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TLC choose a new unique element everytime


I've been trying to specify a simple problem where one process writes arbitrary data to a source datastore and a log and a second process follows and applies that log to a different datastore (similar to MySQL binlogs). The corresponding Pluscal implementation is shown at the end of the email. The implementation is supposed to show a violation of the DataOK invariance as lfread and lfwrite are two steps rather than one atomic steps.

The idea is that the DataUpdater process will write an entry into source. This is done via the `source[currentI] := RandomElement(PossibleRecords)`. In TLC, I have to specify PossibleRecords as a set of finite number of model values. This does not seem like it's "correct" so to speak, as it is possible for `RandomElement(PossibleRecords) = RandomElement(PossibleRecords)` if two of the same values happened to be picked.

Is there a better way to specify this for TLC? I feel this may be impossible as I can't give TLC an infinite set. *In which case: is there a better way to model this so TLC can work on it?

Furthermore, it seems like even if I use RandomElement, the operation of `source[currentI] := RandomElement(PossibleRecords)` causes TLC to somehow loses traces. If I run the below algorithm in TLC with Invariance of DataOK, I will get:

Invariant DataOK is violated.
Failed to recover the initial state from its fingerprint.
This is probably a TLC bug(1).

I see that people have reported this in the past (https://groups.google.com/forum/#!topic/tlaplus/mik45bcujtw). However, after reading the thread, I'm not quite sure if there is a workaround for this. Does anyone know?



EXTENDS TLC, Integers, Sequences


NoRecord == CHOOSE r : r \notin Records

PossibleRecords == Records \cup {NoRecord}

--algorithm foo {
    source = [k \in 1..N |-> NoRecord],
    target = [k \in 1..N |-> NoRecord],
    log = <<>>

  process (DataUpdater = "DataUpdater")
  variable currentI = 1;
    duloop: while (currentI <= N) {
              source[currentI] := RandomElement(PossibleRecords);
              log := Append(log, source[currentI]);
              currentI := currentI + 1;

  process (LogFollower = "LogFollower")
  variable currentR;
    lfloop: while (Len(target) < N) {
    lfread:   currentR := log[Len(log)];
    lfwrite:  target[Len(log)] := currentR;
} ***************************************************************************)

DataOK == (\A self \in ProcSet: pc[self] = "Done") => source = target