sksp2024-mcu/libucw/ucw/doc/trans.txt

Transactions and resource tracking
==================================

LibUCW is equipped with a general system for keeping track of resources
(allocated memory, open files, ...) and freeing them when requested to.

The resource tracker can be used either separately (in the form of explicitly
managed resource pools) or within a transactional layer, which offers
exceptions similar to those in higher-level languages. An exception
then rolls back the transaction, freeing all temporary resources allocated
within the transaction.

- <<respools,Resource pools>>
- <<trans,Transactions>>
- <<exc,Exceptions>>
- <<excnames,Exception names>>

Resource pools: ucw/resource.h [[respools]]
-------------------------------------------

A resource pool contains a stack of resources. When a new resource
is created, it is pushed onto the stack. When freeing the pool, the
resources are freed in the opposite order, which allows a resource
refer to data of previously created resources.

A resource can be also freed separately (which unlinks it from the pool),
or *detached* from the pool (which keeps the real resource, but forgets
its meta-data, so the resource is no longer tracked).

In many cases, a combination of both methods is needed: some resources
are marked as temporary, while some others are permanent. When the
an operation is completed successfully (and @rp_commit() is called),
all temporary resources are freed and the permanent ones detached.
When the operation fails, @rp_delete() deletes all resources. By default,
all resources are created as temporary. You can make a resource permanent
by calling @res_permanent(), or change the default in `resource->default_res_flags`.

For each thread, LibUCW remembers the currently active resource pool.
One pool can be used for at most one thread at a time. All functions
which create resources do so in the active pool. All other functions
operating on resources work on both active and in-active pools.

!!ucw/resource.h

Transactions: ucw/trans.h [[trans]]
-----------------------------------

Upon the resource pools, a transactional mechanism is built. A transaction
consists of a piece of code and a resource pool for temporary objects created
by the code. Whenever the transaction is running, this pool is set as current.
You are allowed to switch to a different pool, but please do so carefully.

When a transaction ends, the pool is destroyed and the previous active
pool is popped off the transaction stack. The fate of the resources
inside the pool depends on the operation used to end the transaction:

* *commit* -- permanent resources are detached from the pool, temporary
  resources are freed
* *rollback* -- all resources are freed
* *fold* -- instead of destroying the pool, it is added as a subpool
  to the parent transaction (which must exist)

A transaction is tied to a thread which has created it. A transaction
can create a sub-transaction, so every thread keeps a stack of running
transactions in its per-thread data. Calling @trans_init() is optional,
but @trans_cleanup() should be used before a thread exits in order to
free resources used by transaction system.

Each transaction also includes a memory pool, from which all temporary
structures (including all resources created by the transaction) are
allocated. Feel free to allocate your temporary data from this pool, too;
they will be freed when the transaction is committed or rolled back.
When the transaction ends with a fold, this pool gets included inside
the parent transaction's pool.

(More precisely, there is actually a shared transaction pool per thread
and the transaction logic uses @mp_push() and @mp_pop() to keep a stack
of per-transaction data.)

=== Exceptions [[exc]] ===

Transactions are commonly used together with exceptions (which are similar
to how exceptions work in other languages, but they differ in subtle details,
so please read carefully). When a failure condition of some kind is detected,
an exception is *raised* ("*thrown*" is also sometimes used). It involves
creating an exception object and jumping out of the transaction by
a `longjmp()`. The exception object (`struct exception`) contains an
identification of the error and possibly additional data.

Usually, creation of an transaction and handling of exceptions is done
using *helper macros* (it is not strictly necessary, but highly recommended):

	TRANS_TRY
	  {
	    // Code that runs inside the transaction.
	  }
	TRANS_CATCH(x)
	  {
	    // When an exception is raised, execution continues here.
	  }
	TRANS_END;

The code inside the transaction ends with an implicit @trans_commit().
If you want to end the transaction in a different way, you can do so,
but you need to use a `break` statement to skip the implicit commit.

The exception handling code gets a local variable `x` pointing to the
exception object. When the exception is handled (for example, an error
message is logged), @trans_caught() is called automatically, which rolls
back the transaction and frees all its resources. Again, you can use the
`break` statement to skip this.

Alternatively, when you are in a *nested transaction*, you can throw a different
exception or re-throw the original one. This raises an exception in the
context of the parent transaction. In this case, the child transaction is
not rolled back, but its pools are folded as sub-pools of the parent transaction
and kept until @trans_caught() is called finally.

When an exception is thrown *outside a transaction*, it is converted to
a plain @die().

*Memory management* and lifetime of various objects and pools deserve special
attention, as usually when non-local jumps are taking place. When an exception
is raised, the exception structure is allocated from the memory pool of the
current transaction. When the exception is propagated through the stack of
transactions, no transaction is ever rolled back -- all of them are folded
and their pools remain accessible until @trans_caught() is called at the end.
Therefore exceptions can carry pointers to the objects which have failed
without a risk of the object becoming invalid. However, you need to avoid
pointing to on-stack data like local variables of functions, because these
are of course destroyed during the `longjmp()`.

=== Functions and structures ===

!!ucw/trans.h

== Exception names [[excnames]] ==

Exception identifiers form a hierarchy. Each identifier consists of dot-separated
components with the most general component at the beginning.

All exceptions raised by LibUCW reside in the `ucw` subtree. So far, the
following exception types are defined:

`ucw.fb`:: <<fastbuf:fbexc,Fastbufs>>