Portals and nested transactions - Mailing list pgsql-hackers

I've been thinking about what to do with cursors in subtransactions.
The problem really includes both cursors (created with DECLARE CURSOR)
and portals (created with the V3-protocol Bind message) since they are
the same kind of animal internally, namely a Portal.  In previous
discussion I think everyone agreed that we would like the following
properties:

1. A Portal created within a successful (committed) subtransaction
remains open and usable by the parent transaction, as well as by subsequent
child subtransactions.

2. If a subtransaction uses (fetches from) a pre-existing Portal, the
Portal state change persists after subxact commit.

What was not totally settled was what to do on subtransaction abort:

Q1: Should Portals successfully created within the failed subxact
be closed?  Or should they remain open?

Q2: If the subxact changed the state of a pre-existing Portal, should
that state change roll back?  In particular, can a Close Portal
operation roll back?

Taking a "transactional" view means answering "yes" to both questions
(so that all portal state returns to what it was at subxact entry).
But there was also support for a "nontransactional" view in which both
questions are answered "no".

The discussion sort of trailed off there because we had no ideas how to
implement either.  I will now sketch some implementation ideas about how
to do the nontransactional way.  We could support the transactional
behavior as well, but not very efficiently (at least not in the first
cut).

An important limitation that I think we must make is that any error
occurring while a specific Portal is executing "kills" that Portal;
you cannot do anything further with it except close it, even if the
Portal would otherwise have survived the subtransaction abort caused
by the error.  The reason for this is that we can't be sure we have
consistent internal state for the Portal when an error occurred at a
random point.  (Example: a btree index scan could have released lock on
one buffer and gotten an error while trying to read the next page of the
index; it's not certain that the scan data structures accurately reflect
this intermediate state.)  Later on we might be able to relax this
restriction, but it will take a lot of care to decide which errors are
"safe".  So for the moment, an error during a FETCH (or protocol-level
Execute) leaves that Portal in a state where any subsequent fetch or
execute draws "ERROR: portal execution cannot be continued".


How to do it non-transactionally
--------------------------------

The key insight I had while thinking about this is that subtransactions
are the wrong units for managing ownership of resources used by queries
(buffers, locks, etc).  When portals can outlive subtransactions, those
resources really need to be thought of as belonging to the portals not
the subtransactions.

However I think we *also* need to allow subtransaction to own resources
--- at least locks.  We usually want to hold table locks until main
transaction end, and it would be bad to have to keep Portals around
just to remember some locks.  It would be better to reassign ownership
of the locks to the current transaction when a Portal is closed.

What I think we ought to do to support this is to invent the concept
of "ResourceOwner" objects, which will be very much like MemoryContexts
except that they represent held buffer pins, table locks, and anything
else that we decide needs to be managed in this fashion (rtree index scans
are one example).  In particular we'll let ResourceOwners have child
ResourceOwner objects so that there can be forests of the things, just
as with MemoryContexts.

There would be a CurrentResourceOwner global variable analogous to
CurrentMemoryContext, which would for instance tell PinBuffer which
ResourceOwner to affix ownership of the pin to.

(I am half tempted to unify ResourceOwners and MemoryContexts completely,
but that's probably overkill, since we have many short-lived
MemoryContexts that would never be appropriate owners of query-level
resources.  In particular I think CurrentResourceOwner would usually be
different from CurrentMemoryContext.)

Depending on the resource in question, we could let ResourceOwners point
to owned objects (for instance, I'm thinking of storing an array of Buffer
numbers in a ResourceOwner to represent buffer pins) or vice versa (for
instance, the best way to keep track of rtree indexscan ownership is
probably to store a pointer to the ResourceOwner object in the rtree
indexscan struct).

This infrastructure shouldn't be much work to create, since we have the
MemoryContext stuff available to serve as a model.  Once we have it,
we'll create a ResourceOwner for each transaction or subtransaction as
well as one for each Portal.  (We need one for a transaction because, for
example, query parsing requires buffer and lock access, and that happens
before we create a Portal to execute the query.)

The reason this solves our problems is that while executing a portal's
query, CurrentResourceOwner will point to the portal's ResourceOwner
not the current subtransaction's.  Therefore any buffers, locks, etc
acquired or released by the query are effectively owned by the portal
and not by the subtransaction.  Subtransaction abort would release only
resources associated with the subtransaction itself, not those associated
with the portals it has happened to touch.

A nice property of this solution is that we can get rid of much of the
subtransaction entry/exit overhead that exists in current CVS tip.
There's no particular reason for the buffer manager to save and restore
buffer pin counts, for example.

One of the reasons why the pre-existing code needed to check and zero
buffer pin counts at transaction abort is that it cannot assume that all
pins held on behalf of a query were properly released by query abort; that
would have required making global assumptions about all code everywhere
being careful to record held pins in places that query abort cleanup would
find out about them.  In the ResourceOwner paradigm, the equivalent
correctness guarantee needs only local correctness in a few routines:
for instance, PinBuffer has to be sure it cannot error out between
acquiring a buffer pin and recording the pin in CurrentResourceOwner.
(Thus, for instance, it should make sure there is room in the
ResourceOwner's buffer-number array *before* it grabs the pin.)
This should give us much the same level of confidence that we have now
for memory management: there are no permanent memory leaks when you use
palloc.


How to do it transactionally
----------------------------

With the above design, we can handle nontransactional Portals easily:
we just don't touch the state of (non-failed) Portals when we exit a
subtransaction.

It's easy to see how to handle Portal creation and deletion
transactionally; it's pretty much the same algorithm we use already
in other places such as OnCommit.  Just stamp each Portal with the
creating or would-be-deleting subxact's XID.

The hard part is rolling back a pre-existing Portal to its prior state
at subxact abort.  It seems completely infeasible to do this at a low
level --- we'd never find all the state involved, and if we could get it
all there would be too much to save/restore efficiently.

What I think we could do, though, is record the Portal's high-level state
as the number of rows fetched from it.  On abort, rewind the Portal and
then fetch that number of rows again (this is the same method used by
MOVE ABSOLUTE).  We could optimize things a little bit by not doing this
repositioning until and unless the Portal is actually used again.  Still,
it wouldn't be cheap...

Of course this only handles SELECT-query portals, not portals that contain
data-modification commands.  But the latter cannot be suspended partway
through anyhow, so there is no scenario where we need to recover to a
partly-executed state.  (Recall what I said before about not allowing
continuation of a portal that itself got an error.)


Comments?
        regards, tom lane