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keep track of portal-related resources separately from transaction-related resources. This allows cursors to work in a somewhat sane fashion with nested transactions. For now, cursor behavior is non-subtransactional, that is a cursor's state does not roll back if you abort a subtransaction that fetched from the cursor. We might want to change that later.
$PostgreSQL: pgsql/src/backend/utils/resowner/README,v 1.1 2004/07/17 03:30:10 tgl Exp $ 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. 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 Currently, ResourceOwners contain direct support for recording ownership of buffer pins, lmgr locks, and catcache and relcache 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.