Intro
-----

ZODB maintains pool of opened-to-DB connections. For each request Zope
opens 1 connection and, after request handling is done, returns the
connection back to ZODB pool (via Connection.close()). The same
connection will be opened again for handling some future next request at
some future time. This next open can happen in different-from-first
request worker thread.

TransactionManager  (as accessed by transaction.{get,commit,abort,...})
is thread-local, that is e.g. transaction.get() returns different
transaction for threads T1 and T2.

When _ZBigFileH hooks into txn_manager to get a chance to run its
.beforeCompletion() when transaction.commit() is run, it hooks into
_current_ _thread_ transaction manager.

Without unhooking on connection close, and circumstances where
connection migrates to different thread this can lead to
dissynchronization between ZBigFileH managing fileh pages and Connection
with ZODB objects. And even to data corruption, e.g.

    T1              T2

    open
    zarray[0] = 11
    commit
    close

                    open                # opens connection as closed in T1
    open
                    zarray[0] = 21
    commit
                    abort

    close           close

Here zarray[0]=21 _will_ be committed by T1 as part of T1 transaction -
because when T1 does commit .beforeCompletion() for zarray is invoked,
sees there is dirty data and propagate changes to zodb objects in
connection for T2, joins connection for T2 into txn for T1, and then txn
for t1 when doing two-phase-commit stores modified objects to DB ->
oops.

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

To prevent such dissynchronization _ZBigFileH needs to be a DataManager
which works in sync with the connection it was initially created under -
on connection close, unregister from transaction_manager, and on
connection open, register to transaction manager in current, possibly
different, thread context. Then there won't be incorrect
beforeCompletion() notification and corruption.

This issue, besides possible data corruption, was probably also exposing
itself via following ways we've seen in production (everywhere
connection was migrated from T1 to T2):

1. Exception ZODB.POSException.ConnectionStateError:
        ConnectionStateError('Cannot close a connection joined to a transaction',)
        in <bound method Cleanup.__del__ of <App.ZApplication.Cleanup instance at 0x7f10f4bab050>> ignored

     T1          T2

                 modify zarray
                 commit/abort    # does not join zarray to T2.txn,
                                 # because .beforeCompletion() is
                                 # registered in T1.txn_manager

     commit                      # T1 invokes .beforeCompletion()
     ...                         # beforeCompletion() joins ZBigFileH and zarray._p_jar (=T2.conn) to T1.txn
     ...                         # commit is going on in progress
     ...
     ...         close           # T2 thinks request handling is done and
     ...                         # and closes connection. But T2.conn is
     ...                         # still joined to T1.txn

2. Traceback (most recent call last):
     File ".../wendelin/bigfile/file_zodb.py", line 121, in storeblk
       def storeblk(self, blk, buf):   return self.zself.storeblk(blk, buf)
     File ".../wendelin/bigfile/file_zodb.py", line 220, in storeblk
       zblk._v_blkdata = bytes(buf)    # FIXME does memcpy
     File ".../ZODB/Connection.py", line 857, in setstate
       raise ConnectionStateError(msg)
   ZODB.POSException.ConnectionStateError: Shouldn't load state for 0x1f23a5 when the connection is closed

   Similar to "1", but close in T2 happens sooner, so that when T1 does
   the commit and tries to store object to database, Connection refuses to
   do the store:

     T1          T2

                 modify zarray
                 commit/abort

     commit
     ...         close
     ...
     ...
     . obj.store()
     ...
     ...

3. Traceback (most recent call last):
     File ".../wendelin/bigfile/file_zodb.py", line 121, in storeblk
       def storeblk(self, blk, buf):   return self.zself.storeblk(blk, buf)
     File ".../wendelin/bigfile/file_zodb.py", line 221, in storeblk
       zblk._p_changed = True          # if zblk was already in DB: _p_state -> CHANGED
     File ".../ZODB/Connection.py", line 979, in register
       self._register(obj)
     File ".../ZODB/Connection.py", line 989, in _register
       self.transaction_manager.get().join(self)
     File ".../transaction/_transaction.py", line 220, in join
       Status.ACTIVE, Status.DOOMED, self.status))
   ValueError: expected txn status 'Active' or 'Doomed', but it's 'Committing'

  ( storeblk() does zblk._p_changed -> Connection.register(zblk) ->
    txn.join() but txn is already committing

    IOW storeblk() was invoked with txn.state being already 'Committing' )

    T1          T2

                modify obj      # this way T2.conn joins T2.txn
                modify zarray

    commit                      # T1 invokes .beforeCompletion()
    ...                         # beforeCompletion() joins only _ZBigFileH to T1.txn
    ...                         # (because T2.conn is already marked as joined)
    ...
    ...         commit/abort    # T2 does commit/abort - this touches only T2.conn, not ZBigFileH
    ...                         # in particular T2.conn is now reset to be not joined
    ...
    . tpc_begin                 # actual active commit phase of T1 was somehow delayed a bit
    . tpc_commit                # when changes from RAM propagate to ZODB objects associated
    .  storeblk                 # connection (= T2.conn !) is notified again,
    .   zblk = ...              # wants to join txn for it thinks its transaction_manager,
                                # which when called from under T1 returns *T1* transaction manager for
                                # which T1.txn is already in state='Committing'

4. Empty transaction committed to NEO

   ( different from doing just transaction.commit() without changing
     any data - a connection was joined to txn, but set of modified
     object turned out to be empty )

   This is probably a race in Connection._register when both T1 and T2
   go to it at the same time:

   https://github.com/zopefoundation/ZODB/blob/3.10/src/ZODB/Connection.py#L988

   def _register(self, obj=None):
        if self._needs_to_join:
            self.transaction_manager.get().join(self)
            self._needs_to_join = False

    T1                          T2

                                modify zarray
    commit
    ...
    .beforeCompletion           modify obj
    . if T2.conn.needs_join      if T2.conn.needs_join      # race here
    .   T2.conn.join(T1.txn)       T2.conn.join(T2.txn)     # as a result T2.conn joins both T1.txn and T2.txn
    .
    commit finishes             # T2.conn registered-for-commit object list is now empty

                                commit
                                 tpc_begin
                                  storage.tpc_begin
                                 tpc_commit
                                  # no object stored, because for-commit-list is empty

/cc @jm, @klaus, @Tyagov, @vpelletier