dejafu- A library for unit-testing concurrent programs.

Copyright(c) 2015--2017 Michael Walker
MaintainerMichael Walker <>
Safe HaskellNone




Internal types and functions for SCT via dynamic partial-order reduction. This module is NOT considered to form part of the public interface of this library.


Dynamic partial-order reduction

data DPOR Source #

DPOR execution is represented as a tree of states, characterised by the decisions that lead to that state.





Eq DPOR Source # 


(==) :: DPOR -> DPOR -> Bool #

(/=) :: DPOR -> DPOR -> Bool #

Show DPOR Source # 


showsPrec :: Int -> DPOR -> ShowS #

show :: DPOR -> String #

showList :: [DPOR] -> ShowS #

NFData DPOR Source # 


rnf :: DPOR -> () #

data BacktrackStep Source #

One step of the execution, including information for backtracking purposes. This backtracking information is used to generate new schedules.




initialState :: DPOR Source #

Initial DPOR state, given an initial thread ID. This initial thread should exist and be runnable at the start of execution.

findSchedulePrefix :: DPOR -> Maybe ([ThreadId], Bool, Map ThreadId ThreadAction) Source #

Produce a new schedule prefix from a DPOR tree. If there are no new prefixes remaining, return Nothing. Also returns whether the decision was added conservatively, and the sleep set at the point where divergence happens.

A schedule prefix is a possibly empty sequence of decisions that have already been made, terminated by a single decision from the to-do set. The intent is to put the system into a new state when executed with this initial sequence of scheduling decisions.

incorporateTrace Source #


:: Bool

Whether the "to-do" point which was used to create this new execution was conservative or not.

-> Trace

The execution trace: the decision made, the runnable threads, and the action performed.

-> DPOR 
-> DPOR 

Add a new trace to the stack. This won't work if to-dos aren't explored depth-first.

findBacktrackSteps Source #


:: BacktrackFunc

Backtracking function. Given a list of backtracking points, and a thread to backtrack to at a specific point in that list, add the new backtracking points. There will be at least one: this chosen one, but the function may add others.

-> Bool

Whether the computation was aborted due to no decisions being in-bounds.

-> Seq ([(ThreadId, Lookahead)], [ThreadId])

A sequence of threads at each step: the list of runnable in-bound threads (with lookahead values), and the list of threads still to try. The reason for the two separate lists is because the threads chosen to try will be dependent on the specific domain.

-> Trace

The execution trace.

-> [BacktrackStep] 

Produce a list of new backtracking points from an execution trace. These are then used to inform new "to-do" points in the DPOR tree.

Two traces are passed in to this function: the first is generated from the special DPOR scheduler, the other from the execution of the concurrent program.

If the trace ends with any threads other than the initial one still runnable, a dependency is imposed between this final action and everything else.

incorporateBacktrackSteps :: [BacktrackStep] -> DPOR -> DPOR Source #

Add new backtracking points, if they have not already been visited and aren't in the sleep set.

DPOR scheduler

data DPORSchedState k Source #

The scheduler state




  • schedSleep :: Map ThreadId ThreadAction

    The sleep set: decisions not to make until something dependent with them happens.

  • schedPrefix :: [ThreadId]

    Decisions still to make

  • schedBPoints :: Seq ([(ThreadId, Lookahead)], [ThreadId])

    Which threads are runnable and in-bound at each step, and the alternative decisions still to make.

  • schedIgnore :: Bool

    Whether to ignore this execution or not: True if the execution is aborted due to all possible decisions being in the sleep set, as then everything in this execution is covered by another.

  • schedBoundKill :: Bool

    Whether the execution was terminated due to all decisions being out of bounds.

  • schedDepState :: DepState

    State used by the dependency function to determine when to remove decisions from the sleep set.

  • schedBState :: Maybe k

    State used by the incremental bounding function.

initialDPORSchedState Source #


:: Map ThreadId ThreadAction

The initial sleep set.

-> [ThreadId]

The schedule prefix.

-> DPORSchedState k 

Initial DPOR scheduler state for a given prefix

type IncrementalBoundFunc k = Maybe k -> Maybe (ThreadId, ThreadAction) -> (Decision, Lookahead) -> Maybe k Source #

An incremental bounding function is a stateful function that takes the last and next decisions, and returns a new state only if the next decision is within the bound.

type BacktrackFunc = [BacktrackStep] -> [(Int, Bool, ThreadId)] -> [BacktrackStep] Source #

A backtracking step is a point in the execution where another decision needs to be made, in order to explore interesting new schedules. A backtracking function takes the steps identified so far and a list of points and thread at that point to backtrack to. More points be added to compensate for the effects of the bounding function. For example, under pre-emption bounding a conservative backtracking point is added at the prior context switch. The bool is whether the point is conservative. Conservative points are always explored, whereas non-conservative ones might be skipped based on future information.

In general, a backtracking function should identify one or more backtracking points, and then use backtrackAt to do the actual work.

backtrackAt Source #


:: (ThreadId -> BacktrackStep -> Bool)

If this returns True, backtrack to all runnable threads, rather than just the given thread.

-> BacktrackFunc 

Add a backtracking point. If the thread isn't runnable, add all runnable threads. If the backtracking point is already present, don't re-add it UNLESS this would make it conservative.

dporSched Source #


:: IncrementalBoundFunc k

Bound function: returns true if that schedule prefix terminated with the lookahead decision fits within the bound.

-> Scheduler (DPORSchedState k) 

DPOR scheduler: takes a list of decisions, and maintains a trace including the runnable threads, and the alternative choices allowed by the bound-specific initialise function.

After the initial decisions are exhausted, this prefers choosing the prior thread if it's (1) still runnable and (2) hasn't just yielded. Furthermore, threads which will yield are ignored in preference of those which will not.

Dependency function

dependent :: DepState -> ThreadId -> ThreadAction -> ThreadId -> ThreadAction -> Bool Source #

Check if an action is dependent on another.

This is basically the same as dependent', but can make use of the additional information in a ThreadAction to make better decisions in a few cases.

dependent' :: DepState -> ThreadId -> ThreadAction -> ThreadId -> Lookahead -> Bool Source #

Variant of dependent to handle Lookahead.

Termination of the initial thread is handled specially in the DPOR implementation.

dependentActions :: DepState -> ActionType -> ActionType -> Bool Source #

Check if two ActionTypes are dependent. Note that this is not sufficient to know if two ThreadActions are dependent, without being so great an over-approximation as to be useless!

Dependency function state

data DepState Source #




initialDepState :: DepState Source #

Initial dependency state.

updateDepState :: DepState -> ThreadId -> ThreadAction -> DepState Source #

Update the dependency state with the action that has just happened.

updateCRState :: ThreadAction -> Map CRefId Bool -> Map CRefId Bool Source #

Update the CRef buffer state with the action that has just happened.

updateMVState :: ThreadAction -> Set MVarId -> Set MVarId Source #

Update the MVar full/empty state with the action that has just happened.

updateMaskState :: ThreadId -> ThreadAction -> Map ThreadId MaskingState -> Map ThreadId MaskingState Source #

Update the thread masking state with the action that has just happened.

isBuffered :: DepState -> CRefId -> Bool Source #

Check if a CRef has a buffered write pending.

isFull :: DepState -> MVarId -> Bool Source #

Check if an MVar is full.

canInterrupt :: DepState -> ThreadId -> ThreadAction -> Bool Source #

Check if an exception can interrupt a thread (action).

canInterruptL :: DepState -> ThreadId -> Lookahead -> Bool Source #

Check if an exception can interrupt a thread (lookahead).

isMaskedInterruptible :: DepState -> ThreadId -> Bool Source #

Check if a thread is masked interruptible.

isMaskedUninterruptible :: DepState -> ThreadId -> Bool Source #

Check if a thread is masked uninterruptible.


didYield :: ThreadAction -> Bool Source #

Check if a thread yielded.

willYield :: Lookahead -> Bool Source #

Check if a thread will yield.

killsDaemons :: ThreadId -> Lookahead -> Bool Source #

Check if an action will kill daemon threads.