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[Matt Riedemann <mriedem.os@xxxxxxxxx>]

** Also affects: nova/pike
   Importance: Undecided
       Status: New

** Changed in: nova/pike
       Status: New => In Progress

** Changed in: nova/pike
   Importance: Undecided => High

** Changed in: nova/pike
     Assignee: (unassigned) => melanie witt (melwitt)

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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):
  Fix Released
Status in OpenStack Compute (nova) pike series:
  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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