Document FileResponse does not seek its file source

[Claude Paroz]

I'm not sure this is a bug, but I was bitten by some code today:

buffer = io.BytesIO()
buffer.write(b'some binary content')
return FileResponse(buffer, filename='somename.bin')

and the response is empty because the stream position is at the end of the buffer. I can easily fix that by seeking the buffer myself, but shouldn't FileResponse seek itself in _set_streaming_content before calling filelike.read?

[Carlton Gibson]

Hi Claude.

...shouldn't FileResponse seek itself ...

I think not here.

I imagine a situation where you want to tail a log, say. You can't do that if we reset the stream position ourselves.

I then think that resetting the stream makes FileResponse a little bit too clever. (Happy if someone wants to put that in a subclass, but it's beyond "take a buffer, stream it to an HTTP response", which is what I take to be the limit of the scope here.)

So, for me, the maxim in play is, "You pass an open file. Your responsibility to seek accordingly".

As such, I'm going to go with wontfix. I hope that makes sense.

Maybe an addition to the FileResponse docs?

...streaming_content begins from the current cursor position of open_file...

(Or such, somewhere.)

[Claude Paroz]

Something like that?:

diff --git a/docs/ref/request-response.txt b/docs/ref/request-response.txt
index 19593dfb90..3dfbf28cf6 100644
--- a/docs/ref/request-response.txt
+++ b/docs/ref/request-response.txt
@@ -1122,6 +1122,8 @@ Attributes
 
     If ``open_file`` doesn't have a name or if the name of ``open_file`` isn't
     appropriate, provide a custom file name using the ``filename``  parameter.
+    Note that if you pass a file-like object like ``io.BytesIO``, it's your
+    task to ``seek()`` it before passing it to ``FileResponse``.
 
     The ``Content-Length`` and ``Content-Type`` headers are automatically set
     when they can be guessed from contents of ``open_file``.

I suspect the example in ​https://docs.djangoproject.com/en/2.2/howto/outputting-pdf/#write-your-view is missing the seek call.

[Carlton Gibson]

Yep, I'll take that. Perfect. 🙂

[Claude Paroz]

​PR

[Carlton Gibson <carlton.gibson@…>]

In 7203efb7:

Fixed #30694 -- Documented FileResponse does not seek its file source.

[Carlton Gibson <carlton.gibson@…>]

In 556cef1:

[2.2.x] Fixed #30694 -- Documented FileResponse does not seek its file source.

Backport of 7203efb799969b4662ecb58f4cefd2a5f2e0062b from master