Concurrent uploads completely broken since V2 api.

Thanks for the report Stefan! I just tried running two of these though and I'm not seeing the same behavior, using the same version of Python and the Dropbox library.

How are you checking the file size to see the unexpected empty file? Can you add some logging and share the output showing the issue? In particular, it might be good to check the size in three places: in the metadata returned by files_download, of fp itself, and in the metadata returned by files_upload. That way we can see exactly when/where the empty file originates.

Thanks in advance! 

files_download always returns an error code 200 and the correct file size. Calling tell() on the file pointer after writing also always returns the correct file size. The metadata returned by files_upload (occasionally) claims to have resulted in a zero bytes upload.

Based on the behavior i see in my app, i am quite confident that is is the upload that fails.

I also ran a few experiments with the files in different folders / in shared folders, but that doesn't seem to make a difference.

Is there any internel dropbox lib logging i can enable?

You can add the following to enable some debugging output that would show the HTTP requests:

import httplib
httplib.HTTPConnection.debuglevel = 1

Good idea. I modified the inner loop to this:

for i in range(10000):
    with tempfile.NamedTemporaryFile() as fp:
        metadata, restdata = client.files_download(FILE)
        fp.write(restdata.content)
        fp.write("{}\r".format(i))

        fp.seek(0)

        print '=================================='
        httplib.HTTPConnection.debuglevel = 1
        md2 = client.files_upload(fp, FILE, dropbox.files.WriteMode('overwrite', None))
        httplib.HTTPConnection.debuglevel = 0
        assert md2.size

Output:

==================================
send: 'POST /2/files/upload HTTP/1.1\r\nHost: content.dropboxapi.com\r\nConnection: keep-alive\r\nAccept-Encoding: gzip, deflate\r\nAccept: */*\r\nUser-Agent: OfficialDropboxPythonSDKv2/6.8.0\r\nContent-Type: application/octet-stream\r\nDropbox-API-Arg: {"path": "/foo/test.txt", "mode": {".tag": "overwrite"}, "autorename": false, "mute": false}\r\nAuthorization: Bearer CENSORED\r\nContent-Length: 38\r\n\r\n'
send: <open file '<fdopen>', mode 'w+b' at 0x0000000003A57390>
sendIng a read()able
reply: 'HTTP/1.1 200 OK\r\n'
header: Server: nginx
header: Date: Tue, 20 Sep 2016 17:28:13 GMT
header: Content-Type: application/json
header: Transfer-Encoding: chunked
header: Connection: keep-alive
header: pragma: no-cache
header: cache-control: no-cache
header: X-Server-Response-Time: 1122
header: X-Dropbox-Request-Id: 754c6605f013cd1dcad17ad97551c672
header: X-Robots-Tag: noindex, nofollow, noimageindex
header: Content-Encoding: gzip
==================================
send: 'POST /2/files/upload HTTP/1.1\r\nHost: content.dropboxapi.com\r\nConnection: keep-alive\r\nAccept-Encoding: gzip, deflate\r\nAccept: */*\r\nUser-Agent: OfficialDropboxPythonSDKv2/6.8.0\r\nContent-Type: application/octet-stream\r\nDropbox-API-Arg: {"path": "/foo/test.txt", "mode": {".tag": "overwrite"}, "autorename": false, "mute": false}\r\nAuthorization: Bearer CENSORED\r\nContent-Length: 41\r\n\r\n'
send: <open file '<fdopen>', mode 'w+b' at 0x0000000003771E40>
sendIng a read()able
reply: 'HTTP/1.1 200 OK\r\n'
header: Server: nginx
header: Date: Tue, 20 Sep 2016 17:28:14 GMT
header: Content-Type: application/json
header: Transfer-Encoding: chunked
header: Connection: keep-alive
header: pragma: no-cache
header: cache-control: no-cache
header: X-Server-Response-Time: 837
header: X-Dropbox-Request-Id: d716609d9fdfd3cb17001967b6f82009
header: X-Robots-Tag: noindex, nofollow, noimageindex
header: Content-Encoding: gzip
==================================
send: 'POST /2/files/upload HTTP/1.1\r\nHost: content.dropboxapi.com\r\nConnection: keep-alive\r\nAccept-Encoding: gzip, deflate\r\nAccept: */*\r\nUser-Agent: OfficialDropboxPythonSDKv2/6.8.0\r\nContent-Type: application/octet-stream\r\nDropbox-API-Arg: {"path": "/foo/test.txt", "mode": {".tag": "overwrite"}, "autorename": false, "mute": false}\r\nAuthorization: Bearer CENSORED\r\nContent-Length: 44\r\n\r\n'
send: <open file '<fdopen>', mode 'w+b' at 0x0000000003A57390>
sendIng a read()able
reply: 'HTTP/1.1 429 too_many_write_operations/..\r\n'
header: Server: nginx
header: Date: Tue, 20 Sep 2016 17:28:16 GMT
header: Content-Type: application/json
header: Transfer-Encoding: chunked
header: Connection: keep-alive
header: Retry-After: 1
header: X-Dropbox-Request-Id: 99633340ed675344e6a7d07bad3ddb4b
header: X-Robots-Tag: noindex, nofollow, noimageindex
send: 'POST /2/files/upload HTTP/1.1\r\nHost: content.dropboxapi.com\r\nConnection: keep-alive\r\nAccept-Encoding: gzip, deflate\r\nAccept: */*\r\nUser-Agent: OfficialDropboxPythonSDKv2/6.8.0\r\nContent-Type: application/octet-stream\r\nDropbox-API-Arg: {"path": "/foo/test.txt", "mode": {".tag": "overwrite"}, "autorename": false, "mute": false}\r\nAuthorization: Bearer CENSORED\r\nTransfer-Encoding: chunked\r\n\r\n'
send: '0\r\n\r\n'
reply: 'HTTP/1.1 200 OK\r\n'
header: Server: nginx
header: Date: Tue, 20 Sep 2016 17:28:18 GMT
header: Content-Type: application/json
header: Transfer-Encoding: chunked
header: Connection: keep-alive
header: pragma: no-cache
header: cache-control: no-cache
header: X-Server-Response-Time: 598
header: X-Dropbox-Request-Id: 71ff38bee72b24a9afd7172830e85671
header: X-Robots-Tag: noindex, nofollow, noimageindex
header: Content-Encoding: gzip
Traceback (most recent call last):
  File "C:/Users/Stefan/git/dropbox_test.py", line 28, in <module>
    assert md2.size
AssertionError

Seems that the upload fails for some reason (reply: 'HTTP/1.1 429 too_many_write_operations/..\r\n') and when it retries, it sends an "0\r\n\r\n" instead of "<open file '<fdopen>', mode 'w+b' at 0x0000000003A57390>" ?

Thanks! That's very helpful. We're looking into it.