cloud backend: a failed upload can leave chunk objects that prevent subsequent uploads of the same file

[davidsarah]

When attempting to upload a ~10 MiB file using the OpenStack? cloud backend (to a Rackspace account), the upload failed due to a DNS error:

<class 'allmydata.interfaces.UploadUnhappinessError'>:
shares could be placed or found on only 0 server(s). We were asked to place
shares on at least 1 server(s) such that any 1 of them have enough shares to
recover the file.:
[Failure instance: Traceback (failure with no frames):
<class 'allmydata.util.pipeline.PipelineError'>:
<PipelineError error=([Failure instance: Traceback (failure with no frames):
<class 'foolscap.tokens.RemoteException'>:
<RemoteException around '[CopiedFailure instance: Traceback from remote host --
Traceback (most recent call last):
Failure: twisted.internet.error.DNSLookupError: DNS lookup failed: address
'storage101.dfw1.clouddrive.com' not found: [Errno -2] Name or service not known.
]'> ])> ]

That error is not itself the subject of this ticket. The issue for this ticket is that subsequent uploads of the same immutable file also failed, even after the DNS error had resolved itself:

<class 'allmydata.interfaces.UploadUnhappinessError'>:
server selection failed for <Tahoe2ServerSelector for upload sefes>:
shares could be placed or found on only 0 server(s). We were asked to place
shares on at least 1 server(s) such that any 1 of them have enough shares to
recover the file. (placed 0 shares out of 1 total (1 homeless), want to place
shares on at least 1 servers such that any 1 of them have enough shares to
recover the file, sent 1 queries to 1 servers, 0 queries placed some shares,
1 placed none (of which 0 placed none due to the server being full and 1 placed
none due to an error)) (last failure (from <ServerTracker for server kdu2jtww
and SI sefes>) was:
[Failure instance: Traceback (failure with no frames):
<class 'foolscap.tokens.RemoteException'>:
<RemoteException around '[CopiedFailure instance: Traceback from remote host --
Traceback (most recent call last):
[...]
File "/home/davidsarah/tahoe/git/working/src/allmydata/storage/backends/cloud/cloud_common.py", line 339, in _retry
  d2 = self._handle_error(f, 1, None, description, operation, *args, **kwargs)
File "/usr/lib/python2.7/dist-packages/twisted/internet/defer.py", line 551, in _runCallbacks
  current.result = callback(current.result, *args, **kw)
File "/home/davidsarah/tahoe/git/working/src/allmydata/storage/backends/cloud/openstack/openstack_container.py", line 81, in _got_response
  message="unexpected response code %r %s" % (response.code, response.phrase))
allmydata.storage.backends.cloud.cloud_common.CloudError:
("try 1 failed: GET object ('shares/se/sefeslgzc4su3i66b72aytmebm/0',) {}", 404,
'unexpected response code 404 Not Found', None) ]'> ])

On looking at the container contents via the Rackspace Cloud Files WUI, there is only one chunk object stored for this file, with key:

shares/se/sefeslgzc4su3i66b72aytmebm/0.5