Guard input container properties and methods again use after close

Calling `close()` on an input container calls `avformat_close_input`
which will set the `self.ptr` AVFormatContext pointer to NULL. We need
to ensure `self.ptr` is not NULL before using it.

Fixes: #1137

av/container/core.pxd

@@ -37,6 +37,8 @@ cdef class Container:
     cdef readonly StreamContainer streams
     cdef readonly dict metadata
 
+    # Private API.
+    cdef _assert_open(self)
     cdef int err_check(self, int value) except -1
 
     # Timeouts

av/container/core.pyx

@@ -285,6 +285,7 @@ cdef class Container:
         return err_check(value, filename=self.name)
 
     def dumps_format(self):
+        self._assert_open()
         with LogCapture() as logs:
             lib.av_dump_format(self.ptr, 0, "", isinstance(self, OutputContainer))
         return ''.join(log[2] for log in logs)
@@ -298,10 +299,16 @@ cdef class Container:
     cdef start_timeout(self):
         self.interrupt_callback_info.start_time = clock()
 
+    cdef _assert_open(self):
+        if self.ptr == NULL:
+            raise AssertionError("Container is not open")
+
     def _get_flags(self):
+        self._assert_open()
         return self.ptr.flags
 
     def _set_flags(self, value):
+        self._assert_open()
         self.ptr.flags = value
 
     flags = Flags.property(

av/container/input.pyx

@@ -15,6 +15,7 @@ from av.dictionary import Dictionary
 cdef close_input(InputContainer self):
     if self.input_was_opened:
         with nogil:
+            # This causes `self.ptr` to be set to NULL.
             lib.avformat_close_input(&self.ptr)
         self.input_was_opened = False
 
@@ -90,19 +91,25 @@ cdef class InputContainer(Container):
 
     property start_time:
         def __get__(self):
+            self._assert_open()
             if self.ptr.start_time != lib.AV_NOPTS_VALUE:
                 return self.ptr.start_time
 
     property duration:
         def __get__(self):
+            self._assert_open()
             if self.ptr.duration != lib.AV_NOPTS_VALUE:
                 return self.ptr.duration
 
     property bit_rate:
-        def __get__(self): return self.ptr.bit_rate
+        def __get__(self):
+            self._assert_open()
+            return self.ptr.bit_rate
 
     property size:
-        def __get__(self): return lib.avio_size(self.ptr.pb)
+        def __get__(self):
+            self._assert_open()
+            return lib.avio_size(self.ptr.pb)
 
     def close(self):
         close_input(self)
@@ -123,6 +130,7 @@ cdef class InputContainer(Container):
         .. note:: The last packets are dummy packets that when decoded will flush the buffers.
 
         """
+        self._assert_open()
 
         # For whatever reason, Cython does not like us directly passing kwargs
         # from one method to another. Without kwargs, it ends up passing a
@@ -198,6 +206,7 @@ cdef class InputContainer(Container):
             the arguments.
 
         """
+        self._assert_open()
         id(kwargs)  # Avoid Cython bug; see demux().
         for packet in self.demux(*args, **kwargs):
             for frame in packet.decode():
@@ -234,6 +243,7 @@ cdef class InputContainer(Container):
         .. seealso:: :ffmpeg:`avformat_seek_file` for discussion of the flags.
 
         """
+        self._assert_open()
 
         # We used to take floats here and assume they were in seconds. This
         # was super confusing, so lets go in the complete opposite direction
@@ -270,6 +280,8 @@ cdef class InputContainer(Container):
         self.flush_buffers()
 
     cdef flush_buffers(self):
+        self._assert_open()
+
         cdef Stream stream
         cdef CodecContext codec_context
 

tests/test_decode.py

@@ -111,3 +111,16 @@ def test_decode_video_corrupt(self):
 
         self.assertEqual(packet_count, 1)
         self.assertEqual(frame_count, 0)
+
+    def test_decode_close_then_use(self):
+        container = av.open(fate_suite("h264/interlaced_crop.mp4"))
+        container.close()
+
+        # Check accessing every attribute either works or raises
+        # an `AssertionError`.
+        for attr in dir(container):
+            with self.subTest(attr=attr):
+                try:
+                    getattr(container, attr)
+                except AssertionError:
+                    pass