RefIDImpl.Mesa
Copyright © 1985 by Xerox Corporation. All rights reserved.
Doug Terry, October 22, 1985 9:56:18 am PDT
Swinehart, October 25, 1985 6:42:05 pm PDT
This package creates IDs for REFs and maps between the two. An ID is simply a laundered REF, which can be passed across an RPC connection. When a REF is "sealed" into an ID, this package holds onto the REF so that it is not garbage collected. IDs must be explicitly released to indicate that interest in the REF no longer exists.
DIRECTORY
Atom USING [ GetProp, PutProp ],
CardTable USING [Create, Delete, Fetch, Insert, Ref, SeqIndex],
RefID;
RefIDImpl: CEDAR PROGRAM
IMPORTS Atom, CardTable
EXPORTS RefID
= BEGIN OPEN RefID;
TableSize: CardTable.SeqIndex = 2039;
2039 is a good candidate because it is prime and is large enough that collisions should be rare.
refIDTable: CardTable.Ref;
Seal: PUBLIC PROC[r: REF] RETURNS [ID] = TRUSTED {
Returns an ID for the supplied REF and assures that the REF survives. When the ID is no longer needed, it must be explicitly deallocated using Release.
id: ID = LOOPHOLE[r];
IF r=NIL THEN RETURN[nullID];
[] ← CardTable.Insert[refIDTable, id, r];
RETURN[id];
};
Reseal: PROC[r: REF] RETURNS [ID] = INLINE { RETURN[LOOPHOLE[r]]; };
A semi-safe Seal that can be used if one obtained r from Unseal, or knows it was obtained that way. The next Unseal will check the ID's validity again.
Unseal: PUBLIC PROC[id: ID] RETURNS [REF] = {
Returns a REF for the supplied ID. If the ID is not currently valid, the operation returns NIL.
RETURN[CardTable.Fetch[refIDTable, id].val];
};
Release: PUBLIC PROC[id: ID] RETURNS[existed: BOOL] = {
Removes the association between the ID and the target REF, and reduces the reference count to the target by 1.
RETURN[CardTable.Delete[refIDTable, id]];
};
There should be only one refIDTable per machine, whether or not the package is run more than once.
refIDTable ← NARROW[Atom.GetProp[$RefIDTable, $refIDTable]];
IF refIDTable = NIL THEN {
refIDTable ← CardTable.Create[TableSize];
Atom.PutProp[$RefIDTable, $refIDTable, refIDTable];
};
END.