"No backup chains found" after "Deleting /...[] (not authoritative at backend).!" on s3 target (lp:#570586)

[Original report](https://bugs.launchpad.net/bugs/570586) created by **Jock (c-launchpad-dermot-org-uk)**
```
Full detail here: https://answers.launchpad.net/duplicity/+question/107074

After performing a full encrypted backup to S3, subsequent duplicity
operations cause the local cache for that backup to be deleted, preventing
further manipulation or verification of the backup.

However, carrying out the same steps (full backup then verify) using a
local file:// backend rather than S3 works as expected and the local cache
is not deleted on subsequent operations.

Using duplicity 0.6.08b from Debian squeeze (testing) with python-boto
1.2a-1 and python 2.5.2.

After a full encrypted backup to S3, I then run a verify on my newly-
created backup using the following command (again with AWS credentials
exported first):

    duplicity verify \
        --encrypt-key=MYKEYHERE \
        --sign-key=MYKEYHERE \
        --tempdir=/var/tmp \
        "s3+http://my-bucket-name" /

I then get this output from duplicity:

Synchronizing remote metadata to local cache...
Deleting local
/root/.cache/duplicity/96e3afbc9cfaced5608c3540469d2d09/duplicity-full-
signatures.20100411T144157Z.sigtar.gz (not authoritative at backend).
Deleting local
/root/.cache/duplicity/96e3afbc9cfaced5608c3540469d2d09/duplicity-
full.20100411T144157Z.manifest (not authoritative at backend).
Last full backup date: none
Traceback (most recent call last):
  File "/usr/bin/duplicity", line 1251, in <module>
    with_tempdir(main)
  File "/usr/bin/duplicity", line 1244, in with_tempdir
    fn()
  File "/usr/bin/duplicity", line 1200, in main
    verify(col_stats)
  File "/usr/bin/duplicity", line 653, in verify
    collated =
diffdir.collate2iters(restore_get_patched_rop_iter(col_stats),
  File "/usr/bin/duplicity", line 560, in restore_get_patched_rop_iter
    backup_chain = col_stats.get_backup_chain_at_time(time)
  File "/usr/lib/python2.5/site-packages/duplicity/collections.py", line
934, in get_backup_chain_at_time
    raise CollectionsError("No backup chains found")
CollectionsError: No backup chains found

ERROR:duplicity:Traceback (most recent call last):
  File "/usr/bin/duplicity", line 1251, in <module>
    with_tempdir(main)
  File "/usr/bin/duplicity", line 1244, in with_tempdir
    fn()
  File "/usr/bin/duplicity", line 1200, in main
    verify(col_stats)
  File "/usr/bin/duplicity", line 653, in verify
    collated =
diffdir.collate2iters(restore_get_patched_rop_iter(col_stats),
  File "/usr/bin/duplicity", line 560, in restore_get_patched_rop_iter
    backup_chain = col_stats.get_backup_chain_at_time(time)
  File "/usr/lib/python2.5/site-packages/duplicity/collections.py", line
934, in get_backup_chain_at_time
    raise CollectionsError("No backup chains found")
CollectionsError: No backup chains found

My local cache is deleted at the beginning of the process and duplicity is
then unable to verify the backup.

Repopulating the cache manually (decrypting and gzip'ing the manifest and
sigtar as appropriate) and then rerunning the verify operation deletes the
cache again.

Carrying out the same steps (full backup then verify) using a local file://
backend rather than S3 works okay.
```
Original tags: cache local s3

Story points: 38.0