Somehow when my code stops working, my first (and second, and third) reaction is to blame everything except my own code. (“It’s not me, it’s you.”)

And almost always, it’s my own code that’s the problem (hence the title of this post).

I spent the day trying to resolve a bug in my early-in-development R package, qtl2geno. In the process, I blamed

• TravisCI for not handling system.file() properly.
• R-devel for having broken system.file().
• data.table::fread() for treating sep=NULL differently on different operating systems.

Of course, none of these were true. I was just passing sep=NULL to data.table::fread(), and that worked in the previous version, but doesn’t work in the latest release on CRAN, and I hadn’t yet installed the latest version of data.table on my Mac, but Travis and my junky Windows laptop had the latest version.

The debugging process seems a potentially interesting case study, so I thought I’d write down some of the details.

The whole thing started off with my qtl2geno package suddenly failing on TravisCI, even though it was still passing R CMD check on my local machine. (This was on 19 Sept 2015, when a new version of data.table hit CRAN, but I didn’t realize that until the very end of the story.)

The problem line was in a vignette:

grav2 <- read_cross2( system.file("extdata", "grav2.zip", package="qtl2geno") )

The changes I’d been making to my package didn’t seem to have anything to do with this, so I concluded that the problem was system.file(). Why that would have worked before but not now, I couldn’t say, but I figured maybe Travis was installing things differently than before.

I changed that code block to eval=FALSE, but then a bunch of tests using read_cross2( system.file( ... ) ) were failing instead. Again, I figured the problem was system.file(), because R CMD check worked fine on my machine.

I then thought maybe Travis was using R-devel whereas I was using the R-release. So I used R Win-builder to test my package with R-devel, and sure enough it failed there. So it wasn’t Travis. My conclusion was that R-devel had broken system.file().

Poked around some more, and then finally used R Win-builder with R-release. And it failed with that, too. So it wasn’t R-devel either.

I then created a little tester package, testsysfile, testing system.file() with read.csv() and then with data.table::fread(). That all worked fine. So it seemed like it wasn’t actually system.file(), but something within read_cross2().

So finally I poked into my own code, and then I realized that I was passing a NULL value to the sep argument of fread(). (See here and here.)

But why would sep=NULL work on my Mac but not on Travis or R Win-builder? I pulled out my junky Windows laptop (purchased in order to have a better sense of what my students were dealing with), went through all of the Windows updates, then updated R, Rtools, and RStudio, and tried things out there.

With my testsysfile package, I could see that data.table::fread() was throwing an error with sep=NULL on Windows, though it was working fine on my Mac.

I spent some time making a patch to fread(), and I was really close to submitting a pull request. (Treating sep=NULL didn’t seem important, but it did seem like it the behavior should be the same on Windows and Mac.) But then I saw, in the README file, that there were Contribution guidelines. I figured I’d better go through them.

The guidelines say, “squash all your commits together,” so I spent some time getting my two commits combined into one. The guidelines also say to update the README file. In doing so, I realized that they’d put a new version of data.table on CRAN on 19 Sep 2015, which is precisely when my package started failing on Travis.

And so then I finally installed the updated version of data.table on my Mac, and saw that there was no operating system dependence; the differences I was seeing were just because of the old vs new version of data.table.

So data.table was updated in a way where sep=NULL was no longer working, and in my own code, I had relied on that. So six hours, many tweets, lots of blaming of others, and finally I’d worked out that it was all my own fault.