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Counting Weekends and Wrapping Text

I said that I probably wouldn’t have time to get involved with the Perl Weekly Challenge every week and that has, unfortunately, proven to be the case. But I had a few free minutes earlier in the week so I decided to look at this week’s challenges. I’m glad I did because they seemed to fit the way my brain works pretty well and I had solutions written rather quickly.

Challenge 1: Write a script to display months from the year 1900 to 2019 where you find 5 weekends i.e. 5 Friday, 5 Saturday and 5 Sunday.

This would be simple enough to just brute-force. But when I started to think about it, I realised there’s a bit of a trick we can use which can cut down our search space quite significantly.

If we’re looking for a month with five Fridays, Saturdays and Sundays then we need a month with 31 days (as four weeks is twenty-eight days and we need three extra days). Only seven months ever have 31 days – January, March, May, July, August, October and December. There is no point at all in ever looking in any other month. You might also realise that those three extra days need to be Friday 29th, Saturday 30th and Sunday 31st. And that means that the first day of the month also needs to be a Friday.

So, the problem simplifies to “Find months with 31 days where the 1st is a Friday”. And here’s the code I wrote to do that:

I’ve seen a few other solutions published and people seem to split into one group who spotted the shortcuts and another who didn’t. But the actual solutions seem very similar. Some people used DateTime instead of Time::Piece and others used low-level functions like timelocal().

Challenge 2: Write a script that can wrap the given paragraph at a specified column using the greedy algorithm.

Honestly, I didn’t think very hard about this at all. I just read the Wikipedia description of the algorithm and wrote a pretty much word-for-word Perl translation of that.

Next week is all about the European Perl Conference so I very much doubt if I’ll have time to try the Perl Weekly Challenges. But I hope to be able to try more of the problems in the coming weeks.

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Perl Weekly Challenge – 2019-03-25

I’m not sure that I’ll have time to do these every week, but here are my answers to this week’s two Perl Weekly Challenges.

Challenge #1

Write a script to replace the character ‘e’ with ‘E’ in the string ‘Perl Weekly Challenge’. Also print the number of times the character ‘e’ found in the string.

Nothing really complicated here. We can discuss why I reached for tr/.../.../ rather than s/.../.../. I just think it’s silly to invoke the full power of the regex engine when you’re only changing a few letters.

Challenge #2

Write one-liner to solve FizzBuzz problem and print number 1-20. However, any number divisible by 3 should be replaced by the word fizz and any divisible by 5 by the word buzz. Numbers divisible by both become fizz buzz.

This is a bit more interesting. And I have to say that I think this is deeply dirty code. Over the twenty plus years I’ve been writing Perl I’ve trained myself to write code that is use strict and use warnings clean. This code is very much not that and with warnings turned on, you’d see a lot of warnings!

So how does it work?

The core is the (string)[index] construct.  We all know that you can access an individual element in an array with code like \$my_array[\$index]. But you can also do this with a list of values. For example:

returns “bar”. I often use this when I only need some of the values returned by localtime().

Here, I’m using something very similar. (fizz)[\$_ % 3] is looking up a value in a list that contains only a single value (the string “fizz”). So if the index is zero then we get the first (and only) value in the list. If the index is anything other than zero then we’re looking for an element that’s off the end of the list and Perl gives us undef, which is interpreted as an empty string. And the index expression, \$_ % 3, gives us zero if \$_ is exactly divisable by 3. There are two things here that would throw warnings under use strict and use warnings. Firstly, “fizz” is a bareword and secondly, we’re using undef in a concatenation.

We then repeat the same logic using “buzz” and 5 and we concatenate together the results of those two expressions. That gives us either “fizz”, “buzz”, “fizzbuzz” or the empty string. Of those values, only the empty string is seen as false, so we can use || \$_ to replace that with the original number.

There’s one mystery left. Why have I put that + before the first expression? I think I’m going to leave that as an exercise for the reader. If you work it out, please leave a comment.

And there we have it. A solution that is simultaneously both horribly dirty and yet, I think, rather clever.

Have you come up with your own solution yet?

Update:

It’s been pointed out to me that my solution for challenge #2 prints “fizzbuzz” for 15, when the specification clearly asks for “fizz buzz” (with a space). So here’s a version which fixes that:

Categories

A Subtle Bug

Earlier this week, I saw this code being recommended on Stack Overflow. The code contains a nasty, but rather subtle bug. The version I saw has been fixed now, but I thought there were some interesting lessons to learn by looking at the problems in some detail.

Let’s start by working out what the bug is. Here’s the code:

On first glance, it seems fine. It uses the common “open or die” idiom. It uses the modern approach of using a lexical filehandle. It even uses the three-argument version of “open()”. Code like has appeared in huge numbers of Perl programs for years. What can possibly be the problem?

I’ll give you a couple of minutes to have a closer look and work out what you think the problem is.

[ … time passes … ]

So what do you think? Do you see what the problem is?

The problem is that there is no error checking.

“What do you mean, Dave?” I hear you say. “There’s error checking there – I can see it plainly.” Some of you might even be wondering if I’m going senile.

And, yes, it certainly looks like it checks for errors. But the error checking doesn’t work. Let me prove that to you. We can check it with a simple command line program.

You would expect to see the “die” message there. But it doesn’t appear. Ok, perhaps I’m lying. Perhaps I really do have a file called “not.there”. Let’s try another, slightly different, version of the code.

And there we see the error message. That file really doesn’t exist.

So what went wrong with the first version? Of course, a good way to start working that out is to compare the two versions and look at the differences between them. The difference here is that when I put parentheses around the parameters to “open()” it started working. And when you fix things by adding parentheses it’s a pretty sure bet that the problem comes down to precedence.

The order of precedence for Perl operators is listed in perldoc perlop. If you look at that list you’ll see that the “or” operator we used (“||”) is at position 16 on the list. But what other operators are we using in our code? The answer is lurking down at position 21 on the list. When we call a Perl built-in function without using parentheses around the parameters, it’s known as a list operator. And list operators have rather low precedence.

All of which means that our original code is actually parsed as if we had written it like this:

Notice the parentheses that have appeared around \$file and (crucially) the whole “or die” clause. That means that the bracketed expression is evaluated and passed to “open()” as its third argument. And when Perl evaluates that expression, it does that clever “Boolean short-circuiting” thing. An expression of “A || B” evaluates A first and if that is true, it returns it. Only if A is false will it go on to evaluate B and return that. In our case, the filename will always be true (well, unless you have a file called “0”) so the second half of the expression (the “or die…” bit) is never evaluated and, effectively, ignored.

Which is why I said, back at the start, that this code has no error checking at all – that’s literally true as the error checking has no effect at all.

So how do we fix it? Well, we’ve already seen one approach – you can explicitly add parentheses around the arguments to “open()”. But Perl programmers don’t like to use unnecessary punctuation and I’m sure I’ve seen this written without parentheses, so how does that work?

If you take another look at the table of operator precedence and look down below the list operators, you’ll see another “or” operator (the one that’s actually the word “or”, rather than punctuation). It’s right at the bottom of the list – at position 24. And that means we can use that version without needing the parentheses around the parameters to “open()”.

And that’s the version that you’ll see in most codebases. But, as we’ve seen, it’s vitally important to use the correct version of the “or” operator.

The worst thing about this bug is that it appears at the worst time. If your file exists and you can open it successfully, then everything works fine. Things only go wrong when… well, when things go wrong. If you can’t open your file for some reason, you won’t know about it. Which is bad.

So it’s important to test that your code works correctly when things go wrong. And that’s why we have modules like Test::Exception. You could write a test program like this:

And it would fail every time. But if you switched to the other “or” operator, it will work.

There’s one other approach you can take. You can use autodie in your code and just forget about adding “or die” to any of your calls to “open()”.

This is an easy bug to introduce into your code and a hard one to track down. Who’s confident that it doesn’t appear in any of their code?

Categories

Earlier this week, the Perl magazine site, perl.com, published an article about writing web applications using CGI.pm. That seemed like a bizarre choice to me, but I’ve decided to use it as an excuse to write an article explaining why I think that’s a really bad idea.

It’s important to start by getting some definitions straight – as, often, I see people conflating two or three of these concepts and it always confuses the discussion.

• The Common Gateway Interface (CGI) is a protocol which defines one way that you can write applications that create dynamic web pages. CGI defines the interface between a web server and a computer program which generates a dynamic page.
• A CGI program is a computer program that is written in a manner that conforms to the CGI specification. The program optionally reads input from its environment and then prints to STDOUT a stream of data representing a dynamic web page. Such programs can be (and have been!) written in pretty much any programming language.
• CGI.pm is a CPAN module which makes it easier to write CGI programs in Perl. The module was included in the Perl core distribution from Perl 5.004 (in 1997) until it was removed from Perl 5.22 (in 2015).

A Brief Introduction to CGI.pm

CGI.pm basically contained two sets of functions. One for input and one for output. There was a set for reading data that was passed into the program (the most commonly used one of these was param()) and a set for producing output to send to the browser. Most of these were functions which created HTML elements like <h1> or <p>. By about 2002, most people seemed to have worked out that these HTML creation functions were a bad idea and had switched to using a templating engine instead. One output function that remained useful was header() which gave the programmer an easy way to create the various headers required in an HTTP response – most commonly the “Content-type” header.

For at least the last ten years that I was using CGI.pm, my programs included the line:

as it was only the param() and header() functions that I used.

I should also point out that there are two different “modes” that you can use the module in. There’s an object-oriented mode (create an object with CGI->new and interact with it through methods) and a function-based mode (just call functions that are exported by the module). As I never needed more than one CGI object in a program, I always just used the function-based interface.

Why Shouldn’t We Use CGI.pm Today?

If you’re using CGI.pm in the way I mentioned above (using it as a wrapper around the CGI protocol and ignoring the HTML generation functions), then it’s not actually a terrible way to write simple web applications. There are two problems with it:

1. CGI programs are slow. They start up a Perl process for each request to the CGI URL. This is, of course, a problem with the CGI protocol itself, not the CGI.pm module. This might not be much of a problem if you have a low-traffic application that you want to put on the web.
2. CGI.pm gives you no help building more complicated features in a web application. For example, there’s no built-in support for request routing. If your application needs to control a number of URLs, then you either end up with a separate CGI program for each URL or you shoe-horn them all into the same program and set up some far-too-clever mod_rewrite magic. And everyone reinvents the same wheels.

Basically, there are better ways to write web applications in Perl these days. It was removed from the Perl code distribution in 2015 because people didn’t want to encourage people to use an outdated technology.

What are these better methods? Well, anything based on an improved gateway interface specification called the Perl Server Gateway Interface (PSGI). That could be a web framework like Dancer2, Catalyst or Web::Simple or you could even just use raw PSGI (by using the toolkit in the Plack distribution).

Often when I suggest this to people, they think that the PSGI approach is going to be far more complex than just whipping up a quick CGI program. And it’s easy to see why they might think that. All too often, an introduction to PSGI starts by building a relatively powerful (and, therefore, complicated) web application using Catalyst. And while Catalyst is a fine web framework, it’s not the simplest way to write a basic web application.

But it doesn’t need to be that way. You can write PSGI programs in “raw PGSI” without reaching for a framework. Sure, you’ll still have the problems listed in my point two above, but when you want to address that, you can start looking at the various web frameworks. Even so, you’ll have three big benefits from moving to PSGI.

The Benefits of PSGI

As I see it, there are three huge benefits that you get from PSGI.

Software Ecosystem

The standard PSGI toolkit is called Plack. You’ll need to install that. That will give you adapters enabling you to use PSGI programs in pretty much any web deployment environment. It also includes a large number of plugins and extensions (often called “middleware”) for PSGI. All of this software can be added to your application really simply. And any bits of your program that you don’t have to write is always a big advantage.

Testing and Debugging

How do you test your CGI program? Probably, you use something like Selenium (or, perhaps, just LWP) to fire requests at the server and see what results you get back.

And how about debugging any problems that your testing finds? All too often, the debugging that I see is warn() statements written to the web server error log. Actually, when answering questions on StackOverflow, often the poster has no idea where to find the error log and we need to resort to something like use CGI::Carp 'fatalsToBrowser', which isn’t exactly elegant.

A PSGI application is just a subroutine. So it’s trivial for testing tools to call the subroutine with the correct parameters. This makes testing PSGI programs really easy (and all of the tools to do this are part of the Plack distribution I mentioned above). Similarly, there are tools debugging a PSGI program far easier than the equivalent CGI program.

Deployment Flexibility

This, to me, is the big one. I talked earlier about the performance problems that the CGI environment leads to. You have a CGI program that is only used by a few people on your internal network. And that’s fine. The second or so it takes to respond to each request isn’t a problem. But it proves useful and before you know it, many more people start to use it. And then someone suggests publishing it to external users too. The one-second responses stretch to five or ten seconds, or even longer and you start getting complaints about the system. You know you should move it to a persistent environment like FastCGI or mod_perl, but that would require large-scale changes to the code and how are you ever going to find the time for that?

With a PSGI application, things are different. You can start by deploying your PSGI code in a CGI environment if you like (although, to be honest, it seems that very few people do that). Then when you need to make it faster, you can move it to FastCGI or mod_perl. Or you can run it as a standalone web service and configure your web proxy to redirect requests to it. Usually, you’ll be able to use exactly the same code in all of these environments.

In Conclusion

I know why people still write CGI programs. And I know why people still write them using CGI.pm – it’s what people know. It’s seen as the easy option. It’s what twenty-five years of web tutorials are telling them to do.

But in 2018 (and, to be honest, for most of the last ten years) that’s simply not the best approach to take. There are more powerful and more flexible options available.

Please don’t write code using CGI.pm. And please don’t write tutorials encouraging people to do that.

Categories

Fixing a Bug

I fixed a bug earlier this week. Ok, actually, I introduced a bug and then spent the next few hours tracking it down and fixing it – but that doesn’t sound quite so positive, does it?

I thought it might be interesting to talk you through the bug and the fix. I should point out that this is client code, so all identifiers have been changed to protect the innocent.

We had some code that was being called in a loop while building the response to an API call. Usually, the loop would have a few dozen iterations, but in a few cases it could have something like 6,000 iterations. And in those cases we wanted to speed things up. One of the approaches I took was to eliminate all unnecessary database calls from the code in the loop. So I set DBIC_TRACE and made a request. And saw about a dozen database queries for each iteration. They had to go, so I started looking at the code.

One line I saw, looked like this:

The \$widgets variable ends up storing a reference to an array containing details of three commonly used widget types. But they are always the same three widget types. So we can cache this.

This is basically the same code, but we now only run one query against the database. So that’s a big win. There was one other step. It turned out I was building the same cache in two different subroutines, so I moved the cache to a package-level lexical variable.

I made a few more similar changes and then submitted the code for review by some other developers in the team before releasing it. One of them asked why I hadn’t used the existing Widget::Cache module instead of inventing my own cache. The answer was that I hadn’t heard of it but on investigation it seemed to be just what I wanted. As the server starts, this module reads details of all of the widgets and stores them in a hash. It ends up looking like this:

The module also exports helper functions which gave access to the various parts of the cache – things like get_widget_by_code() and get_widget_by_id(). I added a new subroutine called get_common_widgets() and called it from my code like this:

I also removed the package-level lexical variable that I had previously been using to store my widget cache.

At this point, I had introduced the bug. But it wasn’t found until a few hours later when the fix was in QA. The API response that I had been working on was still working fine, but a few other unrelated pages on the site had stopped working. And the error messages all came from functions in the Widget::Cache module. They all complained that the cache variable, \$widgets, was being used as a hash reference when it was actually an array reference.

It was clear that I’d broken something. But what? Can you see it?

Eventually, I resorted to checking every piece of code that touched the \$widgets widgets variable. And there were two places that set a variable called \$widgets. One was the initialisation code that I hadn’t touched and the other was my code where I get the list of common widgets. Remember? It looks like this:

And get_common_widgets() returns an array reference – so that would certainly explain the behaviour I’m seeing. But this variable isn’t the cache variable from Widget::Cache. It’s not even in the same file. Oh. Wait!

Remember I said that Widget::Cache exports some helper functions that give access to parts of the cache? Well, it also exports the \$widget variable itself. I guess that’s in case you want to access a bit of it that isn’t exposed by a helper function. But in that situation, you should do what I did – you should add a new helper function. You shouldn’t just go rummaging in the innards of a global variable. In fact you shouldn’t have access to that global variable at all.

So, the fix was simple.

We now have a local, lexical variable and we don’t go trashing a global variable that various parts of the codebase rely on. The next step would be to remove that dangerous export and check that all code that uses it is replaced with code that uses helper functions instead. But it’s a large codebase and I’m not sure I want to open that particular can of worms.

Global variables are a terrible idea. Only use them if it’s absolutely necessary (and, even then, you’re probably wrong about it being absolutely necessary).

So now you all know how I nearly broke a client’s site this week and how the problem was only caught at the last minute in QA. Remember this when I’m next asking you to employ me.

(And, yes, I know I make the codebase sound very “procedural”. Lots of it is, I’m afraid, and we just have to do the best with what we have.)