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[melanie witt <1722404@xxxxxxxxxxxxxxxxxx>]

Public bug reported:

I found this while working on a change to improve the usage of the
CellDatabases test fixture by defaulting to the 'cell0' database instead
of the 'cell1' database.

In the set_target_cell method, we synchronize access to the cached cell
database transaction context manager objects to prevent more than one 
thread from using a cell's transaction context manager at the same
time.

In scatter_gather_cells, we're calling target_cell in such a way that
the lock is acquired and released BEFORE the green thread actually
accesses the database in the spawned function. So multiple threads can 
access a cell's database transaction context manager and it's possible
for a database transaction to fail with the error:

  TypeError: Can't upgrade a READER transaction to a WRITER
             mid-transaction

because the in-scope transaction context might be in the middle of a
read when a concurrent green thread tries to do a write.

I saw this happen in the test:

nova.tests.unit.compute.test_compute.DisabledInstanceTypesTestCase.test_can_resize_to_visible_instance_type

where a parallel read of instances during a quota check accessed the
cell's database transaction context manager at the same time as an
instance.save() in the compute/api, causing the instance.save() to fail
with "TypeError: Can't upgrade a READER transaction to a WRITER mid-
transaction."

** Affects: nova
     Importance: Undecided
     Assignee: melanie witt (melwitt)
         Status: In Progress


** Tags: cells

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https://bugs.launchpad.net/bugs/1722404

Title:
  Database transactions can fail with "TypeError: Can't upgrade a READER
  transaction to a WRITER mid-transaction" because of
  scatter_gather_cells

Status in OpenStack Compute (nova):
  In Progress

Bug description:
  I found this while working on a change to improve the usage of the
  CellDatabases test fixture by defaulting to the 'cell0' database
  instead of the 'cell1' database.

  In the set_target_cell method, we synchronize access to the cached cell
  database transaction context manager objects to prevent more than one 
  thread from using a cell's transaction context manager at the same
  time.

  In scatter_gather_cells, we're calling target_cell in such a way that
  the lock is acquired and released BEFORE the green thread actually
  accesses the database in the spawned function. So multiple threads can 
  access a cell's database transaction context manager and it's possible
  for a database transaction to fail with the error:

    TypeError: Can't upgrade a READER transaction to a WRITER
               mid-transaction

  because the in-scope transaction context might be in the middle of a
  read when a concurrent green thread tries to do a write.

  I saw this happen in the test:

  nova.tests.unit.compute.test_compute.DisabledInstanceTypesTestCase.test_can_resize_to_visible_instance_type

  where a parallel read of instances during a quota check accessed the
  cell's database transaction context manager at the same time as an
  instance.save() in the compute/api, causing the instance.save() to
  fail with "TypeError: Can't upgrade a READER transaction to a WRITER
  mid-transaction."

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