corprewland

So, for those of you who don’t know, I’ve been working part-time at a local company to help pay my way through grad school. That’s actually a simplification of the actual truth, as I’m a part-owner of the company and I also am mostly getting benefits more than cash, but for now I’m the main system administrator on one of the main systems they run.

For the last bit, I’ve been tracking down problems in the spam checking software that we use, and it’s been a merry time. Most of the problems have been getting everything on the server to be in a single known compatible state, which is a concept I greatly commend to you if you’re running a server and don’t want to spend lots of time messing with it.

Today’s project was figuring out the source of and eliminating a bunch of error messages that get mailed out to the administrators’ mailbox every night. They’re known harmless, but it’s just aggravating and it might hide other problems.

So, I was looking through the codebase, and I found this little gem:

SPAMD=`ps aux | awk –posix ‘{ if (($1 ~ /popuser/) && ($0 ~ /\/spamd[[:blank:]]/)) print $2; }’ | wc -l | awk ‘{print $1}’`

You might ask yourself what that does. It’s pretty easy to figure out… it counts the number of instances of processes match ‘spamd ‘ followed by ‘popuser’, which is useful for figuring out whether or not spamassassin is running on your server. It’s part of 4psa server assistant. However, this may not work depending on how your server is configured. On my server, this never works because of how ps does its output.

My main point here is that that’s a crazy way to write that code. What the person is actually trying to do is make sure that they’re only getting the main spamassassin process and not any of the child processes. The child processes display as “spamd child”, the main spamassassin process displays as something like “/usr/bin/spamd -u popuser -d -m NUMBER -x –virtual-config-dir=/MAIL/DIR/FOR/YOUR/SERVER/%d/%l –socketpath=/tmp/spamd_full.sock”. So, they’ve got to distinguish between those two lines, and they’ve decided to check for random text in the first one, and written a fairly complex little shell script (calling awk twice!) to do so. They can’t check for just the word ‘popuser’ because it might appear in the path, in case you were wondering.

I replaced this with the following line:

SPAMD=`ps ax | grep -v “grep\|spamd child” | grep -i “spamd ” | wc -l | awk ‘{print $1}’`

This checks for all spamd processes, and just eliminates the ‘spamd child’ processes first. Why this way? If you’re trying to choose between two things, and one of them changes from system to system, and one of them is fixed and simple, you probably should try to select the fixed one.

So, here I didn’t want the fixed ones, so I eliminated (‘grep -v’) them. It saved me from having to try to pick the one I wanted. It’s generally as easy to select for elimination as it is to select for further processing in computer programs. This is also true in card tricks, incidentally. Just in case you want to do some card tricks.

The basic idea behind a lot of card tricks where you choose between two things is that the magician knows which one of the two things that (s)he wants you to have before had. So, the magician decides whether you’re selecting an item or selecting an item for elimination at the time you make the choice, to make sure that you get the right item.

Actually, it’s typically mostly used in really bad card tricks. The ‘decisive moments’ blog describes how a similar process to the magician’s force is used in many video games to keep the plot moving in a somewhat linear fashion transparently to the user, and why it fails.

I wonder how much the folks who do massive interactive games like 4orty2wo use this tactic, and whether they’ve found good ways to disguise that it’s happening.