bool ConflictsWithReader(XStatus me, ICollection readers)
{
foreach (XStatus reader in readers)
{
if (reader.State == XState.ACTIVE && reader != me)
{
return(true);
}
}
return(false);
}
/// <summary>
/// Open object with intention to modify.
/// </summary>
/// <returns>Private version of object</returns>
public ICloneable OpenWrite()
{
XStatus me = Xaction.XStatus; // my transaction
IContentionManager manager = Xaction.Manager; // my manager
// allocate successor
Locator newLocator = new Locator();
newLocator.writer = me;
ICloneable oldVersion = null;
ICloneable newVersion = null;
while (true)
{
retry:
// read locator
Locator oldLocator = (Locator)this.start;
XStatus writer = oldLocator.writer;
switch (writer.State)
{
case XState.ACTIVE:
case XState.WAITING:
// abort or wait?
manager.ResolveConflict(me, writer);
goto retry; // try again
case XState.COMMITTED:
oldVersion = newLocator.oldObject = oldLocator.newObject;
break;
case XState.ABORTED:
oldVersion = newLocator.oldObject = oldLocator.oldObject;
break;
default:
Panic.SystemError("Unknown transaction state: {0}", me.State);
break; // not reached
}
switch (me.State)
{
case XState.ABORTED:
throw new AbortedException();
case XState.COMMITTED:
return(oldVersion);
case XState.ACTIVE:
// check for read conflicts
if (ConflictsWithReader(me, oldLocator.readers))
{
manager.ResolveConflict(me, oldLocator.readers);
goto retry;
}
// no conflict
newVersion = newLocator.newObject = (ICloneable)oldVersion.Clone();
// try to install
if ((Locator)(Interlocked.CompareExchange(
ref start,
newLocator,
oldLocator)) == oldLocator)
{
#if DEBUG
Xaction.myMisses++;
#endif
Xaction.AddToCache(readCache, newVersion);
Xaction.AddToCache(writeCache, newVersion);
return(newVersion);
}
break;
default:
Panic.SystemError("Unknown transaction state: {0}", me.State);
break; // not reached
}
}
}
/// <summary>
/// If object hasn't changed since snapshot, upgrade to read access.
/// </summary>
/// <returns>Shared version of object</returns>
public bool Upgrade(ICloneable snapshot)
{
XStatus me = Xaction.XStatus; // my transaction
IContentionManager manager = Xaction.Manager; // my manager
// allocate successor
Locator newLocator = new Locator();
newLocator.writer = XStatus.COMMITTED;
while (true)
{
retry:
// read locator
Locator oldLocator = (Locator)this.start;
XStatus writer = oldLocator.writer;
switch (writer.State)
{
case XState.ACTIVE:
// abort or wait?
manager.ResolveConflict(me, writer);
goto retry; // try again
case XState.COMMITTED:
if (snapshot != oldLocator.newObject)
{
return(false);
}
break;
case XState.ABORTED:
if (snapshot != oldLocator.oldObject)
{
return(false);
}
break;
default:
Panic.SystemError("Unknown transaction state: {0}", me.State);
break; // not reached
}
switch (me.State)
{
case XState.ABORTED:
throw new AbortedException();
case XState.ACTIVE:
newLocator.newObject = snapshot;
// copy live readers into new locator
newLocator.readers.Clear();
newLocator.readers.Add(me);
foreach (XStatus t in oldLocator.readers)
{
if (t.State == XState.ACTIVE)
{
newLocator.readers.Add(t);
}
}
if (Interlocked.CompareExchange(
ref start,
newLocator,
oldLocator) == oldLocator)
{
#if DEBUG
Xaction.myMisses++;
#endif
Xaction.AddToCache(readCache, snapshot);
return(true);
}
break;
default:
Panic.SystemError("Unknown transaction state: {0}", me.State);
break; // not reached
}
}
}
