Infinite Loops. Or: Anything that Can Possibly Go Wrong, Does.

Lukas EderJanuary 20th, 2015Last Updated: January 19th, 2015

0 27 3 minutes read

A wise man once said:

Anything that can possibly go wrong, does
– Murphy

Some programmers are wise men, thus a wise programmer once said:

A good programmer is someone who looks both ways before crossing a one-way street.
– Doug Linder

In a perfect world, things work as expected and you may think that it is a good idea to keep consuming things until the end. So the following pattern is found all over in every code base:

Java

for (;;) {
    // something
}

while (1) {
    // something
}

BASIC

10 something
20 GOTO 10

Want to see proof? Search github for while(true) and check out the number of matches: https://github.com/search?q=while+true&type=Code

Never use possibly infinite loops

There is a very interesting discussion in computer science around the topic of the “Halting Problem”. The essence of the halting problem as proved by Alan Turing a long time ago is the fact that it is really undecidable. While humans can quickly assess that the following program will never stop:

for (;;) continue;

… and that the following program will always stop:

for (;;) break;

… computers cannot decide on such things, and even very experienced humans might not immediately be able to do so when looking at a more complex algorithm.

Learning by doing

In jOOQ, we have recently learned about the halting problem the hard way: By doing.

Before fixing issue #3696, we worked around a bug (or flaw) in SQL Server’s JDBC driver. The bug resulted in SQLException chains not being reported correctly, e.g. when the following trigger raises several errors:

CREATE TRIGGER Employee_Upd_2  ON  EMPLOYEE FOR UPDATE
AS 
BEGIN

    Raiserror('Employee_Upd_2 Trigger called...',16,-1)
    Raiserror('Employee_Upd_2 Trigger called...1',16,-1)
    Raiserror('Employee_Upd_2 Trigger called...2',16,-1)
    Raiserror('Employee_Upd_2 Trigger called...3',16,-1)
    Raiserror('Employee_Upd_2 Trigger called...4',16,-1)
    Raiserror('Employee_Upd_2 Trigger called...5',16,-1)

END
GO

So, we explicitly consumed those SQLExceptions, such that jOOQ users got the same behaviour for all databases:

consumeLoop: for (;;)
    try {
        if (!stmt.getMoreResults() && 
             stmt.getUpdateCount() == -1)
            break consumeLoop;
    }
    catch (SQLException e) {
        previous.setNextException(e);
        previous = e;
    }

This has worked for most of our customers, as the chain of exceptions thus reported is probably finite, and also probably rather small. Even the trigger example above is not a real-world one, so the number of actual errors reported might be between 1-5.

Did I just say … “probably” ?

As our initial wise men said: The number might be between 1-5. But it might just as well be 1000. Or 1000000. Or worse, infinite. As in the case of issue #3696, when a customer used jOOQ with SQL Azure. So, in a perfect world, there cannot be an infinite number of SQLException reported, but this isn’t a perfect world and SQL Azure also had a bug (probably still does), which reported the same error again and again, eventually leading to an OutOfMemoryError, as jOOQ created a huge SQLException chain, which is probably better than looping infinitely. At least the exception was easy to detect and work around. If the loop ran infinitely, the server might have been completely blocked for all users of our customer.

The fix is now essentially this one:

consumeLoop: for (int i = 0; i < 256; i++)
    try {
        if (!stmt.getMoreResults() && 
             stmt.getUpdateCount() == -1)
            break consumeLoop;
    }
    catch (SQLException e) {
        previous.setNextException(e);
        previous = e;
    }

True to the popular saying:

640 KB ought to be enough for anybody

The only exception

So as we’ve seen before, this embarassing example shows that anything that can possibly go wrong, does. In the context of possibly ininite loops, beware that this kind of bug will take entire servers down.

The Jet Propulsion Laboratory at the California Institute of Technology has made this an essential rule for their coding standards:

Rule 3 (loop bounds)
All loops shall have a statically determinable upper-bound on the maximum number of loop iterations. It shall be possible for a static compliance checking tool to affirm the existence of the bound. An exception is allowed for the use of a single non-terminating loop per task or thread where requests are received and processed. Such a server loop shall be annotated with the C comment: /* @non-terminating@ */.

So, apart from very few exceptions, you should never expose your code to the risk of infinite loops by not providing upper bounds to loop iterations (the same can be said about recursion, btw.)

Conclusion

Go over your code base today and look for any possible while (true), for (;;), do {} while (true); and other statements. Review those statements closely and see if they can halt – e.g. using break, or throw, or return, or continue (an outer loop).

Chances are, that you or someone before you who wrote that code was as naive as we were, believing that…