BUG: close standard streams after webcam access (#711)

imageio/plugins/ffmpeg.py

@@ -224,18 +224,24 @@ def _get_cam_inputname(self, index):
                     "-i",
                     "dummy",
                 ]
-                # Set `shell=True` in sp.Popen to prevent popup of a command line
-                # window in frozen applications. Note: this would be a security
-                # vulnerability if user-input goes into the cmd.
-                proc = sp.Popen(
-                    cmd, stdin=sp.PIPE, stdout=sp.PIPE, stderr=sp.PIPE, shell=True
+                # Set `shell=True` in sp.run to prevent popup of a command
+                # line window in frozen applications. Note: this would be a
+                # security vulnerability if user-input goes into the cmd.
+                # Note that the ffmpeg process returns with exit code 1 when
+                # using `-list_devices` (or `-list_options`), even if the
+                # command is successful, so we set `check=False` explicitly.
+                completed_process = sp.run(
+                    cmd,
+                    stdout=sp.PIPE,
+                    stderr=sp.PIPE,
+                    encoding="utf-8",
+                    shell=True,
+                    check=False,
                 )
-                proc.stdout.readline()
-                proc.terminate()
-                infos = proc.stderr.read().decode("utf-8", errors="ignore")
+
                 # Return device name at index
                 try:
-                    name = parse_device_names(infos)[index]
+                    name = parse_device_names(completed_process.stderr)[index]
                 except IndexError:
                     raise IndexError("No ffdshow camera at index %i." % index)
                 return "video=%s" % name

tests/test_ffmpeg.py

@@ -13,7 +13,7 @@
 
 import numpy as np
 
-from pytest import raises, skip
+from pytest import raises, skip, warns
 from imageio.testing import run_tests_if_main, get_test_dir
 
 import imageio
@@ -561,6 +561,34 @@ def test_webcam_process_termination():
         skip("no webcam")
 
 
+def test_webcam_resource_warnings():
+    """
+    Test for issue #697. Ensures that ffmpeg Reader standard streams are
+    properly closed by checking for ResourceWarning "unclosed file".
+
+    todo: use pytest.does_not_warn() as soon as it becomes available
+     (see https://github.com/pytest-dev/pytest/issues/9404)
+    """
+    try:
+        with warns(None) as warnings:
+            with imageio.get_reader("<video0>"):
+                pass
+    except IndexError:
+        skip("no webcam")
+
+    import imageio_ffmpeg
+
+    if imageio_ffmpeg.__version__ == "0.4.5":
+        # We still expect imagio_ffmpeg 0.4.5 to generate (at most) one warning.
+        # todo: remove this assertion when a fix for imageio_ffmpeg issue #61
+        #  has been released
+        assert len(warnings) < 2
+        return
+
+    # There should not be any warnings, but show warning messages if there are.
+    assert not [w.message for w in warnings]
+
+
 def show_in_console():
     reader = imageio.read("imageio:cockatoo.mp4", "ffmpeg")
     # reader = imageio.read('<video0>')