Replace shared global _empty_stream with separate instances

[chromakode]

The use of a shared _empty_stream instance can cause subtle failures when a test or process closes the shared stream.

For example, if a test initializes a request with a file upload FileStorage with a falsy stream argument, the FileStorage will use the shared _empty_stream:

werkzeug/werkzeug/datastructures.py

Line 2642 in ac230a2

self.stream = stream or _empty_stream

When the request is torn down, it will close the shared _empty_stream:

werkzeug/werkzeug/wrappers.py

Line 416 in ac230a2

value.close()

werkzeug/werkzeug/datastructures.py

Line 2736 in ac230a2

self.stream.close()

Subsequently, if a test environment is created with no specified input_stream, it will use the closed _empty_stream:

werkzeug/werkzeug/test.py

Line 599 in 816a1e9

input_stream = _empty_stream

Now calls to request.get_data() and other operations on the stream will fail.

In the case where we discovered this behavior at Patreon, two unrelated tests were causing a failure: one which initialized an FileStorage with _empty_stream, and one which called request.get_data().

I believe that replacing this shared instance with individually created BytesIO buffers would reduce the chances of such spooky shared global bugs without creating much overhead. What do you think?

[davidism]

Makes sense. Would you add a changelog entry?

[chromakode]

Thanks @davidism. I've amended the commit with a changelog entry and the commit is now passing lint.

[davidism]

Thanks!