Entries tagged less

Related tags: bugs, debian, misc, public bugfixing, testing.

On writing test-cases and testsuites.

Wednesday, 22 October 2014

Last night I mostly patched my local copy of less to build and link against the PCRE regular expression library.

I've wanted to do that for a while, and reading Raymond Chen's blog post last night made me try it out.

The patch was small and pretty neat, and I'm familiar with GNU less having patched it in the past. But it doesn't contain tests.

Test cases are hard. Many programs, such as less, are used interactively which makes writing a scaffold hard. Other programs suffer from a similar fate - I'm not sure how you'd even test a web browser such as Firefox these days - mangleme would catch some things, eventually, but the interactive stuff? No clue.

In the past MySQL had a free set of test cases, but my memory is that Oracle locked them up. SQLite is famous for its decent test coverage. But off the top of my head I can't think of other things.

As a topical example there don't seem to be decent test-cases for either bash or openssl. If it compiles it works, more or less.

I did start writing some HTTP-server test cases a while back, but that was just to automate security attacks. e.g. Firing requests like:

GET /../../../etc/passwd HTTP/1.0
GET //....//....//....//etc/passwd HTTP/1.0

(It's amazing how many toy HTTP server components included in projects and products don't have decent HTTP-servers.)

I could imagine that being vaguely useful, especially because it is testing the protocol-handling rather than a project-specific codebase.

Anyway, I'm thinking writing test cases for things is good, but struggling to think of a decent place to start. The project has to be:

  • Non-interactive.
  • Open source.
  • Widely used - to make it a useful contribution.
  • Not written in some fancy language.
  • Open to receiving submissions.

Comments welcome; but better yet why not think about the test-coverage of any of your own packages and projects...?



Listen to me when I'm telling you

Thursday, 14 February 2008

So today I'm a little bit lazy and I've got the day off work. As my previous plan suggested I wanted to spend at least some of the day tackling semi-random bugs. Earlier I picked a victim: less.

less rocks, and I use it daily. I even wrote an introduction to less once upon a time.

So lets take a look at two bugs from the long-neglected pile. These two issues are basically the same:

They seem like simple ones to fix, with the same root cause. Here's an example if you want to play along at home:

 cp /dev/null testing
 gzip testing
 zless testing.gz

What do you see? I see this:

"testing.gz" may be a binary file.  See it anyway?

When I select "y" I see the raw binary of the compressed file.

So, we can reproduce it. Now to see why it happens. /bin/zless comes from the gzip package and is a simple shell script:

# snipped a lot of text
LESSOPEN="|gzip -cdfq -- %s"; export LESSOPEN
exec less "$@"

So what happens if we run that?

$ LESSOPEN="|gzip -cdfq -- ~/testing.gz" /usr/bin/less ~/testing.gz
"/home/skx/testing.gz" may be a binary file.  See it anyway?

i.e. it fails in the same way. Interestingly this works just fine:

gzip -cdfq -- ~/testing.gz | less

So we've learnt something interesting and useful. We've learnt that when LESSOPEN is involved we get the bug. Which suggests we should "apt-get source less" and then "rgrep LESSOPEN ~/less-*/".

Doing so reveals the following function in filename.c:

	public char *
open_altfile(filename, pf, pfd)
	char *filename;
	int *pf;
	void **pfd;

/* code to test whether $LESSOPEN is set, and attempt to run the
   command if it is */

		 * Read one char to see if the pipe will produce any data.
		 * If it does, push the char back on the pipe.
		f = fileno(fd);

		if (read(f, &c, 1) != 1)
			 * Pipe is empty.  This means there is no alt file.
			return (NULL);
		*pfd = (void *) fd;
		*pf = f;
		return (save("-"));

That might not be digestible, but basically less runs the command specified in $LESSOPEN. If it may read a single character of output from that command it replaces the file it was going to read with the output of the command instead!

(i.e. Here less failed to read a single character, because our gzipped file was zero-bytes long! So instead it reverted to showing the binary gzipped file.)

So we have a solution: If we want this to work we merely remove the "read a single character test". I can't think of circumstance in which that would do the wrong thing, so I've submitted a patch to do that.

Bug(s) fixed.

Incidentally if you like these kind of "debuggin by example" posts, or hate them, do let me know. So I'll know whether to take notes next time or not..



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