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Test-first an infinite loop

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  • Bill Caputo
    Hi All, This is one of those test-first situations that I m never happy with: How to implement a requirement that a process not terminate (or possibly until
    Message 1 of 31 , Apr 4 2:50 PM
      Hi All,

      This is one of those test-first situations that I'm never happy with:
      How to implement a requirement that a process not terminate (or
      possibly until some external stimuli like a signal or kill file is
      triggered) test-first?

      Over the years, I've tried the following:

      1) Write a test to assert code is running, then have test kill it in
      some way perhaps in the way prescribed by the task at hand (probably
      using multiple threads, yuck). And yuck for the whole approach. A)
      doesn't show that the process will run indefinitely, only that it is
      running when asserted to be running. B) generally slow (often needs
      some kind of sleep or other timer to ensure process is actually
      running before assertion).

      2) Write a test to assert code is running for n amount of time, else
      fail. Similar to the previous one, but this just fails if the process
      is not running at the end of the test. Again, slow (there's some sleep
      time built into the test) and still doesn't specify the actual
      behavior (run until stopped), and probably using multiple threads
      again (double yuck).


      3) Don't test it - treat the "run forever" bit as part of the UI and
      ensure the loop is empty of all but a call to tested code, then test
      the hell out of the loop contents. In some ways this feels like a
      failure, but is least expensive (both in terms of test run time, and
      maintenance knowledge burden for the tests). The other problem with
      this approach is that it requires either having the kill event left
      untested as well, or faked and mocked somehow (this last part I've
      never done, but I'd like to give it a go some day).

      In short: This type of thing smells to high heaven, but all
      long-running processes (think services, daemons, etc) have this type
      of behavior, so its something that comes up a lot (but not often
      enough that I've seen a lot of discussion of it, hence my post).

      Thoughts, ideas, approaches, criticisms, all welcome.

      Best,
    • Bill Michell
      Well, an infinite loop is definitely a special case construct - not something that you want to see in normal usage. Sounds like what we are looking for is a
      Message 31 of 31 , Apr 9 4:46 AM
        Well, an infinite loop is definitely a special case construct - not something that you want to see in normal usage.

        Sounds like what we are looking for is a good use case - maybe a daemon that keeps running some processing method even when given no time by the current thread.

        So I'd make the processing method simply increment a counter.

        The first test just sets the counter to 0 and spawns a thread that runs our main method. The test then checks that the counter value eventually becomes at least 1 - and hence that our processing method got called. Note that spawning the thread is probably part of the test, not the method under test.

        No loop required - just a call to our processing method.

        The next test spawns the thread again, but this time checks the counter eventually becomes at least, say, 10.

        I'd posit that the simplest way to pass the test is to surround the worker method call with a while(1) construct. If you wanted to go via ten calls to the worker method first, knock yourself out.

        For completeness, I'd probably write a test that checks the counter with a decent time interval between the checks, and show that the counter value was continuing to increment. If you haven't reached while(1) already, then this test will surely get you there...

        Yes, it isn't a fast test, and yes, it involves spawning threads. Not pretty. But I'd argue that you are working with a special use case here - even though on the face of it, the construct is a really simple one. It also gives a nice clean design (separating the "never stop" from the "processing" stuff) that lets you do nice things like checking error conditions and termination conditions if those ever become interesting.

        On 8 Apr 2010, at 23:09, Tom wrote:

        > Eggzackly - hence my hesitation.
        >
        > --- In extremeprogramming@yahoogroups.com, Adam Sroka <adam.sroka@...> wrote:
        > >
        > > On Thu, Apr 8, 2010 at 11:06 AM, Tom <rossentj@...> wrote:
        > > >
        > > >
        > > >
        > > > I hesitate to suggest such ugly complexity - but ... maybe have the test start the target in a separate thread which it can kill when it's satisfied ... in a "finally" clause, of course....
        > > >
        > >
        > > At which point it may no longer be the simplest thing that could
        > > possibly work ;-)
        > >
        > > How would you change the test so that it was?
        > >
        >
        >

        --
        Bill Michell
        billmichell@...






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