RefID.Mesa
Copyright Ó 1985, 1992 by Xerox Corporation. All rights reserved.
Doug Terry, October 24, 1985 2:20:48 pm PDT
Swinehart, October 25, 1985 6:41:28 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. An ID must be explicitly released to indicate that interest in the REF no longer exists.
Fine point: Verifying that an ID corresponds to a valid REF is a potentially difficult problem. Normally, a REF will not be garbage collected since the package keeps a copy of REFs for all current IDs. However, if a machine restarts (i.e. is rollbacked or booted) a remote machine may have an ID that is no longer valid or valid for a REF to a different object. We do not worry about this problem since, in such a case, the restart must be known to the remote machine so that it can reestablished the RPC connection. For our purposes, the duration of an RPC connection is the time until one has to field RPC.CallFailure[unbound]. ANY IDS SHOULD BE TREATED AS VALID ONLY FOR THE DURATION OF A PARTICULAR RPC CONNECTION.
~
BEGIN
ID:
TYPE ~
LONG
CARDINAL;
An object's ID is a sanitized REF, which can be safely stored in other address spaces.
nullID: ID = LOOPHOLE[NIL[REF], LONG CARDINAL];
All of the routines below are idempotent operations; that is, multiple calls to a given procedure for the same REF or ID are treated as a single call. Except Release: it returns TRUE only once for a given argument.
Seal:
PROC[r:
REF]
RETURNS [ID];
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.
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:
PROC[id: ID]
RETURNS [
REF];
Returns a REF for the supplied ID. If the ID is not currently valid, the operation returns NIL. This operation should only be performed on the machine that sealed the particular ID!
Release:
PROC[id:
ID]
RETURNS [existed:
BOOL];
Removes the association between the ID and the target REF (thereby reducing the reference count to the target by 1). Returns TRUE if the ID was found.