par2+file does not work on mounted volumes on OSX: Warning, found signatures but no corresponding backup files (lp:#1609966)

[Original report](https://bugs.launchpad.net/bugs/1609966) created by **Lars Volker (lv)**
```
Duplicity 0.7.06 (installed with homebrew)
Python 2.7.10
OSX 10.11.6
Target filesystem: par2+file to /dev/disk5s1 on /Volumes/DupTest (hfs,
local, nodev, nosuid, journaled, noowners, mounted by lv)

A full backup works with file:// to both the root disk (hfs, local,
journaled) and to /Volumes/DupTest. With par2+file:// it works to the root
disk, but *not* to /Volumes/DupTest.

It seems to be the specific combination of par2, file and mounted Volume.
It doesn't matter where the data resides that I try to backup, only the
target filesystem seems to make a difference.

Here is the log of a failed run:

✔ ~/tmp/duptest$ find test
test
test/1
✔ ~/tmp/duptest$ duplicity full -v9 test
par2+file:///Volumes/DupTest/backup
Using archive dir:
/Users/lv/.cache/duplicity/5a09dcb3de2ecb2901ecf1c600926ad6
Using backup name: 5a09dcb3de2ecb2901ecf1c600926ad6
Import of duplicity.backends.azurebackend Succeeded
Import of duplicity.backends.b2backend Succeeded
Import of duplicity.backends.botobackend Succeeded
Import of duplicity.backends.cfbackend Succeeded
Import of duplicity.backends.copycombackend Succeeded
Import of duplicity.backends.dpbxbackend Succeeded
Import of duplicity.backends.gdocsbackend Succeeded
Import of duplicity.backends.giobackend Succeeded
Import of duplicity.backends.hsibackend Succeeded
Import of duplicity.backends.hubicbackend Succeeded
Import of duplicity.backends.imapbackend Succeeded
Import of duplicity.backends.lftpbackend Succeeded
Import of duplicity.backends.localbackend Succeeded
Import of duplicity.backends.megabackend Succeeded
Import of duplicity.backends.multibackend Succeeded
Import of duplicity.backends.ncftpbackend Succeeded
Import of duplicity.backends.onedrivebackend Succeeded
Import of duplicity.backends.par2backend Succeeded
Import of duplicity.backends.pydrivebackend Succeeded
Import of duplicity.backends.rsyncbackend Succeeded
Import of duplicity.backends.ssh_paramiko_backend Succeeded
Import of duplicity.backends.ssh_pexpect_backend Succeeded
Import of duplicity.backends.swiftbackend Succeeded
Import of duplicity.backends.sxbackend Succeeded
Import of duplicity.backends.tahoebackend Succeeded
Import of duplicity.backends.webdavbackend Succeeded
Main action: full
================================================================================
duplicity 0.7.06 (December 07, 2015)
Args: /usr/local/Cellar/duplicity/0.7.06_1/libexec/bin/duplicity full -v9
test par2+file:///Volumes/DupTest/backup
Darwin MacBook-Pro.local 15.6.0 Darwin Kernel Version 15.6.0: Thu Jun 23
18:25:34 PDT 2016; root:xnu-3248.60.10~1/RELEASE_X86_64 x86_64 i386
/usr/bin/python 2.7.10 (default, Oct 23 2015, 19:19:21)
[GCC 4.2.1 Compatible Apple LLVM 7.0.0 (clang-700.0.59.5)]
================================================================================
Using temporary directory
/var/folders/pz/kczql79d54l1fbb9b9k11lch0000gp/T/duplicity-n2Wodz-tempdir
Registering (mkstemp) temporary file
/var/folders/pz/kczql79d54l1fbb9b9k11lch0000gp/T/duplicity-n2Wodz-
tempdir/mkstemp-gPCEWY-1
Temp has 65250316288 available, backup will use approx 34078720.
Synchronizing remote metadata to local cache...
Deleting local
/Users/lv/.cache/duplicity/5a09dcb3de2ecb2901ecf1c600926ad6/duplicity-full-
signatures.20160804T185221Z.sigtar.gpg (not authoritative at backend).
Deleting local
/Users/lv/.cache/duplicity/5a09dcb3de2ecb2901ecf1c600926ad6/duplicity-
full.20160804T185221Z.manifest (not authoritative at backend).
0 files exist on backend
2 files exist in cache
Extracting backup chains from list of files: []
Last full backup date: none
Collection Status
-----------------
Connecting with backend: BackendWrapper
Archive dir: /Users/lv/.cache/duplicity/5a09dcb3de2ecb2901ecf1c600926ad6

Found 0 secondary backup chains.
No backup chains with active signatures found
No orphaned or incomplete backup sets found.
PASSPHRASE variable not set, asking user.
GnuPG passphrase:
PASSPHRASE variable not set, asking user.
Retype passphrase to confirm:
Using temporary directory
/Users/lv/.cache/duplicity/5a09dcb3de2ecb2901ecf1c600926ad6/duplicity-
fuEWCa-tempdir
Registering (mktemp) temporary file
/Users/lv/.cache/duplicity/5a09dcb3de2ecb2901ecf1c600926ad6/duplicity-
fuEWCa-tempdir/mktemp-AeSjBd-1
Using temporary directory
/Users/lv/.cache/duplicity/5a09dcb3de2ecb2901ecf1c600926ad6/duplicity-
_cSMKl-tempdir
Registering (mktemp) temporary file
/Users/lv/.cache/duplicity/5a09dcb3de2ecb2901ecf1c600926ad6/duplicity-
_cSMKl-tempdir/mktemp-7lXd53-1
AsyncScheduler: instantiating at concurrency 0
Registering (mktemp) temporary file
/var/folders/pz/kczql79d54l1fbb9b9k11lch0000gp/T/duplicity-n2Wodz-
tempdir/mktemp-hL4hWy-2
Selecting test
Comparing . and None
Getting delta of (. dir) and None
A .
Selection: examining path test/1
Selection:     + no selection functions found. Including
Selecting test/1
Comparing 1 and None
Getting delta of (1 reg) and None
A 1
Removing still remembered temporary file
/Users/lv/.cache/duplicity/5a09dcb3de2ecb2901ecf1c600926ad6/duplicity-
fuEWCa-tempdir/mktemp-AeSjBd-1
Removing still remembered temporary file
/Users/lv/.cache/duplicity/5a09dcb3de2ecb2901ecf1c600926ad6/duplicity-
_cSMKl-tempdir/mktemp-7lXd53-1
AsyncScheduler: running task synchronously (asynchronicity disabled)
Writing duplicity-full.20160804T202254Z.vol1.difftar.gpg
Making directory
/var/folders/pz/kczql79d54l1fbb9b9k11lch0000gp/T/duplicity-n2Wodz-
tempdir/duplicity_temp.1
Create Par2 recovery files
Deleting /var/folders/pz/kczql79d54l1fbb9b9k11lch0000gp/T/duplicity-n2Wodz-
tempdir/duplicity_temp.1/duplicity-full.20160804T202254Z.vol1.difftar.gpg
Deleting tree
/var/folders/pz/kczql79d54l1fbb9b9k11lch0000gp/T/duplicity-n2Wodz-
tempdir/duplicity_temp.1
Selecting
/var/folders/pz/kczql79d54l1fbb9b9k11lch0000gp/T/duplicity-n2Wodz-
tempdir/duplicity_temp.1
Selection: examining path
/var/folders/pz/kczql79d54l1fbb9b9k11lch0000gp/T/duplicity-n2Wodz-
tempdir/duplicity_temp.1/duplicity-
full.20160804T202254Z.vol1.difftar.gpg.par2
Selection:     + no selection functions found. Including
Selecting
/var/folders/pz/kczql79d54l1fbb9b9k11lch0000gp/T/duplicity-n2Wodz-
tempdir/duplicity_temp.1/duplicity-
full.20160804T202254Z.vol1.difftar.gpg.par2
Deleting /var/folders/pz/kczql79d54l1fbb9b9k11lch0000gp/T/duplicity-n2Wodz-
tempdir/duplicity_temp.1/duplicity-
full.20160804T202254Z.vol1.difftar.gpg.par2
Selection: examining path
/var/folders/pz/kczql79d54l1fbb9b9k11lch0000gp/T/duplicity-n2Wodz-
tempdir/duplicity_temp.1/duplicity-
full.20160804T202254Z.vol1.difftar.gpg.vol0+6.par2
Selection:     + no selection functions found. Including
Selecting
/var/folders/pz/kczql79d54l1fbb9b9k11lch0000gp/T/duplicity-n2Wodz-
tempdir/duplicity_temp.1/duplicity-
full.20160804T202254Z.vol1.difftar.gpg.vol0+6.par2
Deleting /var/folders/pz/kczql79d54l1fbb9b9k11lch0000gp/T/duplicity-n2Wodz-
tempdir/duplicity_temp.1/duplicity-
full.20160804T202254Z.vol1.difftar.gpg.vol0+6.par2
Deleting /var/folders/pz/kczql79d54l1fbb9b9k11lch0000gp/T/duplicity-n2Wodz-
tempdir/duplicity_temp.1
Deleting /var/folders/pz/kczql79d54l1fbb9b9k11lch0000gp/T/duplicity-n2Wodz-
tempdir/mktemp-hL4hWy-2
Forgetting temporary file
/var/folders/pz/kczql79d54l1fbb9b9k11lch0000gp/T/duplicity-n2Wodz-
tempdir/mktemp-hL4hWy-2
AsyncScheduler: task completed successfully
Processed volume 1
Making directory
/Users/lv/.cache/duplicity/5a09dcb3de2ecb2901ecf1c600926ad6/duplicity_temp.1
Create Par2 recovery files
Deleting
/Users/lv/.cache/duplicity/5a09dcb3de2ecb2901ecf1c600926ad6/duplicity_temp.1/duplicity-
full-signatures.20160804T202254Z.sigtar.gpg
Deleting tree
/Users/lv/.cache/duplicity/5a09dcb3de2ecb2901ecf1c600926ad6/duplicity_temp.1
Selecting
/Users/lv/.cache/duplicity/5a09dcb3de2ecb2901ecf1c600926ad6/duplicity_temp.1
Selection: examining path
/Users/lv/.cache/duplicity/5a09dcb3de2ecb2901ecf1c600926ad6/duplicity_temp.1/duplicity-
full-signatures.20160804T202254Z.sigtar.gpg.par2
Selection:     + no selection functions found. Including
Selecting
/Users/lv/.cache/duplicity/5a09dcb3de2ecb2901ecf1c600926ad6/duplicity_temp.1/duplicity-
full-signatures.20160804T202254Z.sigtar.gpg.par2
Deleting
/Users/lv/.cache/duplicity/5a09dcb3de2ecb2901ecf1c600926ad6/duplicity_temp.1/duplicity-
full-signatures.20160804T202254Z.sigtar.gpg.par2
Selection: examining path
/Users/lv/.cache/duplicity/5a09dcb3de2ecb2901ecf1c600926ad6/duplicity_temp.1/duplicity-
full-signatures.20160804T202254Z.sigtar.gpg.vol0+7.par2
Selection:     + no selection functions found. Including
Selecting
/Users/lv/.cache/duplicity/5a09dcb3de2ecb2901ecf1c600926ad6/duplicity_temp.1/duplicity-
full-signatures.20160804T202254Z.sigtar.gpg.vol0+7.par2
Deleting
/Users/lv/.cache/duplicity/5a09dcb3de2ecb2901ecf1c600926ad6/duplicity_temp.1/duplicity-
full-signatures.20160804T202254Z.sigtar.gpg.vol0+7.par2
Deleting
/Users/lv/.cache/duplicity/5a09dcb3de2ecb2901ecf1c600926ad6/duplicity_temp.1
Making directory
/Users/lv/.cache/duplicity/5a09dcb3de2ecb2901ecf1c600926ad6/duplicity_temp.1
Create Par2 recovery files
Deleting
/Users/lv/.cache/duplicity/5a09dcb3de2ecb2901ecf1c600926ad6/duplicity_temp.1/duplicity-
full.20160804T202254Z.manifest.gpg
Deleting tree
/Users/lv/.cache/duplicity/5a09dcb3de2ecb2901ecf1c600926ad6/duplicity_temp.1
Selecting
/Users/lv/.cache/duplicity/5a09dcb3de2ecb2901ecf1c600926ad6/duplicity_temp.1
Selection: examining path
/Users/lv/.cache/duplicity/5a09dcb3de2ecb2901ecf1c600926ad6/duplicity_temp.1/duplicity-
full.20160804T202254Z.manifest.gpg.par2
Selection:     + no selection functions found. Including
Selecting
/Users/lv/.cache/duplicity/5a09dcb3de2ecb2901ecf1c600926ad6/duplicity_temp.1/duplicity-
full.20160804T202254Z.manifest.gpg.par2
Deleting
/Users/lv/.cache/duplicity/5a09dcb3de2ecb2901ecf1c600926ad6/duplicity_temp.1/duplicity-
full.20160804T202254Z.manifest.gpg.par2
Selection: examining path
/Users/lv/.cache/duplicity/5a09dcb3de2ecb2901ecf1c600926ad6/duplicity_temp.1/duplicity-
full.20160804T202254Z.manifest.gpg.vol0+5.par2
Selection:     + no selection functions found. Including
Selecting
/Users/lv/.cache/duplicity/5a09dcb3de2ecb2901ecf1c600926ad6/duplicity_temp.1/duplicity-
full.20160804T202254Z.manifest.gpg.vol0+5.par2
Deleting
/Users/lv/.cache/duplicity/5a09dcb3de2ecb2901ecf1c600926ad6/duplicity_temp.1/duplicity-
full.20160804T202254Z.manifest.gpg.vol0+5.par2
Deleting
/Users/lv/.cache/duplicity/5a09dcb3de2ecb2901ecf1c600926ad6/duplicity_temp.1
1 file exists on backend
6 files exist in cache
Extracting backup chains from list of files: [u'duplicity-
full.20160804T202254Z.vol1.difftar.gpg']
File duplicity-full.20160804T202254Z.vol1.difftar.gpg is not part of a
known set; creating new set
Warning, found incomplete backup sets, probably left from aborted session
--------------[ Backup Statistics ]--------------
StartTime 1470342175.75 (Thu Aug  4 21:22:55 2016)
EndTime 1470342175.76 (Thu Aug  4 21:22:55 2016)
ElapsedTime 0.01 (0.01 seconds)
SourceFiles 2
SourceFileSize 107 (107 bytes)
NewFiles 2
NewFileSize 107 (107 bytes)
DeletedFiles 0
ChangedFiles 0
ChangedFileSize 0 (0 bytes)
ChangedDeltaSize 0 (0 bytes)
DeltaEntries 2
RawDeltaSize 5 (5 bytes)
TotalDestinationSizeChange 227 (227 bytes)
Errors 0
-------------------------------------------------

Releasing lockfile
/Users/lv/.cache/duplicity/5a09dcb3de2ecb2901ecf1c600926ad6/lockfile.lock
Removing still remembered temporary file
/var/folders/pz/kczql79d54l1fbb9b9k11lch0000gp/T/duplicity-n2Wodz-
tempdir/mkstemp-gPCEWY-1
```

Story points: 10.0