src/backend/utils/resowner/README

Notes About Resource Owners
===========================

ResourceOwner objects are a concept invented to simplify management of
query-related resources, such as buffer pins and table locks.  These
resources need to be tracked in a reliable way to ensure that they will
be released at query end, even if the query fails due to an error.
Rather than expecting the entire executor to have bulletproof data
structures, we localize the tracking of such resources into a single
module.

The design of the ResourceOwner API is modeled on our MemoryContext API,
which has proven very flexible and successful in preventing memory leaks.
In particular we allow ResourceOwners to have child ResourceOwner objects
so that there can be forests of the things; releasing a parent
ResourceOwner acts on all its direct and indirect children as well.

(It is tempting to consider unifying ResourceOwners and MemoryContexts
into a single object type, but their usage patterns are sufficiently
different that this is probably not really a helpful thing to do.)

We create a ResourceOwner for each transaction or subtransaction as
well as one for each Portal.  During execution of a Portal, the global
variable CurrentResourceOwner points to the Portal's ResourceOwner.
This causes operations such as ReadBuffer and LockAcquire to record
ownership of the acquired resources in that ResourceOwner object.

When a Portal is closed, any remaining resources (typically only locks)
become the responsibility of the current transaction.  This is represented
by making the Portal's ResourceOwner a child of the current transaction's
ResourceOwner.  resowner.c automatically transfers the resources to the
parent object when releasing the child.  Similarly, subtransaction
ResourceOwners are children of their immediate parent.

We need transaction-related ResourceOwners as well as Portal-related ones
because transactions may initiate operations that require resources (such
as query parsing) when no associated Portal exists yet.


API Overview
------------

The basic operations on a ResourceOwner are:

* create a ResourceOwner

* associate or deassociate some resource with a ResourceOwner

* release a ResourceOwner's assets (free all owned resources, but not the
  owner object itself)

* delete a ResourceOwner (including child owner objects); all resources
  must have been released beforehand

Locks are handled specially because in non-error situations a lock should
be held until end of transaction, even if it was originally taken by a
subtransaction or portal.  Therefore, the "release" operation on a child
ResourceOwner transfers lock ownership to the parent instead of actually
releasing the lock, if isCommit is true.

Currently, ResourceOwners contain direct support for recording ownership of
buffer pins, lmgr locks, and catcache, relcache, plancache, tupdesc, and
snapshot references.  Other objects can be associated with a ResourceOwner by
recording the address of the owning ResourceOwner in such an object.  There is
an API for other modules to get control during ResourceOwner release, so that
they can scan their own data structures to find the objects that need to be
deleted.

Whenever we are inside a transaction, the global variable
CurrentResourceOwner shows which resource owner should be assigned
ownership of acquired resources.  Note however that CurrentResourceOwner
is NULL when not inside any transaction (or when inside a failed
transaction).  In this case it is not valid to acquire query-lifespan
resources.

When unpinning a buffer or releasing a lock or cache reference,
CurrentResourceOwner must point to the same resource owner that was current
when the buffer, lock, or cache reference was acquired.  It would be possible
to relax this restriction given additional bookkeeping effort, but at present
there seems no need.

Code that transiently changes CurrentResourceOwner should generally use a
PG_TRY construct to ensure that the previous value of CurrentResourceOwner
is restored if control is lost during an error exit.
