Context switching with TxnLocal

[Sciss]

hi,

which is the recommended way to execute a block with temporary change of a TxnLocal:

abstract sealed class Transition
case object NoTransition extends Transition
case class XFade( time: Float ) extends Transition
case class Ramp( time: Float ) extends Transition

val transition: new TxnLocal[ Transition ] {
override def initialValue( t: Txn ) = NoTransition
}

STM.atomic {
...
fade( 33 ) {
...
operationWhichCausesTxnToFail
...
}
...
}

where

def fade( time: Float )( thunk: => Unit )( implicit t: Txn ) {
val oldTrns = transition.swap( XFade( time ))
try {
thunk
}
finally {
if( t.status.mightCommit ) transition.set( oldTrns )
}
}

like this?

i assume i must add "if( t.status.mightCommit )" because otherwise if the inner operation fails, it might not be possible to call set any more. correct?

also i wonder what happens to transition if concurrent code is calling fade( ) { ... } ? will that be a problem, or will one fail immediately as transition has already done a write-action?

thanks, -sciss-

[Nathan Bronson]

A TxnLocal-s values are actually stored in the Txn, so there will be no
interference between concurrent calls to fade().

At the moment, TxnLocal.set doesn't check whether the current
transaction is doomed. This might need to change after I add partial
rollback, because TxnLocal values will be reverted if a nested
transaction fails. Regardless, it should always be okay to just revert
it directly

val old = aLocal.get
aLocal.set(newV)
try {
stuff
} finally {
aLocal.set(old)
}

When TxnLocal.set checks the transaction status in the future, it will
just regenerate the RollbackError if it finds a doomed transaction.

- Nathan

[Sciss]

ok, thanks for clarifying!