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

Assorted notes:

- 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.

- Every transaction keeps a resource pool containing resources, which
  have been created during the transaction. 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 -- all resources are detached from the pool
	* rollback -- all resources are freed
	* fold -- instead of destroying the pool, it is added as a subpool
	  to the parent transaction (which must exist)

- 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.
+
  (To be true, 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.)

- Transactions are usually 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. 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()`.

FIXME: Interaction between exceptions, pools and other libucw modules.