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Re: TLC choose a new unique element everytime

The request to include the model in a TLC error report was for future reference.  There is no reason to examine your problem further.  RandomElement should be used only to obtain statistical information on a specification that has no errors.  When you represent nondeterminism the way you should, you will get the error trace that you expect.

On Friday, July 14, 2017 at 11:06:52 AM UTC-7, Shuhao Wu wrote:

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