About
Android, Linux, FLOSS etc.

Code
My code

Subscribe
Subscribe to a syndicated RSS feed of my blog.

Tue, 22 Dec 2009

I have become interested in the poppler library lately (and one of the
programs that depends on it, evince). Poppler is a PDF rendering library. I have been looking through Ubuntu's bug tracking system on launchpad, and people have been complaining how they run Evince on a PDF file and it crashes, or at least it doesn't display the file right away.

The particular bug that was reported through Ubuntu that I am looking at is #497175 . The user tried to use evince to look at his PDF< but it did not work. Text that should have been displayed was not displayed. He said Xpdf did work on the PDF file, displaying all text. I downloaded the sample PDF file, and saw indeed it displayed the text with Xpdf and not evince 2.28.1 (using poppler 0.12.0) on Ubuntu 9.10. I tried displaying it with evince 2.22.1.1 based on the poppler 0.6.4 library. That worked. So I figured some time between those early, working evince/poppler versions, and the more recent evince/poppler versions which broke for the bug reporter (and myself), something must have changed that broke this.

I wasted time in two respects looking at this. One is I looked at both evince and poppler. From my kibitzing of evince and poppler over the past months, I have seen over and over that most reported bugs on Ubuntu dealing with PDFs and evince are due to bugs in poppler, not evince. So trying different evince versions was a waste of time.

The second was how I dealt with poppler versions. I knew poppler 0.6.4 worked and poppler 0.12.0 didn't, so I downloaded poppler 0.10.0 (as the bug was reported recently, I leaned towards a more recent poppler version), then compiled evince against it, ran it, saw it worked, then began manually downloading other poppler versions, compiling them, compiling evince against it, testing it and so on. Eventually I saw that poppler 0.11.1 worked and poppler 0.11.2 did not.

However, I was doing a lot of unnecessary work. Poppler uses a git repository. I heard about git when it was announced in 2005, and I have checked out code from git repositories, and have browsed some git source trees over the web, but I have never looked much into it. Git has a cool feature called "bisect". Poppler has each release version tagged with the release name. So what I could have done was a git bisect - marking 0.6.4 as a good version, and 0.12.0 as a bad version. Git would have bisected all the commits between these two tags. I would test it to see if it was good or bad. If it was bad, it would bisect at the 25% mark between 0.6.4 and 0.12.0, if it is good, it would bisect at the 75% mark between 0.6.4 and 0.12.0. You keep bisecting until you get to the bad commit.

I am doing this now, and am down to my last test. I will mark it good or bad, after which we will know which commit caused this problem.

[...]

There, I'm done. Commit ad26e34bede53cb6300bc463cbdcc2b5adf101c2 broke it. Changes to the CairoOutputDev.cc file. Before that commit, the text displays, after the commit, it does not. I changed the Ubuntu bug report and reported it to the poppler upstream.

[/poppler] permanent link