picam2: encoding mp4       #Threads: 29
picam2: encoder stopped    #Threads: 29
test_1_2.mp4
picam2: encoding mp4       #Threads: 38
picam2: encoder stopped    #Threads: 38
test_1_3.mp4
picam2: encoding mp4       #Threads: 47
picam2: encoder stopped    #Threads: 47
test_1_4.mp4
picam2: encoding mp4       #Threads: 56
picam2: encoder stopped    #Threads: 56
picam2: Stopped            #Threads: 55
picam2: closed             #Threads: 48
picam2: del'd              #Threads: 12
Press <Enter> to continue or <Ctrl+C> to terminate


Capturing mp4
picam2: None               #Threads: 12
picam2: Instantiated       #Threads: 18
picam2: configured         #Threads: 18
picam2: started            #Threads: 20
test_2_0.mp4
picam2: encoding mp4       #Threads: 29
picam2: encoder stopped    #Threads: 20
test_2_1.mp4
picam2: encoding mp4       #Threads: 29
picam2: encoder stopped    #Threads: 20
test_2_2.mp4
picam2: encoding mp4       #Threads: 29
picam2: encoder stopped    #Threads: 20
test_2_3.mp4
picam2: encoding mp4       #Threads: 29
picam2: encoder stopped    #Threads: 20
test_2_4.mp4
picam2: encoding mp4       #Threads: 29
picam2: encoder stopped    #Threads: 20
picam2: Stopped            #Threads: 19
picam2: closed             #Threads: 12
picam2: del'd              #Threads: 3
Press <Enter> to continue or <Ctrl+C> to terminate

Hardware

RPi 5
Attached cameras:

• (0): module 2 camera ("Model": "imx708")
• (1): module HQ camera ("Model": "imx477")

OS: Bookworm
Debian version: 12.5

Additional Context