Most of the cost, effort and time of a software project is spent on the maintenance phase, the modification of a software product after delivery. If you think about all these resources as “negative investments” or debt settlement and try to associate your spendings with specific code areas or even single lines of code, you’ll probably find that the maintenance cost per line is not equally distributed. There are lots of lines of code that outlast the test of time without any maintenance work at all, a fair amount of lines that require moderate attention and some lines that seem to require constant and excessive developer care.

If you transfer this image to another metaphor, your code presents itself like a minefield for maintenance effort: Most of the area is harmless and safe to travel. But there are some positions that will just blow up once touched. The difference is that as a software developer, you don’t tread on the minefield, but you catch the flak if something happens.

You should try to deliver your code free of maintenance mines.

Spotting a maintenance mine

Identifying a line of code as a maintenance mine after the fact is easy. You probably already recognize the familiar code as “troublesome” because you’ve spent hours trying to understand and fix it. The commit history of your version control system can show you the “hottest” lines in your code – the areas that were modified most often. If you add tests for each new bug, you’ll find that the code is probably tested really well, with tests motivated by different bug issues. In hindsight, you can clearly distinguish low-effort code from high maintenance code.

But before delivery, all code looks the same. Or does it?

An example of a maintenance mine

Let’s look at an example. Our system monitors critical business data and sends out alerts if certain conditions are met. One implementation of the part sending the alerts is a simple e-mail sender. The code is given here:

public class SendEmailService {

  public void sendTo(
                Person person,
                String subject,
                String body) {
    execCmd(
         buildCmd(
               person.email(), subject, body));
  }

  private String buildCmd(String recipientMailAdress, String subject, String body){
    return "'/usr/bin/mutt -t " + recipientMailAdress + " -u " + subject + " -m " + body + "'";
  }

  private int execCmd(String command) throws IOException{
    return Runtime.getRuntime()
                  .exec(command).exitValue();
  }
}

This code has two interesting problems:

• The first problem is that it is written in Java, a platform agnostic programming language, but depends on being run on a linux (or sufficiently similar unixoid) operating system. The system it runs on needs to supply the /usr/bin/mutt program and have the e-mail sending settings properly configured or else every try to run the send command will result in an error. This implicit dependency on the configuration of the production system isn’t the best way to deal with the situation, but it’s probably a one-time pain. The problem clearly presents itself and once the system is set up in the right way, it is gone (until somebody tampers with the settings again). And my impression is that this code separates two concerns between development and operations rather nicely: Development provides software that can send specific e-mails if operations provides a system that is capable of sending e-mails. No need to configure the system for e-mail sending and doing it again for the software on said system.
• The second problem looks like a maintenance mine. In the line where the code passes the command line to the operating system (by calling Runtime.getRuntime().exec()), a Process object is returned that is only asked for its exitValue(), implicating a wait for the termination of the system command. The line looks straight and to the point. No need to store and handle intermediate objects if you aren’t interested in them. But perhaps, you should care:

By default, the created process does not have its own terminal or console. All its standard I/O (i.e. stdin, stdout, stderr) operations will be redirected to the parent process, where they can be accessed via the streams obtained using the methods getOutputStream(), getInputStream(), and getErrorStream(). The parent process uses these streams to feed input to and get output from the process. Because some native platforms only provide limited buffer size for standard input and output streams, failure to promptly write the input stream or read the output stream of the process may cause the process to block, or even deadlock.

Emphasize by me, see also: https://docs.oracle.com/en/java/javase/11/docs/api/java.base/java/lang/Process.html

This means that the Process object’s stdout and stderr outputs are stored in buffers of unknown (and system dependent) size. If one of these buffers fills up, the execution of the command just stops, as if somebody had paused it indefinitely. So, depending on your call’s talkativeness, your alert e-mail will not be sent, your system will appear to have failed to recognize the condition and you’ll never see a stacktrace or error exit value. All other e-mails (with less chatter) will go through just fine. This is a guaranteed source of frantic telephone calls, headaches and lost trust in your system and your ability to resolve issues.

And all the problems originate from one line of code. This is a maintenance mine with a stdout fuse.

The fix for this line might lie in the use of the ProcessBuilder class or your own utility code to drain the buffers. But how would you discover the mine before you deliver it?

Mines often lie at borders

One thing that stands out in this line of code is that it passes control to the “outside”. It acts as a transit point to the underlying operating system and therefor has a lot of baggage to check. There are no safety checks implemented, so the transit must be regarded as unsafe. If you look out for transit points in your code (like passing control to the file system, the network, a database or another external system), make sure you’ve read the instructions and requirements thoroughly. The problems of a maintenance mine aren’t apparent in your code and only manifest themselves during the interaction with the external system. And this is a situation that happens disproportionately often in production and comparably seldom during development.

So, think of your code as a maintenance minefield and be careful around its borders.

What is your minesweeper story? Drop us a comment.