Add unit tests that simulates broken frames and catches the correct error.

tests/test_camera.py

@@ -0,0 +1,58 @@
+"""Use the Simulation camera to test base camera functionality."""
+
+import tempfile
+from contextlib import contextmanager
+
+import pytest
+from fastapi.testclient import TestClient
+import numpy as np
+from PIL import Image
+
+import labthings_fastapi as lt
+
+from openflexure_microscope_server.things.camera.simulation import SimulatedCamera
+
+
+@contextmanager
+def camera_server(camera: SimulatedCamera) -> lt.ThingClient:
+    """Add the camera to a ThingServer and start a TestClient application.
+
+    The test client application is needed for the camera to have a blocking portal.
+    """
+    with tempfile.TemporaryDirectory() as tmpdir:
+        server = lt.ThingServer(settings_folder=tmpdir)
+        server.add_thing(camera, "/camera/")
+        with TestClient(server.app):
+            yield server
+
+
+def test_handle_broken_frame():
+    """Monkey patch the the mjpeg steam so 1 in 5 frames are broken, then test operation.
+
+    This simulates the very occasional broken frames that can occur when grabbing
+    directly from the MJPEG stream.
+    """
+    camera = SimulatedCamera()
+
+    # Money patch the mjpeg_stream grab_frame to break 1 in 5 frames.
+    frame_number = 0
+    original_grabber = camera.mjpeg_stream.grab_frame
+
+    async def flaky_grabber():
+        """Break 1 in 5 frames."""
+        # Use a non-local variable to know the frame count.
+        nonlocal frame_number
+        frame = await original_grabber()
+        if frame_number % 5 == 2:
+            # Make a weird broken frame
+            frame = frame[:2000] + frame[:2000]
+        frame_number += 1
+        return frame
+
+    camera.mjpeg_stream.grab_frame = flaky_grabber
+    with camera_server(camera):
+        portal = lt.get_blocking_portal(camera)
+        with pytest.raises(OSError, match="broken data stream when reading image file"):
+            for _i in range(15):
+                jpeg = camera.grab_jpeg(portal)
+                np.asarray(Image.open(jpeg.open()))

tests/test_cameras.py

@@ -1,4 +1,8 @@
-"""Generic tests for cameras, specific camera hardware reqipres hardware specific tests."""
+"""Tests for camera classes. These tests focus on checking the camera APIs are equivalent.
+
+Tests for specific camera hardware are in the hardware-specific-tests directory. Tests
+on camera functionality using the simulation camera are in "test_camera".
+"""
 
 import pytest