internal Bucket(BucketSet owningSet)
{
_timedOut = false;
_index = -1;
_size = 1024; // A possible design change here is to have this scale dynamically based on load.
_transactions = new InternalTransaction[_size];
_owningSet = owningSet;
}
internal TransactionTable()
{
// Create a timer that is initially disabled by specifing an Infinite time to the first interval
_timer = new Timer(new TimerCallback(ThreadTimer), null, Timeout.Infinite, _timerInterval);
// Note that the timer is disabled
_timerEnabled = false;
// Store the timer interval
_timerInterval = 1 << TransactionTable.timerInternalExponent;
// Ticks start off at zero.
_ticks = 0;
// The head of the list is long.MaxValue. It contains all of the transactions that for
// some reason or other don't have a timeout.
_headBucketSet = new BucketSet(this, long.MaxValue);
// Allocate the lock
_rwLock = new CheapUnfairReaderWriterLock();
}
// Process a timer event
private void ThreadTimer(Object state)
{
//
// Theory of operation.
//
// To timeout transactions we must walk down the list starting from the head
// until we find a link with an absolute timeout that is greater than our own.
// At that point everything further down in the list is elegable to be timed
// out. So simply remove that link in the list and walk down from that point
// timing out any transaction that is found.
//
// There could be a race between this callback being queued and the timer
// being disabled. If we get here when the timer is disabled, just return.
if (!_timerEnabled)
{
return;
}
// Increment the number of ticks
_ticks++;
_lastTimerTime = DateTime.UtcNow.Ticks;
//
// First find the starting point of transactions that should time out. Every transaction after
// that point will timeout so once we've found it then it is just a matter of traversing the
// structure.
//
BucketSet lastBucketSet = null;
BucketSet currentBucketSet = _headBucketSet; // The list always has a head.
// Acquire a writer lock before checking to see if we should disable the timer.
// Adding of transactions acquires a reader lock and might insert a new BucketSet.
// If that races with our check for a BucketSet existing, we may not timeout that
// transaction that is being added.
WeakReference nextWeakSet = null;
BucketSet nextBucketSet = null;
nextWeakSet = (WeakReference)currentBucketSet.nextSetWeak;
if (nextWeakSet != null)
{
nextBucketSet = (BucketSet)nextWeakSet.Target;
}
if (nextBucketSet == null)
{
_rwLock.EnterWriteLock();
try
{
// Access the nextBucketSet again in writer lock to account for any race before disabling the timeout.
nextWeakSet = (WeakReference)currentBucketSet.nextSetWeak;
if (nextWeakSet != null)
{
nextBucketSet = (BucketSet)nextWeakSet.Target;
}
if (nextBucketSet == null)
{
//
// Special case to allow for disabling the timer.
//
// If there are no transactions on the timeout list we can disable the
// timer.
if (!_timer.Change(Timeout.Infinite, Timeout.Infinite))
{
throw TransactionException.CreateInvalidOperationException(
TraceSourceType.TraceSourceLtm,
SR.UnexpectedTimerFailure,
null
);
}
_timerEnabled = false;
return;
}
}
finally
{
_rwLock.ExitWriteLock();
}
}
// Note it is slightly subtle that we always skip the head node. This is done
// on purpose because the head node contains transactions with essentially
// an infinite timeout.
do
{
do
{
nextWeakSet = (WeakReference)currentBucketSet.nextSetWeak;
if (nextWeakSet == null)
{
// Nothing more to do.
return;
}
nextBucketSet = (BucketSet)nextWeakSet.Target;
if (nextBucketSet == null)
{
// Again nothing more to do.
return;
}
lastBucketSet = currentBucketSet;
currentBucketSet = nextBucketSet;
}while (currentBucketSet.AbsoluteTimeout > _ticks);
//
// Pinch off the list at this point making sure it is still the correct set.
//
// Note: We may lose a race with an "Add" thread that is inserting a BucketSet in this location in
// the list. If that happens, this CompareExchange will not be performed and the returned abortingSetsWeak
// value will NOT equal nextWeakSet. But we check for that and if this condition occurs, this iteration of
// the timer thread will simply return, not timing out any transactions. When the next timer interval
// expires, the thread will walk the list again, find the appropriate BucketSet to pinch off, and
// then time out the transactions. This means that it is possible for a transaction to live a bit longer,
// but not much.
WeakReference abortingSetsWeak =
(WeakReference)Interlocked.CompareExchange(ref lastBucketSet.nextSetWeak, null, nextWeakSet);
if (abortingSetsWeak == nextWeakSet)
{
// Yea - now proceed to abort the transactions.
BucketSet abortingBucketSets = null;
do
{
if (abortingSetsWeak != null)
{
abortingBucketSets = (BucketSet)abortingSetsWeak.Target;
}
else
{
abortingBucketSets = null;
}
if (abortingBucketSets != null)
{
abortingBucketSets.TimeoutTransactions();
abortingSetsWeak = (WeakReference)abortingBucketSets.nextSetWeak;
}
}while (abortingBucketSets != null);
// That's all we needed to do.
break;
}
// We missed pulling the right transactions off. Loop back up and try again.
currentBucketSet = lastBucketSet;
}while (true);
}
private void AddIter(InternalTransaction txNew)
{
//
// Theory of operation.
//
// Note that the head bucket contains any transaction with essentially infinite
// timeout (long.MaxValue). The list is sorted in decending order. To add
// a node the code must walk down the list looking for a set of bucket that matches
// the absolute timeout value for the transaction. When it is found it passes
// the insert down to that set.
//
// An importent thing to note about the list is that forward links are all weak
// references and reverse links are all strong references. This allows the GC
// to clean up old links in the list so that they don't need to be removed manually.
// However if there is still a rooted strong reference to an old link in the
// chain that link won't fall off the list because there is a strong reference held
// forward.
//
BucketSet currentBucketSet = _headBucketSet;
while (currentBucketSet.AbsoluteTimeout != txNew.AbsoluteTimeout)
{
BucketSet lastBucketSet = null;
do
{
WeakReference nextSetWeak = (WeakReference)currentBucketSet.nextSetWeak;
BucketSet nextBucketSet = null;
if (nextSetWeak != null)
{
nextBucketSet = (BucketSet)nextSetWeak.Target;
}
if (nextBucketSet == null)
{
//
// We've reached the end of the list either because nextSetWeak was null or
// because its reference was collected. This code doesn't care. Make a new
// set, attempt to attach it and move on.
//
BucketSet newBucketSet = new BucketSet(this, txNew.AbsoluteTimeout);
WeakReference newSetWeak = new WeakReference(newBucketSet);
WeakReference oldNextSetWeak = (WeakReference)Interlocked.CompareExchange(
ref currentBucketSet.nextSetWeak, newSetWeak, nextSetWeak);
if (oldNextSetWeak == nextSetWeak)
{
// Ladies and Gentlemen we have a winner.
newBucketSet.prevSet = currentBucketSet;
}
// Note that at this point we don't update currentBucketSet. On the next loop
// iteration we should be able to pick up where we left off.
}
else
{
lastBucketSet = currentBucketSet;
currentBucketSet = nextBucketSet;
}
}while (currentBucketSet.AbsoluteTimeout > txNew.AbsoluteTimeout);
if (currentBucketSet.AbsoluteTimeout != txNew.AbsoluteTimeout)
{
//
// Getting to here means that we've found a slot in the list where this bucket set should go.
//
BucketSet newBucketSet = new BucketSet(this, txNew.AbsoluteTimeout);
WeakReference newSetWeak = new WeakReference(newBucketSet);
newBucketSet.nextSetWeak = lastBucketSet.nextSetWeak;
WeakReference oldNextSetWeak = (WeakReference)Interlocked.CompareExchange(
ref lastBucketSet.nextSetWeak, newSetWeak, newBucketSet.nextSetWeak);
if (oldNextSetWeak == newBucketSet.nextSetWeak)
{
// Ladies and Gentlemen we have a winner.
if (oldNextSetWeak != null)
{
BucketSet oldSet = (BucketSet)oldNextSetWeak.Target;
if (oldSet != null)
{
// prev references are just there to root things for the GC. If this object is
// gone we don't really care.
oldSet.prevSet = newBucketSet;
}
}
newBucketSet.prevSet = lastBucketSet;
}
// Special note - We are going to loop back to the BucketSet that preceeds the one we just tried
// to insert because we may have lost the race to insert our new BucketSet into the list to another
// "Add" thread. By looping back, we check again to see if the BucketSet we just created actually
// got added. If it did, we will exit out of the outer loop and add the transaction. But if we
// lost the race, we will again try to add a new BucketSet. In the latter case, the BucketSet
// we created during the first iteration will simply be Garbage Collected because there are no
// strong references to it since we never added the transaction to a bucket and the act of
// creating the second BucketSet with remove the backward reference that was created in the
// first trip thru the loop.
currentBucketSet = lastBucketSet;
lastBucketSet = null;
// The outer loop will iterate and pick up where we left off.
}
}
//
// Great we found a spot.
//
currentBucketSet.Add(txNew);
}