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src/backend/utils/resowner/README
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Notes About Resource Owners
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===========================
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ResourceOwner objects are a concept invented to simplify management of
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query-related resources, such as buffer pins and table locks. These
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resources need to be tracked in a reliable way to ensure that they will
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be released at query end, even if the query fails due to an error.
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Rather than expecting the entire executor to have bulletproof data
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structures, we localize the tracking of such resources into a single
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The design of the ResourceOwner API is modeled on our MemoryContext API,
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which has proven very flexible and successful in preventing memory leaks.
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In particular we allow ResourceOwners to have child ResourceOwner objects
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so that there can be forests of the things; releasing a parent
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ResourceOwner acts on all its direct and indirect children as well.
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(It is tempting to consider unifying ResourceOwners and MemoryContexts
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into a single object type, but their usage patterns are sufficiently
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different that this is probably not really a helpful thing to do.)
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We create a ResourceOwner for each transaction or subtransaction as
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well as one for each Portal. During execution of a Portal, the global
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variable CurrentResourceOwner points to the Portal's ResourceOwner.
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This causes operations such as ReadBuffer and LockAcquire to record
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ownership of the acquired resources in that ResourceOwner object.
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When a Portal is closed, any remaining resources (typically only locks)
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become the responsibility of the current transaction. This is represented
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by making the Portal's ResourceOwner a child of the current transaction's
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ResourceOwner. resowner.c automatically transfers the resources to the
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parent object when releasing the child. Similarly, subtransaction
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ResourceOwners are children of their immediate parent.
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We need transaction-related ResourceOwners as well as Portal-related ones
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because transactions may initiate operations that require resources (such
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as query parsing) when no associated Portal exists yet.
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The basic operations on a ResourceOwner are:
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* create a ResourceOwner
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* associate or deassociate some resource with a ResourceOwner
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* release a ResourceOwner's assets (free all owned resources, but not the
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* delete a ResourceOwner (including child owner objects); all resources
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must have been released beforehand
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Locks are handled specially because in non-error situations a lock should
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be held until end of transaction, even if it was originally taken by a
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subtransaction or portal. Therefore, the "release" operation on a child
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ResourceOwner transfers lock ownership to the parent instead of actually
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releasing the lock, if isCommit is true.
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Currently, ResourceOwners contain direct support for recording ownership of
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buffer pins, lmgr locks, and catcache, relcache, plancache, tupdesc, and
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snapshot references. Other objects can be associated with a ResourceOwner by
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recording the address of the owning ResourceOwner in such an object. There is
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an API for other modules to get control during ResourceOwner release, so that
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they can scan their own data structures to find the objects that need to be
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Whenever we are inside a transaction, the global variable
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CurrentResourceOwner shows which resource owner should be assigned
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ownership of acquired resources. Note however that CurrentResourceOwner
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is NULL when not inside any transaction (or when inside a failed
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transaction). In this case it is not valid to acquire query-lifespan
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When unpinning a buffer or releasing a lock or cache reference,
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CurrentResourceOwner must point to the same resource owner that was current
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when the buffer, lock, or cache reference was acquired. It would be possible
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to relax this restriction given additional bookkeeping effort, but at present
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Code that transiently changes CurrentResourceOwner should generally use a
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PG_TRY construct to ensure that the previous value of CurrentResourceOwner
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is restored if control is lost during an error exit.