true nesting

[nbronson]

To properly support retry/orElse we must implement partial rollback, at least when rolling back with retry. There are several components

• child vs. top-level commit and rollback handlers
• dynamic lookup of the parent txn (see dynamic access to the current txn #3)
• capture of the partial read set during partial rollback for retry
• capture or release of locks whose handle is removed from the write buffer during partial rollback

It would also be nice to generate partial rollback on conflict when possible.

[nbronson]

Note that true nesting is also required to properly implement rollback without retry on exception.

[nbronson]

How will this affect TxnLocal?

[nbronson]

It seems that maybe we don't need any additional handlers. beforeCommit and afterCommit should always refer to globally visible effects, hence they need only report top-level commit. afterRollback should report when writes performed by the particular nesting level have been undone, which may be a partial rollback.

ReadResources from a child txn may be discarded on partial rollback.

WriteResources from a child txn may be discarded on partial rollback after their performRollback method has been called.

[nbronson]

Hmm. Maybe we should include enough hooks that they could implement TxnLocal. Alternately, we could let them override the required hooks in a TxnLocal instance?

[nbronson]

Either partial rollback and partial retry both remove the current invariant that a txn that is no longer Active can never again be Active, or a pending partial rollback should not be encoded as the transaction status.