Change in vdsm[master]: migration: make progress reporting reliable

[Code Review]

From Dan Kenigsberg :

Dan Kenigsberg has submitted this change and it was merged.

Change subject: migration: make progress reporting reliable
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migration: make progress reporting reliable

The rhbz#1414626 demonstrates some issues in our current progress
handling. Since we use the progress also for logic inside the
SourceThread class, this is a more serious issue than just showing
a value.

The first issue is that we update the progress percentage in two
places: getStat(), just before reporting it, and finishSuccesfully,
to make sure the progress is 100% after migration ended (rounding
errors or missing updates may case it to stay at 99%).
Unfortunately, getStat() can run concurrently to the migration code,
so nothing prevents it from overwrite the value finishSuccessfully set.

The second issue is more complex. First let's point out that
there could be legitimate cases for progress going
backwards. If a migration is stalling because the guess is making pages
dirty too fast, the amount of data_remaining could increase.

However, because both lack of guarantees about
monotonic increase and the race on write, we could end up with
progress = 99 after migration ended, and this could mistakenly trigger
another migration attempt - which indeed happen in rhbz#1414626.

The definitive fix for this harmful retry is provided in change
Ie45553bf3ec3db76e520d11a68c67b5b9664dc32

Change-Id: I2663382b6b1b2b58f8e4980a23ace36f4736930d
Bug-Url: https://bugzilla.redhat.com/1414626
Backport-To: 4.1
Backport-To: 4.0
Signed-off-by: Francesco Romani 
---
M lib/vdsm/virt/migration.py
M tests/vmmigration_test.py
2 files changed, 88 insertions(+), 7 deletions(-)

Approvals:
  Dan Kenigsberg: Looks good to me, approved
  Francesco Romani: Verified; Passed CI tests
  Martin Polednik: Looks good to me, but someone else must approve
  Milan Zamazal: Looks good to me, but someone else must approve