Save the next one in temporary file, then move over to destination file.
This way when writing the file to disk fails (e.g. out of disk space),
user still have old file version intact.
This is somehow related to #757, but only first (easier) step. Actual
change of QubesAdminVm base class requires somehow more changes, for
example qvm-ls needs to know how to display this type of VM (none of
template, appvm, netvm).
Make this first step change now, because starting with R2Beta3 dom0 will
be stored in qubes.xml (for new backups purposes) so this rename would
be complicated later.
Template is saved as single archive of the whole VM directory. Preserve
backup directory structure regardless of its content - in this case it
means we need "." archive (with template directory content) placed in
"vm-tempates/<template-name>/" backup directory. This allows restore
process to select right files to restore regardless of VM type.
Also some major cleanups: Reduce some more code duplication
(verify_hmac, simplify backup_restore_prepare). Rename
backup_dir/backup_tmpdir variables to better match its purpose. Rename
backup_do_copy back to backup_do. Require QubesVm object (instead of VM
name) as appvm param.
This way backup process won't need more than 1GB for temporary files and
also will give more precise progress information. For now it looks like
the slowest element is qrexec, so without such limit, all the data would
be prepared (basically making second copy of it in dom0) while only
first few files would be transfered to the VM.
Also backup progress is calculated based on preparation thread, so when
it finishes there is some other time needed to flush all the data to the
VM. Limiting this amount makes progress somehow more accurate (but still
off by 1GB...).
We can't wait for tar next volume prompt using stderr.readline(),
because tar don't output EOL marker after this prompt. The other way
would be switching file descriptor to non-blocking mode and using lower
level os.read(), but this looks like more error-prone way (races...).
So change idea of handling such archives: after switching to next
archive volume, simply send '\n' to tar (which will receive when
needed). When getting "*.000" file, assume that previous archive was
over and wait for previous tar process. Then start the new one.
Also don't give explicit tape length, only turn multi-volume mode on. So
will correctly handle all multi-volume archives, regardless of its size.
This is mostly revert of "3d1b40f backups: keep file without path in
inner tar archive" in terms of archive format, but the code is more
robust than old one. Especially reuse already computed dir paths. Also
restore only requested files (based on selected VMs and its qubes.xml
data). Change the restore workflow to restore files first to temporary
directory, then move to final dirs. This approach:
- will be compatible with hashed vm name in the archive path
- is required to handle dom0 home backup (directory outside of
/var/lib/qubes)
- it should be also more defensive - make any changes in /var/lib/qubes
only after successful extraction of files and creating Qubes*Vm object
Second change in this commit is implement of dom0 home backup/restore.
As qubes.xml now contains data about dom0, we have information whether
it is included in the backup (before getting actual files).
Ensure that outer tar/encryptor gets all the data *and EOF* before
signalling inner tar to continue. Previously it could happen that inner
tar begins to write next data chunk, while qvm-backup still holds
previous data chunk open.
