Concuerring Concuerror

… and some useful lessons for testing complex applications.

Developers learn the term ‘heisenbug’ when they run their concurrent code and see it succeeding and failing ‘randomly’. Concurrent programming is hard and errors in concurrent programs are difficult to trigger and debug. Changing the program makes the bug go away. Tools that can detect such errors are complex, usually have limited abilities and are difficult to use. Hope must be abandoned.

Concuerror, a stateless model checking tool capable of detecting concurrency errors in Erlang programs, recently started showing symptoms of suffering from a heisenbug itself. Some of the continuous integration jobs on Travis CI would get stuck and timeout due to inactivity (here are a few). Starting those jobs again would sometimes make them succeed. Other, local runs of the test suite, would sometimes crash with Concuerror’s own timeout message (‘a process did not respond…’). These crashes would notably show up at different points within Concuerror’s exploration of program schedulings.

These are all symptoms of heisenbugs.

A fairly common challenge for a program whose input is some other program (e.g. compilers, code analyzers, etc) is to run such program with itself as input. Especially for analysis tools, such as Concuerror, the goal is to assert that the program does not suffer from the errors it can detect.

This is also known as “eating one’s own dog food”.

Could Concuerror find the concurrency bug it had? As its main developer I wanted the answer to be an easy “yes”. Sadly, the complexity of the tool itself was initially too high for such a task.

Thus began project dogfood. Concuerror should be able to analyze itself, either by supporting more features or by not using them at all. As the ‘Concuerror under test’ would still need an input, the simplest Erlang function was picked: test() -> ok..

The stage was set for a (very nerdy) action film!

Instrumentation and Code loading

One of Concuerror’s most useful features is its ability to find source code, instrument it, recompile and reload it on an Erlang VM in a totally automatic and seamless way. Doing this as part of a test, however, would require handling files and this is a very hard task in stateless model checking. In Erlang, this could entail unloads, resets and other similar horrors.

Fortunately, Concuerror lets programs communicate with existing registered Erlang processes and allows them to read public ETS tables. Concuerror is using such an ETS table to mark code that has been already instrumented and loaded. By appending a run of the simple test input before the run of ‘Concuerror under test’ on that input, any code that needed instrumentation would be readily instrumented when inspected by the inner Concuerror.

It is unclear how this trick can be generalized. Using files or ports is still hard in code under stateless model checking.

Unsupported operations

The next challenge was in the use of several Erlang builtin operations whose handling has not yet been implemented in Concuerror. Examples include operations that are fairly problematic to begin with (e.g. os:timestamp) and others that are simply not yet implemented (e.g. ets:update_element). Such operations were either simplified in the tool or minimal support for them was added. The last approach was easy, as Concuerror simulates very few parts of Erlang; letting the tool ‘just run’ the operations (and consider them racing with everything else) is a good first approximation.

Full sequentialization

An early goal of the self-analysis was to assert that Concuerror would be a fully sequential tool: only one process could run at anytime, a constraint that could be enforced by synchronous requests between the components of the application. This approach was abandoned early as full sequentialization of the communication with the logger component was deemed too silly.

Concurrent operations would instead be allowed, but any racing operations would be eliminated.

Looking at the magic

Once the first issues were handled, tests started showing a trace of events before getting stuck here or crashing there. This was the first time I saw how much is going on in a run of Concuerror. ETS tables were written and read, messages and signals were flying. I was dazzled.

A number of simplifications were done using this info. When the analysis is over, processes can be gently signaled to exit (Concuerror used to kill them, something which is discouraged in systematic testing, as such kill signals race with almost every other operation). Some synchronization was added in places where it would retain simplicity in the code.

Improving the user experience

Eating one’s own dog food reveals small frustrating details that can ruin user experience. Badly formed options that are ignored. Unclear messages to the user. Sanding those hard edges is trivial once they are detected. Using the tool for real is the only way to do so.

Simplifying the code

In the end, solving a tricky knot involving the marking of ‘processes under Concuerror’ and the handling of receive statements split the commit history of the ‘dog food’ branch into two parts:

  • Before the fix, Concuerror could not analyze itself: messages would be mysteriously lost between the outer and inner instances and the outer instance would get always stuck. This was not surprising: recursive use of the tool is (still) an esoteric endeavor and getting the instrumentation right is tricky.
  • After the fix, Concuerror could analyze itself! However, the analysis would terminate after exploring just one scheduling, which was correct. No races existed.

Was it the case that the bug was still there, but could only be triggered with more complex input? That’s what I initially thought. Stateless model checking is a very practical technique for finding concurrency errors, but cannot cover all inputs of a program under test.

The reason to believe that the bug had not been fixed was that the fix was just preventing a send operation from being instrumented by moving it elsewhere. To avoid introducing a race due to the movement, another send operation was also moved. Their order remained the same. Or so I thought.

Fixing the bug

Concuerror instruments and reloads code in a way that is completely transparent, unless a process has been ‘marked’ by the tool. Marked processes mostly execute their original code, but wait for permission from Concuerror’s scheduler before performing any operations that could be involved in races and also notify the scheduler about the result of any such operation.

When a process under Concuerror receives a message, Concuerror’s instrumentation intercepts it and places it in a separate queue. This is done because Concuerror’s scheduler itself communicates with processes via messages and this interception keeps channels of communication clean.

When a process under Concuerror is about to execute a receive statement, the instrumentation inspects the patterns, the existence of an after clause and the messages in the queue. If a matching message is found it’s “sent again” (the first send in the story above), so that the actual receive statement can then be executed, finding the message in the mailbox. If no intercepted message matches but an after statement should be executed, no message should be placed in the mailbox.

In either case, the process would notify Concuerror’s scheduler, sending a message to it (the second send in the story above) before executing the actual receive statement.

This opened up the possibility that the scheduler would send a message of its own, asking for the next event, before the receive statement was really executed. This was fine and well if a message was about to be received (the message would already be in the mailbox), but if the process was supposed to execute the after clause, it could ‘accidentally’ receive Concuerror’s scheduler message.

The process might then crash. Or it might ignore the message and reach the point before the next operation for which it would have to notify the scheduler. It would then wait forever for the scheduler’s message (which was already accidentally consumed).

The scheduler would also either crash, blaming the process for not responding within reasonable time to its request for a next event, or wait forever (depending on the --timeout option used).

Having to do the possible “send again” as the absolutely last thing (as this was the operation moved into uninstrumented code), forced the notification to also be moved after the receive statement (with the timeout) was completed. The scheduler’s message could no longer be lost. The test had only one interleaving.

The bug was fixed.

… and a philosophical question

In the version before the fix, Concuerror could not cleanly find this bug, but was also prevented from running correctly: there was too much instrumentation on the “sent again” handling of a message. After a partial fix the bug could still not really be found: the receive statement whose after clause was vulnerable would be uninstrumented (all the inspection would have happened beforehand).

Yet, in the version after the fix, Concuerror did not have the bug.

So, at the end of the day, did Concuerror find the bug, or was it me? :-)