Correctly handle unavailable .fileno() support for io.BytesIO instances

[wbolster]

The changes introduced in 98c9e3b#diff-6607b435d6ee868f251dbdd1a6d0edecL357 are wrong and lead to crashes for io.BytesIO instances which do have a .fileno() method but raise another exception instead:

>>> import io
>>> io.BytesIO().fileno()
Traceback (most recent call last):
  File "<ipython-input-3-7062c09d0c52>", line 1, in <module>
    io.BytesIO().fileno()
UnsupportedOperation: fileno

This exception was already caught properly in the earlier code, a few lines below.

[wbolster]

I guess this is a critical bug since it breaks a lot of cases like flask.send_file() (and other frameworks with similar features) with a io.BytesIO() instance, which is a rather common pattern.

[benoitc]

@wbolster thanks for the ticket. Can you test the change above?

@tilgovi what do you think about it?

[wbolster]

fwiw, BytesIO is specifically intended to be a file like object (according to the vague python definition of that term), so having a function is_fileobject() return False violates the principle of least surprise. :)

I guess we were both looking at this at the same time, and I just posted #1176 with an alternative...

[benoitc]

@wbolster Thanks for the patch!

The solutions are similar but I quite like the idea of having a function apart for it, so it may be later unit-tested eventually.

File objects were better defined in old versions (and Gunicorn starts to get old) [1] . I see now that it has been reduced. Although this tests is done there to test if the file-like object return a usable file descriptior so we can use sendfile(2) .So indeed we should change the name of the function to make it more descriptive. Thoughts?

[1] https://docs.python.org/2.5.2/lib/bltin-file-objects.html
[2] https://docs.python.org/3/glossary.html#term-file-object

[wbolster]

is_usable_for_sendfile(), or has_fileno(). or just obtain_fileno() in true eafp style but that requires a well defined exception.

[DCudmore]

Hi,

I'm not sure if this is exactly related, but I'm having a similar issue with the following Traceback:

Traceback (most recent call last):
File "/home/dale/.local/lib/python2.7/site-packages/gunicorn/workers/sync.py", line 130, in handle
self.handle_request(listener, req, client, addr)
File "/home/dale/.local/lib/python2.7/site-packages/gunicorn/workers/sync.py", line 174, in handle_request
resp.write_file(respiter)
File "/home/dale/.local/lib/python2.7/site-packages/gunicorn/http/wsgi.py", line 398, in write_file
if not self.sendfile(respiter):
File "/home/dale/.local/lib/python2.7/site-packages/gunicorn/http/wsgi.py", line 357, in sendfile
fileno = respiter.filelike.fileno()

Here is a git copy if it helps: https://github.com/DCuddies/simplecontract

I'm using BytesIO in order to store a docx object, and then let the users download that docx object.

I'm at a loss at this point as to how to fix the issue, any help would be appreciated.

[benoitc]

@DCuddies sorry have been side tracked. Patch is now is master. If everything goes fine, i will make a release over the week-end.