Add system-wide subvol exclusion mechanism (#2223)

[phillxnet]

Adds an additional hard coded subvolume exclusion list, akin to the existing system drive only one. All subvolume matches, by name, to list members are ignored. List pre-populated by ".beeshome" and it's possible boot-to-snapshot counterpart "@/.beeshome".

Includes a unit test mocking a typical data drive including the target ".beeshome" subvolume to prove the exclusion mechanisms function.

Fixes #2223
Ready for review.

Testing

The additional unit test was added to prove this exclusion by subvol name:

Failure case:

Pre exclusion mechanism.

./bin/test --settings=test-settings -v 3 -p test_btrfs*

======================================================================
FAIL: test_shares_info_systemwide_exclusion_datapool (fs.tests.test_btrfs.BTRFSTests)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/opt/rockstor-dev/src/rockstor/fs/tests/test_btrfs.py", line 1290, in test_shares_info_systemwide_exclusion_datapool
    expected))
AssertionError: Failed data pool subvol exclusion:
returned {'netstat-config': '0/259', 'sftp-test': '0/257', 'rock-ons-root': '0/258', '.beeshome': '0/275'},
expected {'netstat-config': '0/259', 'sftp-test': '0/257', 'rock-ons-root': '0/258'}.


----------------------------------------------------------------------

Success case:

Post exclusion mechanism.

test_shares_info_systemwide_exclusion_datapool (fs.tests.test_btrfs.BTRFSTests) ... ok

All-test run was as expected:

./bin/test -v 2
...
Ran 213 tests in 35.364s

OK

[FroggyFlox]

@phillxnet ,

I manually created a .beeshome subvolume in my data_pool:
btrfs subvolume create /mnt2/data_pool/.besshome

Pre-PR:

• share is listed in Rockstor webUI

Post-PR:

• share is not listed in Rockstor webUI anymore
• logs show:

[29/Jan/2021 16:35:51] DEBUG [system.osi:466] --- Inheriting base_root_disk info ---
[29/Jan/2021 16:35:52] DEBUG [fs.btrfs:772] Skipping system-wide excluded subvol: name=(.beeshome).

I also could confirm that all 213 tests ran OK on my end.

Looks all good to me!

[phillxnet]

@FroggyFlox Re:

I manually created a .beeshome subvolume in my data_pool:

Yes, In my tests I used Rockstor to create this subvol and confirmed that it was identically created, i.e. as a top level subvol.
As per @ubenmacki issue text forum link and the associated btrfs subvol list output.

Thanks for the review. Most helpful and good to have a double check, especially on critical path code changes such as this. But it's essentially a very simple exclusion "if" so again I think it's a safe addition in our release candidate phase.